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Reconstitution of an Allophycocyanin Trimer Complex Containing the C-Terminal 2 1 -2 3 kDa Domain of the Core-Membrane Linker Polypeptide Lcm L o th a r G o ttsc h a lk 3, Friedrich L ottspeichb and H ugo Scheera a Botanisches Institut der U niversität, Menzinger Straße 67, D-80638 München, Bundesrepublik Deutschland b M ax-Planck-Institut für Biochemie, Am Klopferspitz, D-82152 M artinsried, Bundesrepublik Deutschland Z. Naturforsch. 49c, 331-336 (1994); received February 18, 1994 Allophycocyanin, Linker, Anchor Protein, Phycobilisome Core, Mastigocladus laminosus Allophycocyanin (AP) was isolated from extracts of the cyanobacterium Mastigocladus laminosus. A fraction enriched in AP-associated polypeptides with apparent molecular masses of 2 1 -2 3 kD a in SDS-PAGE, was isolated on a preparative scale and identified as a hom olo gous mixture of C-terminal fragments o f the core-membrane linker polypeptide Lcm. The com plex (aAPßAP)3-21 - 2 3 kD a was reconstituted and characterized by sucrose density gradient ultracentrifugation, absorption, fluorescence emission and circular dichroism spectroscopy. The 2 1 -2 3 kD a polypeptides were found to induce spectral changes in AP similar to those induced by the small core linker polypeptide Lc8 9. Possible functions of the complex in phycobilisomes are discussed. Introduction Phycobilisom es, the light-harvesting an tennae o f cyanobacteria, red algae and cyanelles, are supram olecular, highly-structured p ro tein com plex es located on the thylakoid surface (W ehrm eyer, 1983; G a n tt, 1986). They are com posed o f chrom opho re-b earin g phycobiliproteins (Scheer, 1982; G lazer, 1985; M acC oll an d G u a rd -F ria r, 1987) abso rb in g light over a wide spectral range, and o f linker polypeptides which regulate and sta bilize the phycobilisom e structure, b u t also m odify the lig ht-ab so rp tio n properties o f the phycobili proteins (T an d eau de M arsac an d C ohen-B azire, 1977; B ryant, 1991). Abbreviations: PC, phycocyanin; PEC, phycoerythrocyanin; AP, allophycocyanin; APB, allophycocyanin B; a, ß, biliprotein subunits; L, linker polypeptide, subscripts denoting origin and function in phycobilisomes (r, rod; c, core; rc, rod core; cm, core membrane), superscripts denoting molecular weight in kDa; OD, optical density; OD •ml, unit of protein quantity, OD multiplied by sam ple volume in ml, subscripts denoting wavelength in nm; SDS-PAGE, sodium dodecylsulfate-polyacrylamide-gelelectrophoresis; CD, circular dichroism; p/s, peak (^max)to-shoulder (600 nm) absorbance ratio; AA /A m!ix, ratio of CD signal intensity to absorbance value at >.max. 0 9 3 9 -5 0 7 5 /9 4 /0 5 0 0 -0 3 3 1 $ 0 3.00 © V erlag der Z eitschrift für N atu rfo rsc h u n g , D-72072 T übingen In hem idiscoidal phycobilisom es, an array o f rods is linked by one or m ore rod-core linker poly peptides to the phycobilisom e core. The bi- o r tricylindrical core substructures are form ed by allo phycocyanin (A P), tw o m inor biliproteins stru c turally related to A P, and the tw o linkers Lc an d Lcm. The latter, a large polypeptide o f 127 k D a m olecular m ass in the cyanobacterium M astigo cladus laminosus (E steban, 1993), contains several internal repeats and plays a singular role. It a n chors the core to the m em brane (R edlinger and G a n tt, 1982; Rusckow ski and Zilinskas, 1982; R euter and W ehrm eyer, 1990), an d is involved in the organization o f the core, b u t also carries a chrom ophore which is p robably the term inal em it ter to photosystem II (G indt et al., 1992; Z h ao et al., 1992). The key role o f the linker polypeptides in the ag gregate form ation and the fine-tuning o f the a b sorption characteristics o f the phycobiliproteins has been widely studied, b ut is still only poorly u n derstood. A technique for the isolation o f linker polypeptides and their fragm ents and for the re constitution to biliprotein-linker com plexes re cently developed in o u r lab o rato ry (G o ttschalk et al., 1991 and 1993), has facilitated the study o f these proteins. H ere, we present results on a com plex o f A P-trim er w ith C -term inal fragm ents o f the Lcm polypeptide, w hich show s a strong hyper- Unauthenticated Download Date | 6/18/17 6:15 PM 332 L. G o ttsch alk et al. ■ R e c o n stitu tio n o f an A llophycocyanin T rim er chrom ism as com p ared to A P-trim ers w ithout linker. O u r findings m ay be significant in the present discussion o f the phycobilisom e architec ture o f this organism . Materials and Methods Absorption and CD m easurem ents were m ade at an optical density o f 0 .5 -0 .7 5 . Fluorescence em is sion was m easured at an optical density o f 0 . 1 at the a b so rp tio n m axim um in 1 cm cells; excitation was at 580 nm . Allophycocyanin was isolated from M astigocladus laminosus on D E 52 (W hatm an), pre-purified on hydroxy lap atite Bio-Gel H T (BioR ad), an d purified o n F ractogel T SK D E A E 650(2) (M erck) from all co n tam in atio n s o f other biliproteins an d o f linker polypeptides by the m ethods previously described (G o ttsch alk et al., 1991 an d 1993; G o ttsch alk , 1993). 2 1 - 2 3 kD a polypeptides were isolated w ith buffers containing 3.5 m urea (Serva, p.a. grade) from pre-purified A P on S-Sepharose F a st Flow (P harm acia) as de scribed previously for L c 8 9 (G o ttsch alk et al., 1993). Reconstitution experim ents - 6 O D 6 2 0 ml A P (in 230 (j.1 30 m M p otassium p h o sp h ate buffer, pH 7, con tain in g 3.5 m urea) and 1.2 O D 276-ml en riched fractio n o f 2 1 - 2 3 k D a polypeptides (in 400 (j.1 10 m M T ris/H C l, pH 8.9, co ntaining 3.5 m urea an d 100 m M KC1) were m ixed, reconstituted in p o tassium p h o sp h ate buffer (700 m M , pH 7.5), purified by sucrose density g radient u ltracen trifu gation an d characterized by m ethods described previously fo r com plexes co ntaining L c 8 9 (G ottschalk et al., 1993). T he aggregation state was de term ined by 3 - 1 2 % sucrose density gradient u ltracen trifu g atio n in 700 m M potassium phos ph ate buffer (pH 7.5) as described previously (G o ttsch alk et al., 1993). Am ino acid sequences LysC digestion o f p ro tein bands in polyacrylam ide m atrix was follow ed by elution and H P L C separa tion o f the fragm ents (E ckerskorn and L ottspeich, 1989). N -term inal sequences o f the fragm ents were determ ined as described previously (E ckerskorn e ta l., 1988; L ottspeich, 1985). S D S -P A G E was done according to Läm m li (1970), using a 5% stacking gel an d a 15% sep aratio n gel. Results In a previous p ublication, a new m ethod for the isolation an d p u rification o f biliproteins and link er polypeptides an d for reconstitution o f biliprotein-linker com plexes has been introduced and a p plied to the reconstitution o f (a APß A P ) 3 and of (a APß A P ) 3 • L c 8 9 (G o ttsch alk et al., 1993). D uring this w ork, a n o th e r reconstituted com plex, (a APßAP)3-2 1 -2 3 k D a , had been prepared (G ottschalk et al., 1993), b ut could n ot be further c h a r acterized then because the three 2 1 -2 3 k D a poly peptides were N -term inally blocked. The related polypeptides were now enriched by ch ro m a to graphy on S-Sepharose F F . T he fraction o f 2 1 -2 3 k D a polypeptides eluting at ab o u t 100 m M KC1 was still co n tam in ated by o ther polypeptides (Fig. 1, lane 1). H ow ever, the m ajo r three bands at ap p aren t m olecular m asses o f 21 - 23 k D a were the only ones in this fraction reconstituting stable com plexes w ith A P (Fig. 1, lane 3), whereas the contam inations precipitated and were lost during the reconstitution an d subsequent purification procedure. T herefore, under circum stances fa vouring specific interactions, reconstitution can be used to obtain pure and specific com plexes from im pure sam ples. T he polypeptides o f the three bands form ing a com plex w ith A P were separately cleaved w ith LysC, and some o f the fragm ents were sequenced (T able I). All bands contain cleav age p roducts identified as fragm ents o f the C -term inal p a rt o f Lcm, by com parison w ith the W' V'* — -116 — 66 - 45 W 0 _~ 2369 — 24 — 20.1 — 14.2 1 Fig. 1. SDS-PAGE of isolated polypeptides and recon stituted complexes. Lane 1: fraction of 21 -2 3 kD a poly peptides isolated from S-Sepharose F F column. Lane 2: (aAPßA P ) 3 complex reconstituted as a control. Lane 3: re constituted (aAPßAP)3-21- 2 3 kDa complex. Lane 4: m o lecular weight markers (14.2, 20.1, 24, 29, 36, 45, 6 6 and 116 kDa). Unauthenticated Download Date | 6/18/17 6:15 PM 333 L. G o ttsch alk et al. ■ R eco n stitu tio n o f an A llophycocyanin T rim er Table I. Sequences of LysC fragments of the upper, middle and lower band of 21 —23 kD a polypeptides in the reconstituted complex with AP (Fig. 1, lane 3), and amino acid positions of identical sequences of Lcm from Mastigocladus laminosus (Esteban, 1993). Band Sequences Position upper L L A D G G LK AAADLVK 1063-1070 1110-1116 middle FY TPY PN TK LL A D G G L K 1029-1037 1063-1070 lower FY TPY PN TK LLA D G G L K A AVEAM VNSP 1029-1037 1063-1070 1071-1080 am ino acid sequence deduced from the D N A se quence (E steban, 1993). E steban (1993) has recently also identified a 23 k D a polypeptide as a C -term inal fragm ent o f Lcm startin g at am ino acid position 923. The simi lar size and sequence overlaps show th a t o u r poly peptides are closely related to th e latter. Since they are N -term inally blocked, it could n o t be d eter m ined w hether they differ from each oth er N - or C -term inally. T he finding o f this fam ily o f closely related Lcm fragm ents can be rationalized by an attack o f an endogenous p rotease o r M . laminosus on regions o f Lcm n o t pro tected inside an A P tri m er after subjecting phycobilisom es to dissociat ing conditions. 2 1 - 2 3 k D a seems to be a com m on size for stable fragm ents o f linker polypeptides protected inside biliprotein trim ers against fu rther d eg rad atio n (Y u an d G lazer, 1982; Lundell et al., 1981; Lundell an d G lazer, 1983 a; G o ttsch alk et al., 1991; G lau ser et al., 1992b); we suggest th a t the C -term inal d o m ain o f Lcm is follow ing the same principle. The basis for the N -term in al block o f o u r iso lates is unclear. N -term inally blocked linker poly peptides have been rep o rted from o th er organism s (Lundell et al., 1981; G lauser, 1991; G lauser et al., 1992 b), b u t are n o t expected for proteolytic frag m ents. A block could be due to reaction w ith cyanate ions in the 3.5 m urea solution used th ro u g h o ut the isolation, alth o u g h no such effect has been observed previously in o u r w ork w ith biliproteins and linker polypeptides o f M . laminosus isolated under sim ilar conditio n s (G o ttsch alk et al., 1991 and 1993; G o ttsch alk , 1993). C om plete co m plexation o f trim eric A P w ith the 2 1 -2 3 k D a polypeptides was ensured by the use o f an excess o f the latter, as discussed in G o ttsch alk et al. (1993). T he reconstituted (a APßAP) 3 - 2 1 23 k D a com plex was predom inantly ( > 85% ) tri meric; only traces o f m onom eric ( < 8 % ) an d hexameric (< 7% ) aggregates could be detected by sucrose density gradient u ltracentrifugation. Only the trim eric fraction from the gradient w as used for the spectra show n in Fig. 2. N o co n tam in atio n by APB or the chrom ophore-bearing N -term inus o f Lcm was detected by spectroscopic m ethods or SD S-PA G E. The red ab so rp tio n b an d o f the reconstituted com plex was at A.max = 651.5 nm , and h ad an ex tremely high p/s-ratio o f up to 2.95; the fluores cence em ission w as at A.max = 663 nm (Fig. 2 a). The com plex gave very stro n g C D signals w ith AA /A max = 4 .7 - 10 ~ 4 (Fig. 2). W hen com pared to the spectra o f reconstituted (a APß A P ) 3 and (a APß A P ) 3 • L c 8 9 (G ottschalk et al., 1993), the a b sorption and fluorescence em ission m axim a o f the new com plex are a t interm ediate values. X [nm] Fig. 2. Absorption (A, heavy line, left ordinate), flu orescence emission (A, thin line, right ordinate) and CD spectra (B) o f the reconstituted (aAPßAP)3-21- 2 3 kDa complex. Unauthenticated Download Date | 6/18/17 6:15 PM 334 L. G o ttsch alk et al. • R eco n stitu tio n o f an A llophycocyanin T rim er H ow ever, p/s = 2.95 is the highest value m easured for any A P-com plex as yet. T he overall shape and the signal positions o f the C D spectrum are sim ilar to (a APß A P ) 3 • L c 8 9, but the signal intensities are ra th e r different. (a APßAP)3-2 1 -2 3 k D a shows the highest value o f AA / A max for the three com plexes, while the o th er linker-containing com plex, (a APßAP)3-Lc89, gives the low est am plitudes (AA / A max = 1 .5 -10“4; G o ttsch alk e t al., 1993). In (a APßAP)3-2 1 - 2 3 k D a, the negative signal at a ro u n d 635 nm is m ore p ro n o u n ced th a n th a t at a ro u n d 594 nm ; in (a APß A P ) 3 • L c 8 9, the intensities o f these signals are reversed. Discussion Lcm is the largest an d m ost com plex linker in phycobilisom es. It plays a central role in the ener gy tran sfer from the rods th ro u g h the core to the photosystem s. Specific interactio n s w ith the bilip roteins are necessary to explain the functions o f Lcm as a stru ctu ral protein o f the core, as the a n ch o r protein to the thylakoid m em brane, as a link er m odifying the spectral properties o f the asso ciated biliproteins, an d as the presum ed term inal em itter o f the phycobilisom e (R edlinger and G a n tt, 1982; R usckow ski an d Zilinskas, 1982; L undell and G lazer, 1983a/b/c; H o u m ard et al., 1990; C a p u an o e t al., 1991; G in d t e t al., 1992; Z h ao e t al., 1992). C om plexes co n tain in g Lcm have been isolated only as large core fragm ents c o n ta in ing also o th er linkers an d biliproteins (Lundell and G lazer, 1983a/c; Isono an d K a to h , 1987; R euter and W ehrm eyer, 1990). A P-com plexes containing a single defined linker have been described only for L c 8 9 an d rod-core linkers (L undell and G lazer, 1983b; F üglistaller e t al., 1987; G lauser et al., 1990; G o ttsch alk e t al., 1993). W e have show n here, th a t the C -term inal d o m ain o f Lcm is capable o f form ing specific com plexes w ith A P trim ers. This d o m ain is n o t singu lar in Lcm. Follow ing a N -term inal biliprotein d o m ain, Lcm polypeptides co n tain tw o to four such repeats, which are also hom ologous to the N -term inal dom ains o f rod and rod-core linkers (B ryant, 1988; H o u m ard e t al., 1990). In phycobil isomes containing Lcm w ith fo u r repeats, the C-term inal dom ain has been suggested to link PC (C apuano et al., 1991) or A P-hexam ers (Isono and K atoh, 1987; B ryant, 1991) at the basis o f tw o o f the rods to the core. T his w ould define the C -term inus o f the Lcm o f M . lam inosus an d A n a b a en a sp. PC C 7120 as a rod-core linker, and w ould have to be taken into account w ith regard to the proposed stoichiom etries o f these polypeptides (G lauser et al., 1992a). R ods w ith A P at their base w ould either require the interaction o f L r34-5, pc w ith A P, binding the outer PC -hexam ers to the basal AP-disc. A lternatively, one o f the presum ed rodcore linkers w ould have to play this role, being in reality a m isnam ed rod linker. In this context it is interesting th a t an (a ß )6PC • L r c 2 9 5 •(a ß )3AP • L c 8 9 com plex o f unknow n m o rphology and location w ithin the phycobilisom e has been isolated and re constituted by G lauser et al. (1993). This so-called rod-core com plex could then also be p a rt o f a spe cial rod type. It should be pointed o ut, how ever, th a t the ex perim ental evidence su p p o rtin g this specific phy cobilisom e m odel (B ryant et al., 1991; G lauser et al., 1992a/b; E steban, 1993) is still am biguous. O ur results are also com patible w ith o th e r models: the com plex containing the C -term inal dom ain o f Lcm could be involved in the in teractio n between neighbouring phycobilisom es, and it could also be an integral p a rt o f the core. A cknow ledgem en ts This w ork was su p p o rted by the D eutsche F o r schungsgem einschaft, Bonn (Sonderforschungsbereich 143, project A l) . T he a u th o rs wish to th an k A. E steban and W. Sidler (E T H Zürich) foi m aking available im p o rtan t d a ta p rio r to publica tion. Unauthenticated Download Date | 6/18/17 6:15 PM 335 L. G ottsch alk et al. • R eco n stitu tio n o f an A llophycocyanin T rim er Bryant D. A. (1988), Genetic analysis o f phycobilisome biosynthesis, assembly, structure, and function in the cyanobacterium Synechococcus sp. PCC7002. In: Light-energy transduction in photosynthesis: Higher plant and bacterial models (S. E. Stevens Jr. and D. A. Bryant, eds.). 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