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Holding it all together? Candidate proteins for the plant Golgi matrix Maita Latijnhouwers1, Chris Hawes2 and Claudine Carvalho2 A combination of electron microscopy and fluorescence microscopy has provided us with a global picture of the structure of the plant Golgi apparatus. However, the components that shape this structure remain elusive. In other organisms, members of the golgin family of coiled-coil proteins are essential for Golgi structure and organisation. Putative Arabidopsis and rice homologues of some golgin family members can be identified using database searches. Likewise, the heterogeneous group of multi-subunit-tethering complexes is responsible for crucial transport steps that affect Golgi structure and cisternal organisation in animals and yeasts. The Arabidopsis genome harbours possible homologues for the majority of the subunits of these complexes, suggesting that they also operate in the plant kingdom. Addresses 1 Cell-to-Cell Communication programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK 2 Research School of Biological & Molecular Sciences, Oxford Brookes University, Headington, Oxford OX3 BP, UK Corresponding author: Latijnhouwers, Maita ([email protected]) Current Opinion in Plant Biology 2005, 8:1–8 This review comes from a themed issue on Cell Biology Edited by Patricia C Zambryski and Karl Oparka 1369-5266/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2005.09.014 The structure and function of the plant Golgi The Golgi apparatus in plants plays crucial roles in the glycosylation of proteins and lipids and in the production of polysaccharides for cell wall biosynthesis. Proteins are imported from the endoplasmic reticulum (ER), entering the Golgi apparatus on the cis side. On their way through the organelle to its opposite face, the trans Golgi, proteins and lipids may be processed by Golgi-resident enzymes that modify attached glycan chains [2,3]. The products are sorted and packaged into vesicles for transport to the vacuole or to the plasma membrane. This sorting mainly occurs in the trans Golgi or the trans-Golgi network (TGN), a vesiculo-tubular structure that has a certain degree of autonomy and is located at the trans-side of the Golgi. The TGN may also receive vesicles travelling in the opposite direction, which enter the Golgi apparatus from endosomal compartments, although this pathway has to date received very little attention in plants. In mammalian cells the Golgi stacks are very often arranged end-to-end around the nucleus. In plant cells, however, the Golgi apparatus consists of stacks of cisternae that are distributed throughout the cytoplasm. The use of fluorescent marker proteins has revealed that, in some cell types, the stacks are highly motile and move along ER strands [4–6]. Actin-depolymerising drugs have been used to show that Golgi distribution and motility is actin-dependent [4]. The structural relationship between the ER and the Golgi apparatus in plants is still a matter for debate [6–8], but a key difference between the Golgi of plants and other eukaryotes is that the ER and Golgi often appear connected in plants [8–10]. For that reason, we could predict that a set of (structural) proteins is involved in maintaining this close ER–Golgi relationship in plants. Introduction How the plant Golgi apparatus can maintain its structure of discrete, motile and polarised stacks of membranebounded cisternae is an intriguing question. The Golgi matrix of mammalian cells has been characterised in much more detail than that in plants. Its most notable element is the existence of a class of coiled-coil proteins, called golgins, which define the structure of the Golgi and also play a role in the tethering of vesicles. In this review, these proteins are taken as the starting point in the search for possible factors that give the plant Golgi its structure and organisation. At the same time, it is worth reiterating a note of caution: models of Golgi structure and function that are derived from mammalian and yeast systems must be interpreted with care and investigated experimentally before applying them to plants [1]. www.sciencedirect.com COPLBI 295 What holds the stacks together? Each Golgi stack consists of five to eight cisternae [3], depending on cell type. The existence of some sort of surrounding matrix that holds these together has been postulated [1,3]. Earlier electron microscopy studies identified a zone surrounding plant Golgi stacks from which ribosomes were absent, suggesting the presence of a matrix that has a higher density than the surrounding cytoplasm [11]. Although the existence and nature of this so-called ribosome-exclusion zone remains to be determined, it could be envisaged that it is involved in maintaining the integrity of the stack. A second component of the Golgi apparatus that probably assists in holding the cisternae together are ‘intercisternal elements’. These electron-opaque, fibrous elements have a regular strucCurrent Opinion in Plant Biology 2005, 8:1–8 2 Cell biology ture and are mainly detected between the trans cisternae of plant Golgi stacks (Figure 1; [3,12]). associated with Arabidopsis Golgi and might be required to tether the stacks to the actin cytoskeleton [19]. In the mammalian Golgi apparatus, the first indication that cisternal adhesion is dependent on proteinaceous substances came from in vitro studies of stacks from rat liver cells. Golgi stacks disintegrated into individual cisternae after treatment with proteinase K, chymotrypsin, subtilisin or elastase but remained intact after treatment with carboxypeptidase, trypsin or amylase [13]. Detergent extraction of Golgi stacks leaves an insoluble structure that is referred to as the Golgi ‘ghost’ or ‘skeleton’. In animal cells, this skeleton has been reported to contain cytoskeletal components, including spectrin and ankyrin-like proteins. Spectrins are structural proteins that form networks parallel to membrane surfaces that maintain the structure and integrity of membrane sheets [14]. The introduction of dominant negative isoforms of the Golgi-located, spectrin-like protein Syne-1 caused the collapse of the Golgi complex and the perturbation of various trafficking pathways [15,16]. It remains to be firmly established whether cytoskeletal proteins have a structural role in plant Golgi organisation. One report has, however, identified a spectrin-like protein that is associated with Golgi vesicles in an alga [17]. More recently, a kinesin 13a protein has been reported to be associated with Golgi stacks in the cortical cytoplasm of Arabidopsis leaves, but this protein is not responsible for movement [18]. An actin-associated protein, KATAMARI1, is also Golgins Recently, a family of proteins called golgins that are important for maintaining the integrity of the Golgi apparatus has been identified in animals and yeast (Figure 2). Golgins are large proteins with extensive coiled-coil domains [20–22]. The coiled-coil is a very common protein motif that consists of heptad repeats, which form amphipatic a-helices that twist into a supercoil and form a long rod-like structure [23]. Long coiledcoil proteins often have structural roles; for example, in nuclear organisation, as scaffolding proteins in centrosomes, or as part of the cytoskeleton [24]. Some golgins have carboxy-terminal domains, such as the GRIP and the GRAB (GRIP-related Arf-binding) domains, that show significant conservation across kingdoms. The GRAB domain was recently shown to bind activated ADP-ribosylation factor 1 (ARF1) [25], a small GTPase that is involved in, amongst numerous other processes, the formation of coat protein complex I (COPI)-coated vesicles [26]. The GRIP domain binds a related GTPase called ARF-like1 (ARL1), which is located on the trans Golgi or TGN [27–29]. The interaction of these domains with their respective GTPases is required for these golgins to be recruited to the Golgi. An indication that golgins have an important structural role in the Golgi apparatus was postulated by Seemann Figure 1 Electron micrograph showing a Golgi stack from a tobacco root meristem cell, high-pressure frozen and freeze-substituted. The micrograph shows intercisternal elements (arrows) between the trans-cisternae. The scale bar represents 100 nm. Reproduced from [1] with permission from Blackwell Publishing. Current Opinion in Plant Biology 2005, 8:1–8 www.sciencedirect.com Candidate proteins for the plant Golgi matrix Latijnhouwers, Hawes and Carvalho 3 Figure 2 Golgins and multi-subunit tethering complexes in the mammalian Golgi apparatus. The diagram shows the distribution of known golgins and Golgi-related multi-subunit tethering complexes in the secretory and endosomal pathway in mammalian cells. The golgins are presented in normal black type and the multi-subunit tethering complexes are in bold. The underlined golgins have a putative homologue in plants. et al. [30]. These authors showed that the mammalian golgins GM130, p115 and Giantin were present in the Golgi ‘skeleton’ that remained after treatment with Brefeldin A, a fungal metabolite that causes most Golgiresident enzymes to be transferred to the ER [30]. Further evidence comes from observations that Golgi stacks in a cell line with undetectable levels of the golgin GM130 are disassembled into dispersed vesicles when incubated at higher temperatures (39.5 8C), and can be rescued by transfection with GM130 [31]. Depletion of another golgin, Golgin-84, results in the fragmentation of the Golgi ribbon into separate stacks [32]. The golgins GM130 and Golgin-45 bind to proteins called Golgi reassembly stacking proteins (GRASPs), presumably as a way to associate these golgins with the membrane [22]. GRASPs are required for the stacking of Golgi cisternae after mitosis, as was shown in experiments using the injection of antibodies that recognise these proteins [33,34]. Apart from their role in the stacking of cisternae, golgins are important in vesicle tethering. This is the initial www.sciencedirect.com attachment of vesicles to target membranes, a process that takes place before fusion. Tethering is suggested to provide the first level of recognition and specificity to vesicle binding and to accelerate vesicle fusion by the machinery involving SNARE proteins [35,36]. It is presently hypothesised that Giantin acts in a complex together with p115 and GM130 to form a long tether that binds COPI vesicles to cis Golgi membranes [21,22,37]. Another, more recently discovered golgin tether, is composed of the mammalian golgins Golgin84 and CASP [38]. This tether bound COP1 vesicles that contain cargo that is different from that of the p115bound COPI vesicles, possibly reflecting the different trafficking processes catalysed by these different tethers [38]. The two functions of golgins, in the tethering and the stacking of cisternae, might be intimately related, given that correct vesicle binding and fusion are central to maintaining the size and composition of cisternae. It has also been suggested that golgins hold cisternae Current Opinion in Plant Biology 2005, 8:1–8 4 Cell biology together by forming bridges or networks between them [21], in much the same way as they form tethers between vesicles and cisternae. Interestingly, Saccharomyces cerevisiae has at least five members of the golgin family but its Golgi apparatus is unstacked, implying that golgins perform essential roles beyond cisternal stacking. Golgins in plants Given the importance of golgins in mammals and yeasts, it is possible to envisage a significant role for proteins of this family in plant Golgi. Conventional BLAST searches of both the Arabidopsis and rice genomes using the mammalian golgins as query sequences detected proteins that have sequence similarity to a subset of the mammalian or yeast golgins (Table 1). Because the coiled-coil domains show very low levels of sequence conservation in this class of proteins, most of the possible plant homologues were identified on the basis of the presence of other conserved domains. The first 250 amino acids of the golgin p115 share 40% identity (63% similarity) with the same region in the Arabidopsis protein that is encoded by open reading frame (ORF) At3 g27530. This region forms a globular head to the rod-like structure that is formed by the central region of the protein [39] and binds a guanine-nucleotide exchange factor (GEF) for ARF [40], which might explain the high degree of sequence conservation in this area. In the proteins that are encoded by the ORFs At3 g61570 and At2 g46180, carboxy-terminal GRAB domains were identified that share 32% and 30% identity (62% and 64% similarity), respectively, to the GRAB domain of the mammalian golgin GMAP210 [25]. These two proteins share 70% amino acid identity. The existence of two genes that encode GRAB domain proteins in the Arabidopsis genome is likely to be a result of a recent gene duplication event, as the rice genome only contains one such gene. A carboxy-terminal domain of the protein that is encoded by ORF At5 g66030 is 50% identical to the GRIP domain of Golgin-97. A fusion protein that consists of the this putative Arabidopsis GRIP domain and green fluorescent protein (GFP) was recently reported to colocate with the Golgi marker a-Mannosidase I on Golgi stacks in transformed tobacco protoplasts [41], showing that this domain targets the Golgi. Hence, this protein was named AtGRIP. A full-length AtGRIP–GFP fusion protein also co-located with Golgi markers and is thought to target the far trans face of the Golgi (Figure 3). Moreover, the GRIP domain of AtGRIP interacts directly with an Arabidopsis homologue of the small GTPase ARL1 (Arf-like 1) [42]. Proteins that are encoded by two ORFs (At1 g18190 and At2 g19950) were identified as possible golgins on the basis of sequence similarity between their carboxy-termini and the carboxy-terminal transmembrane (TM) region of Golgin-84 (46% and 50% identity in the TM region, respectively) [24,43]. Similarly, the carboxyl ter- Table 1 Human and yeast golgins and their potential homologues from Arabidopsis and rice. Human Yeast Arabidopsis 1 Rice 1 Domains other than coiled-coil Reference(s) p115 GMAP210 Uso1 RUD3 LOC Os12 g35360 LOC Os08 g29730 – GRAB [52] [25,53,54] Golgin-97 Golgin-245 GCC88 GCC185 Golgin-84 Imh1 – – – – At3 g27530 At3 g61570 At2 g46180 At5 g66030 (AtGRIP) 2 N.D. 2 N.D. 2 N.D. 2 At2 g19950 GRIP GRIP GRIP GRIP TM [55] [56,57] [58] [58] [24,59] CASP Giantin GM130 TMF Bicaudal-D1/2 Golgin-45 Golgin-67 Golgin-160 COY1 – – Sgm1 – – – – At1 g18190 At3 g18480 (AtCASP) N.D. N.D. At1 g79830 N.D. N.D. N.D. N.D. TM TM – – – – – – [24,43,44] [37] [60,61] [45,62] [63] [64] [65] [61,66] LOC Os07 N.D. 2 N.D. 2 N.D. 2 LOC Os03 LOC Os03 LOC Os03 LOC Os04 LOC Os03 N.D. N.D. LOC Os05 N.D. N.D. N.D. N.D. g28940 2 g54130 g54110 3 g54110 3 g55810 g50300 g48620 N.D. = not detected. Arabidopsis locus names: The Arabidopsis Information Resource (TAIR; www.Arabidopsis.org). Rice locus identifiers: TIGR Rice Genome Annotation (http://www.tigr.org/tdb/e2k1/osa1/index.shtml). 2 The family of GRIP domain proteins, comprising four golgin-like proteins in mammals, is represented by only one protein in Arabidopsis and rice. 3 This gene is split into two loci in the rice genome of Oryza sativa spp. japonica cv. Nipponbare. A retrotransposon is inserted in the gene towards the carboxy-terminus, leaving the gene in two parts. There are, however, sequences in the TIGR expressed sequence tag (EST) database that represent the full-length gene (NP906530 in TIGR or AK067648 in National Center for Biotechnology Information [NCBI]). 1 Current Opinion in Plant Biology 2005, 8:1–8 www.sciencedirect.com Candidate proteins for the plant Golgi matrix Latijnhouwers, Hawes and Carvalho 5 Figure 3 Tobacco epidermal cell stably expressing the transmembrane domain of rat sialyl transferase fused to monomeric Red Fluorescent Protein (shown in red). The transiently expressed fusion protein AtGRIP–GFP (shown in green) labels one end of the stacks. In the same study, it was shown that the cis Golgi t-SNARE Memb11–YFP labels the opposite end, indicating that AtGRIP–GFP is a trans Golgi protein. Reproduced from [42], courtesy of Blackwell Publishing. minus of the protein encoded by ORF At3 g18480 shows homology to the TM domain of human CASP (58% identity) [24,43]. The similarity between these two proteins is, however, also evident in other regions of the protein. The membrane topologies of At1 g18190, At2 g19950 and At3 g18480 resemble those of Golgin-84 and of CASP, which possess large amino-terminal cytoplasmic domains and very short carboxy-terminal tails in the Golgi cisternal lumen. When fused to GFP, the protein encoded by ORF At3 g18480 does indeed locate to the Golgi, and the carboxy-terminal 124 amino acids of this protein are sufficient to target GFP to the Golgi stacks. Moreover, mutagenesis of a conserved tyrosine in the TM domain of this protein disrupted the Golgi targeting of this protein [44]. The golgin TMF has no conspicuous domains other than the coiled-coil regions, the most carboxy-terminal of which was reported to bind the small GTPase Rab6 [45] and is conserved in the yeast homologue Sgm1p. The carboxy-terminus of the protein encoded by ORF At1 g79830 shows clear similarity to the Rab6-binding region in TMF and Sgm1p [45]. Although the Arabidopsis and rice ORFs that are presented in Table 1 were identified only on the basis of sequence similarity with mammalian golgins, they might prove very valuable in characterising plant Golgi organisation. Of course, the plant Golgi apparatus differs sigwww.sciencedirect.com nificantly from its yeast and mammalian counterparts, and studying the roles of golgins might reveal alternative, plant-specific functions. In some cases, it was not possible to detect any proteins with significant protein similarity in the Arabidopsis or rice genomes. Of these, Giantin and GM130 probably play a role in binding COPI vesicles to the cis cisternae. Of course, these golgins might be specific for mammalian cells, or alternatively, their sequence might have diverged to such an extent in plants that they are no longer recognised in searches using conventional BLAST algorithms. Another protein family that is particularly notable for its apparent absence in plant Golgi are the GRASP stacking proteins, whose interaction with the golgins is largely responsible for the organisation of mammalian Golgi. Again, this might reflect the differences in the organisation of these two types of Golgi. The plant genome might contain additional, plant-specific, golgin-type proteins that lack detectable similarity to mammalian or yeast golgins, although it will be a more arduous task to find such proteins. To do so, organelle-specific proteomics might be a very useful strategy [46]. Tethering protein complexes Long coiled-coil proteins are not the only vesicle-tethering factors in eukaryotic cells. Several multi-subunit protein complexes have been identified in mammals and yeast in the past decade that are involved in the recognition and tethering of vesicles, and probably in other aspects of vesicle trafficking. In addition, they might perform certain structural roles in the Golgi apparatus. The complexes, called conserved oligomeric Golgi (COG) and transport protein particles (TRAPP I and TRAPPII), facilitate ER–Golgi and intra-Golgi transport. DSL1 is important for Golgi–ER retrograde transport, and the Golgi-associated retrograde protein (GARP) complex accommodates retrograde transport from endosomes to the Golgi/TGN (reviewed in [36,47,48]). The Exocyst and HOPS (Class C) complexes are tethering complexes that are involved in post-Golgi trafficking events and will not be discussed further. In contrast to the coiled-coil tethers, which bind exclusively to activated GTP-bound GTPases, some of the multi-subunit complexes have GEF activity and are involved in the activation of Rab proteins. On the basis of this, Sacher et al. [49] hypothesised that multi-subunit tethering complexes act in the initial phase of the vesicle recognition and docking process to activate small GTPases such as Rab proteins. Activated Rabs might subsequently recruit coiled-coil tethers to facilitate vesicle binding and SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein [SNAP] receptors) to complete the fusion process [49]. The COG complex consists of eight subunits that are divided into two sub-complexes. Each of the subunits possesses short regions that are predicted to be involved in coiled-coil formation. Arabidopsis homologues can be recognised for each of the COG subunits, with levels of Current Opinion in Plant Biology 2005, 8:1–8 6 Cell biology Table 2 Golgi tethering complex subunits from human and yeast and their possible homologues from Arabidopsis. Complex COG TRAPP I TRAPP II GARP Dsl1 Name of human subunit Name of yeast subunit COG1 COG2 COG3 COG4 COG5 COG6 COG7 COG8 TRAPPC3 TRAPPC1 TRAPPC8 TRAPPC2 TRAPPC4 TRAPPC5 TRAPPC6A TRAPPC7 TRAPPC9 TRAPPC10 – Vps52 Vps53 Vps54 ZW10 RINT-1 BNIP1 COG1 COG2 COG3 COG4 COG5 COG6 COG7 COG8 Bet3 Bet5 Trs85 Trs20 Trs23 Trs31 Trs33 Trs65 Trs120 Trs130 Vps51 Vps52 Vps53 Vps54 Dsl1 Tip20 Sec20 Possible Arabidopsis homologue 1 Size in amino acids 1 At5 At4 At1 At4 At1 At1 At5 At5 At5 At1 At5 At1 At5 At5 At3 g16300 g24840 g73430 g01400 g67930 g31780 g51430 g11980 g54750 g51160 g16280 g80500 g02280 g58030 g05000 N.D. N.D. N.D. N.D. At1 g71300 At1 g50500 At4 g19490 N.D. N.D. N.D. % identity to corresponding human protein 2 1068 756 745 1117 832 706 836 569 186 169 1265 81 141 195 173 20 27 34 21 29 31 23 32 51 37 20 28 34 46 29 701 798 1054 26 24 20 1 Arabidopsis locus names and protein sizes according to protein annotation by The Arabidopsis Information Resource (TAIR; www.Arabidopsis.org). 2 Percentage protein identity was determined using ClustalW (Pôle BioInformatique Lyonnais; http://pbil.ibcp.fr/htm/index.php). amino acid identity ranging between 20% and 34% (Table 2). Fusion proteins between the putative Arabidopsis COG5 and COG8 homologues (ORFs At1 g67930 and At5 g11980, respectively) and GFP located to Golgi stacks in tobacco epidermal cells (M Latijnhouwers et al., unpublished), substantiating the significance of the BLAST approach to identify homologues of mammalian Golgi transport proteins. Similarly, the TRAPP I complex appears to be conserved in Arabidopsis, as proteins that have sequence similarity to each of the seven subunits were detected (amino-acid identity ranging between 20% and 51%). The TRAPP II complex contains the same subunits as TRAPP I but has three additional subunits (TRAPPC7, TRAPPC9 and TRAPPC10). No proteins with similarity to these three subunits have been identified in Arabidopsis, suggesting that this complex only exists as one isoform in plants. At least three of the four subunits of the GARP complex seem to be conserved in Arabidopsis [50]. A hemizygous Arabidopsis line with a T-DNA insertion in the gene that encodes a protein with similarity to VPS52 (named POKY POLLEN TUBE[POK]) has short pollen tube growth. A POK–GFP fusion protein localised to Golgi stacks in onion cells, suggesting a conserved function for the GARP complex in plants [50]. The DSL1 complex Current Opinion in Plant Biology 2005, 8:1–8 was only recently described in mammals [51]. It is not, or only poorly, conserved in plants. Conclusions The golgins and multi-subunit transport complexes have emerged as important players in mammalian and yeast Golgi structure and function. As the organisation and distribution of the plant Golgi apparatus differs from that in animal and yeast cells, we might find that plant proteins with similarity to the mammalian or yeast golgins or transport complexes have different functions in the plant Golgi. Exploring their functions in plant cells, however, opens up a new avenue for investigating the structure and function of the Golgi stack, anterograde and retrograde cargo transport, the relationship of the Golgi stack with the endoplasmic reticulum and the unique patterns of movement displayed by individual Golgi stacks. Thus, although caution should be taken when extrapolating findings from the animal to the plant system, the animal Golgi system has given us a launching pad from which to start to understand that in plants. Acknowledgements We thank the Biotechnology and Biological Sciences Research Council (BBSRC) and the Scottish Executive Environment and Rural Affairs Department (SEERAD) for supporting for this work. Ulla Neumann is acknowledged for the electron micrograph (Figure 1). www.sciencedirect.com Candidate proteins for the plant Golgi matrix Latijnhouwers, Hawes and Carvalho 7 References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1. Hawes C: Cell biology of the plant Golgi apparatus. New Phytol 2005, 165:29-44. 2. Keegstra K, Raikhel N: Plant glycosyltransferases. Curr Opin Plant Biol 2001, 4:219-224. 3. Staehelin LA, Moore I: The plant Golgi apparatus: structure, functional organisation and trafficking mechanisms. Annu Rev Plant Physiol Plant Mol Biol 1995, 46:261-288. trichome morphogenesis. The results suggest that the correct distribution of Golgi stacks might require microtubules and this new kinesin. 19. Tamura K, Shimada T, Kondo M, Nishimura M, Hara-Nishimura I: KATAMARI1/MURUS3 is a novel Golgi membrane protein that is required for endomembrane organization in Arabidopsis. Plant Cell 2005, 17:1764-1776. The Arabidopsis mutant katamari1 (kam1) has a severe defect in endomembrane organisation and in cell elongation. The KAM1 protein locates to Golgi stacks, and immunoprecipitation results show that KAM1 binds actin (although unidentified cytosolic factors are also required). KAM1 might be thus required in the actin organisation of endomembranes. 20. Barr FA, Short B: Golgins in the structure and dynamics of the Golgi apparatus. Curr Opin Cell Biol 2003, 15:405-413. 21. Gillingham AK, Munro S: Long coiled-coil proteins and membrane traffic. Biochim Biophys Acta 2003, 1641:71-85. 4. Boevink P, Oparka K, Santa Cruz S, Martin B, Betteridge A, Hawes C: Stacks on tracks: the plant Golgi apparatus traffics on an actin/ER network. Plant J 1998, 15:441-447. 22. Short B, Haas A, Barr FA: Golgins and GTPases, giving identity and structure to the Golgi apparatus. Biochim Biophys Acta 2005, 1744:383-395. 5. Nebenfuhr A, Gallagher LA, Dunahay TG, Frohlick JA, Mazurkiewicz AM, Meehl JB, Staehelin LA: Stop-and-go movements of plant Golgi stacks are mediated by the acto-myosin system. Plant Physiol 1999, 121:1127-1142. 23. Burkhard P, Stetefeld J, Strelkov SV: Coiled coils: a highly versatile protein folding motif. Trends Cell Biol 2001, 11:82-88. 6. Brandizzi F, Snapp EL, Roberts AG, Lippincott-Schwartz J, Hawes C: Membrane protein transport between the endoplasmic reticulum and the Golgi in tobacco leaves is energy dependent but cytoskeleton independent: evidence from selective photobleaching. Plant Cell 2002, 14:1293-1309. 7. Yang YD, Elamawi R, Bubeck J, Pepperkok R, Ritzenthaler C, Robinson DG: Dynamics of COPII vesicles and the Golgi apparatus in cultured Nicotiana tabacum BY-2 cells provides evidence for transient association of Golgi stacks with endoplasmic reticulum exit sites. Plant Cell 2005, 17:1513-1531. 8. daSilva LL, Snapp EL, Denecke J, Lippincott-Schwartz J, Hawes C, Brandizzi F: Endoplasmic reticulum export sites and Golgi bodies behave as single mobile secretory units in plant cells. Plant Cell 2004, 16:1753-1771. 9. Hanton SL, Bortolotti LE, Renna L, Stefano G, Brandizzi F: Crossing the divide — transport between the endoplasmic reticulum and Golgi apparatus in plants. Traffic 2005, 6:267-277. 10. Runions J, Brach T, Kühner S, Hawes C: Photoactivation of GFP reveals protein dynamics within the endoplasmic reticulum membrane. J Exp Bot 2005, in press. 11. Staehelin LA, Giddings TH Jr, Kiss JZ, Sack FD: Macromolecular differentiation of Golgi stacks in root tips of Arabidopsis and Nicotiana seedlings as visualized in high pressure frozen and freeze-substituted samples. Protoplasma 1990, 157:75-91. 12. Kristen U: Ultrastructure and a possible function of the intercisternal elements in dictyosomes. Planta 1978, 138:29-33. 13. Cluett EB, Brown WJ: Adhesion of Golgi cisternae by proteinaceous interactions: intercisternal bridges as putative adhesive structures. J Cell Sci 1992, 103:773-784. 14. Beck KA: Spectrins and the Golgi. Biochim Biophys Acta 2005, 1744:374-382. 15. Gough LL, Beck KA: The spectrin family member Syne-1 functions in retrograde transport from Golgi to ER. Biochim Biophys Acta 2004, 1693:29-36. 16. Gough LL, Fan J, Chu S, Winnick S, Beck KA: Golgi localization of Syne-1. Mol Biol Cell 2003, 14:2410-2424. 17. Holzinger A, De Ruijter N, Emons AM, Lutz-Meindl U: Spectrin-like proteins in green algae (Desmidiaceae). Cell Biol Int 1999, 23:335-344. 18. Lu L, Lee YR, Pan R, Maloof JN, Liu B: An internal motor kinesin is associated with the Golgi apparatus and plays a role in trichome morphogenesis in Arabidopsis. Mol Biol Cell 2005, 16:811-823. A novel cotton kinesin, GhKinesin-13A, and its Arabidopsis homologue, which are divergent from previously identified plant kinesins, are associated to Golgi stacks. The Arabidopsis kinesin-13A plays a role in www.sciencedirect.com 24. Rose A, Manikantan S, Schraegle SJ, Maloy MA, Stahlberg EA, Meier I: Genome-wide identification of Arabidopsis coiled-coil proteins and establishment of the ARABI-COIL database. Plant Physiol 2004, 134:927-939. 25. Gillingham AK, Tong AH, Boone C, Munro S: The GTPase Arf1p and the ER to Golgi cargo receptor Erv14p cooperate to recruit the golgin Rud3p to the cis-Golgi. J Cell Biol 2004, 167:281-292. Following up on studies of the GRIP domain golgins, a related domain is identified in a group of golgins and named GRAB domain. The GRAB domain in the yeast golgin Rud3p binds the small GTPase ARF1. The authors also describe a genetic interaction between RUD3 and YPT6 and between RUD3 and the Golgi cargo receptor ERV14. 26. Donaldson JG, Honda A, Weigert R: Multiple activities for Arf1 at the Golgi complex. Biochim Biophys Acta 2005, 1744:364-373. 27. Munro S: The Arf-like GTPase Arl1 and its role in membrane traffic. Biochem Soc Trans 2005, 33:601-605. 28. Panic B, Whyte JR, Munro S: The ARF-like GTPases Arl1p and Arl3p act in a pathway that interacts with vesicle-tethering factors at the Golgi apparatus. Curr Biol 2003, 13:405-410. 29. Setty SR, Shin ME, Yoshino A, Marks MS, Burd CG: Golgi recruitment of GRIP domain proteins by Arf-like GTPase 1 is regulated by Arf-like GTPase 3. Curr Biol 2003, 13:401-404. 30. Seemann J, Jokitalo E, Pypaert M, Warren G: Matrix proteins can generate the higher order architecture of the Golgi apparatus. Nature 2000, 407:1022-1026. 31. Vasile E, Perez T, Nakamura N, Krieger M: Structural integrity of the Golgi is temperature sensitive in conditional-lethal mutants with no detectable GM130. Traffic 2003, 4:254-272. 32. Diao A, Rahman D, Pappin DJ, Lucocq J, Lowe M: The coiled-coil membrane protein golgin-84 is a novel rab effector required for Golgi ribbon formation. J Cell Biol 2003, 160:201-212. 33. Barr FA, Puype M, Vandekerckhove J, Warren G: GRASP65, a protein involved in the stacking of Golgi cisternae. Cell 1997, 91:253-262. 34. Shorter J, Watson R, Giannakou ME, Clarke M, Warren G, Barr FA: GRASP55, a second mammalian GRASP protein involved in the stacking of Golgi cisternae in a cell-free system. EMBO J 1999, 18:4949-4960. 35. Fasshauer D, Antonin W, Subramaniam V, Jahn R: SNARE assembly and disassembly exhibit a pronounced hysteresis. Nat Struct Biol 2002, 9:144-151. 36. Lupashin V, Sztul E: Golgi tethering factors. Biochim Biophys Acta 2005, 1744:325-339. 37. Sonnichsen B, Lowe M, Levine T, Jamsa E, Dirac-Svejstrup B, Warren G: A role for giantin in docking COPI vesicles to Golgi membranes. J Cell Biol 1998, 140:1013-1021. Current Opinion in Plant Biology 2005, 8:1–8 8 Cell biology 38. Malsam J, Satoh A, Pelletier L, Warren G: Golgin tethers define subpopulations of COPI vesicles. Science 2005, 307:1095-1098. This paper describes a tether of the two interacting golgins Golgin-84 and CASP. The authors show that Golgin-84 and not CASP co-fractionates with COP1 vesicles, demonstrating the asymmetry of the tether. The vesicles that are bound by Golgin-84–CASP differ from those bound by the p115-giantin tether in that they do not contain the p24 family of cargo receptors and contain Golgi enzymes instead. This suggests that this tether is involved in retrograde transport. 39. Sapperstein SK, Walter DM, Grosvenor AR, Heuser JE, Waters MG: p115 is a general vesicular transport factor related to the yeast endoplasmic reticulum to Golgi transport factor Uso1p. Proc Natl Acad Sci USA 1995, 92:522-526. 40. Garcia-Mata R, Sztul E: The membrane-tethering protein p115 interacts with GBF1, an ARF guanine-nucleotide-exchange factor. EMBO Rep 2003, 4:320-325. 41. Gilson PR, Vergara CE, Kjer-Nielsen L, Teasdale RD, Bacic A, Gleeson PA: Identification of a Golgi-localised GRIP domain protein from Arabidopsis thaliana. Planta 2004, 219:1050-1056. The work described in this paper identifies the first plant golgin, AtGRIP. The GRIP domain of AtGRIP fused to GFP co-locates with a-Mannosidase I on Golgi stacks in transformed tobacco protoplasts. 42. Latijnhouwers M, Hawes C, Carvalho C, Oparka K, Gillingham AK, Boevink P: An ArabidopsisGRIP domain protein locates to the transGolgi and binds the small GTPase ARL1. Plant J 2005, in press. Following up on work described in the previous paper [41], the authors show that full-length AtGRIP–GFP also locates to the Golgi and that the cis Golgi t-SNARE Memb11 locates to the opposite side of the stack, indicating that AtGRIP is a trans Golgi protein. An Arabidopsis ARL1 homologue is isolated and shown to interact with the GRIP domain of AtGRIP in vitro. Overexpression of AtARL1 creates more binding sites for GRIP on the Golgi, suggesting that AtARL1 and GRIP also interact in vivo. Mutations that lock ARL1 in the GDP- or GTP-locked form alter the localisation of ARL1. 43. Gillingham AK, Pfeifer AC, Munro S: CASP, the alternatively spliced product of the gene encoding the CCAATdisplacement protein transcription factor, is a Golgi membrane protein related to giantin. Mol Biol Cell 2002, 13:3761-3774. 44. Renna L, Hanton SL, Stefano G, Bortolotti L, Misra V, Brandizzi F: Identification and characterization of AtCASP, a plant transmembrane Golgi matrix protein. Plant Mol Biol 2005, 58:109-122. 45. Fridmann-Sirkis Y, Siniossoglou S, Pelham HR: TMF is a golgin that binds Rab6 and influences Golgi morphology. BMC Cell Biol 2004, 5:18. 46. Dunkley TP, Watson R, Griffin JL, Dupree P, Lilley KS: Localization of organelle proteins by isotope tagging (LOPIT). Mol Cell Proteomics 2004, 3:1128-1134. The authors describe the use of a novel technique to determine the subcellular localisation of membrane proteins without needing to obtain pure organelle preparations. Fractions that are enriched for certain organelles are labelled with different isotope-coded affinity tags (ICATs) and pooled before proteomics analysis. Thanks to the difference in mass between the ICATs, the relative abundance of proteins in the two fractions can be determined, and a likely organelle specificity can be determined from this. 47. Whyte JR, Munro S: Vesicle tethering complexes in membrane traffic. J Cell Sci 2002, 115:2627-2637. The Arabidopsis pok mutant is characterised by short pollen tubes. The POK protein is similar to the yeast GARP complex subunit Vps52p and localises to the Golgi in plant cells. The authors also identify putative Arabidopsis homologues of yeast Vps53p and Vps54p. This paper represents the first characterisation of a plant subunit of a multi-subunit tethering complex. 51. Hirose H, Arasaki K, Dohmae N, Takio K, Hatsuzawa K, Nagahama M, Tani K, Yamamoto A, Tohyama M, Tagaya M: Implication of ZW10 in membrane trafficking between the endoplasmic reticulum and Golgi. EMBO J 2004, 23:1267-1278. 52. Waters MG, Clary DO, Rothman JE: A novel 115-kD peripheral membrane protein is required for intercisternal transport in the Golgi stack. J Cell Biol 1992, 118:1015-1026. 53. Infante C, Ramos-Morales F, Fedriani C, Bornens M, Rios RM: GMAP-210, a cis-Golgi network-associated protein, is a minus end microtubule-binding protein. J Cell Biol 1999, 145:83-98. 54. Kim DW: Characterization of Grp1p, a novel cis-Golgi matrix protein. Biochem Biophys Res Commun 2003, 303:370-378. 55. Griffith KJ, Chan EK, Lung CC, Hamel JC, Guo X, Miyachi K, Fritzler MJ: Molecular cloning of a novel 97-kd Golgi complex autoantigen associated with Sjogren’s syndrome. Arthritis Rheum 1997, 40:1693-1702. 56. Kooy J, Toh BH, Pettitt JM, Erlich R, Gleeson PA: Human autoantibodies as reagents to conserved Golgi components. Characterization of a peripheral, 230-kDa compartmentspecific Golgi protein. J Biol Chem 1992, 267:20255-20263. 57. Fritzler MJ, Lung CC, Hamel JC, Griffith KJ, Chan EK: Molecular characterization of Golgin-245, a novel Golgi complex protein containing a granin signature. J Biol Chem 1995, 270:31262-31268. 58. Luke MR, Kjer-Nielsen L, Brown DL, Stow JL, Gleeson PA: GRIP domain-mediated targeting of two new coiled-coil proteins, GCC88 and GCC185, to subcompartments of the trans-Golgi network. J Biol Chem 2003, 278:4216-4226. 59. Bascom RA, Srinivasan S, Nussbaum RL: Identification and characterization of golgin-84, a novel Golgi integral membrane protein with a cytoplasmic coiled-coil domain. J Biol Chem 1999, 274:2953-2962. 60. Nakamura N, Rabouille C, Watson R, Nilsson T, Hui N, Slusarewicz P, Kreis TE, Warren G: Characterization of a cis-Golgi matrix protein, GM130. J Cell Biol 1995, 131:1715-1726. 61. Fritzler MJ, Hamel JC, Ochs RL, Chan EK: Molecular characterization of two human autoantigens: unique cDNAs encoding 95- and 160-kD proteins of a putative family in the Golgi complex. J Exp Med 1993, 178:49-62. 62. Siniossoglou S, Pelham HR: An effector of Ypt6p binds the SNARE Tlg1p and mediates selective fusion of vesicles with late Golgi membranes. EMBO J 2001, 20:5991-5998. 63. Matanis T, Akhmanova A, Wulf P, Del Nery E, Weide T, Stepanova T, Galjart N, Grosveld F, Goud B, De Zeeuw CI et al.: Bicaudal-D regulates COPI-independent Golgi–ER transport by recruiting the dynein–dynactin motor complex. Nat Cell Biol 2002, 4:986-992. 48. Oka T, Krieger M: Multi-component protein complexes and Golgi membrane trafficking. J Biochem 2005, 137:109-114. 64. Short B, Preisinger C, Korner R, Kopajtich R, Byron O, Barr FA: A GRASP55-rab2 effector complex linking Golgi structure to membrane traffic. J Cell Biol 2001, 155:877-883. 49. Sacher M, Barrowman J, Wang W, Horecka J, Zhang Y, Pypaert M, Ferro-Novick S: TRAPP I implicated in the specificity of tethering in ER-to-Golgi transport. Mol Cell 2001, 7:433-442. 65. Jakymiw A, Raharjo E, Rattner JB, Eystathioy T, Chan EK, Fujita DJ: Identification and characterization of a novel Golgi protein, golgin-67. J Biol Chem 2000, 275:4137-4144. 50. Lobstein E, Guyon A, Ferault M, Twell D, Pelletier G, Bonhomme S: The putative Arabidopsis homolog of yeast vps52p is required for pollen tube elongation, localizes to Golgi, and might be involved in vesicle trafficking. Plant Physiol 2004, 135:1480-1490. 66. Misumi Y, Sohda M, Yano A, Fujiwara T, Ikehara Y: Molecular characterization of GCP170, a 170-kDa protein associated with the cytoplasmic face of the Golgi membrane. J Biol Chem 1997, 272:23851-23858. Current Opinion in Plant Biology 2005, 8:1–8 www.sciencedirect.com