Download ASCORBATE-DEPENDENT CYTOCHROMES b

ASCORBATE-DEPENDENT CYTOCHROMES b-561 IN PLANTS: A MORE COMPLEX
SCENARIO THAN PREVIOUSLY EXPECTED
Preger V, Scagliarini S, Pupillo P, Trost P
Department of Biology – Laboratory of Molecular Plant Physiology – University of Bologna – Italy
Plants contain four genes homologous to animal cytochromes b-561 (named ACYB for Ascorbatedependent CYtochrome B-561). The basic features of the in silico translated proteins are fully
conserved between animal and plants: ACYB’s are formed by about 250 residues, are predicted to
contain six trans-membrane helices (named I to VI) and to bind two hemes by means of two couples
of conserved histidines, each His being located at the N-terminus of four consecutive helices (II to V).
As a result, the two hemes are predicted to face opposite sides of the membrane, thereby promoting a
transmembrane electron transfer between soluble electron donors (ascorbate) and soluble electron
acceptors (monodehydroascorbate, ferrichelates) located in different cellular compartments.
Plant plasma membranes (PM) contain a major ascorbate (Asc)-reducible b-type cytochrome able to
reduce monodehydroascorbate (MDA) on the apoplastic side. The apparent MW of this purified
cytochrome (55 kDa) does not match the predicted MW of plant and animal ACYB (27 kDa).
Recently, however, several genes identified in the Arabidopsis genome have been found to be
constituted by a cyt b-561-like domain fused with a putative catecholamine binding domain (DoH).
Catecholamines such as dopamine are also present in plants, although their function is unknown.
Since the predicted DoH-cyt b-561 proteins are about 45 kDa in size, it seems plausible that the
glycosylated cytochrome of the plant PM (55 kDa) might derive from these class of genes. But if plant
PM do not contain ACYB, where are the products of ACYB genes located?
To answer this question we started looking for ascorbate-reducible cytochromes b in a microsomal
membrane fraction from bean hypocotyls. After detergent solubilization, anion-exchange and size
exclusion chromatography we could separate to major peaks (peak I and peak II) showing distinct
biochemical features. Peak I is fully Asc-reducible and shows a symmetric -band at 561 nm, it can be
oxidized by MDA but not by ferrichelates, it is insensitive to diethylpyrocarbonate (DEPC, inhibitor of
the ascorbate site in chromaffine granules ACYB), it is glycolyslated and has apparent MW of 55 kDa.
Sucrose gradient separations demonstrate that peak I is associated with the PM. All these features are
consistent with peak I being the already described high-potential cytochrome of plant PM and
underline some important differences between this protein and animal ACYB. On the contrary, peak
II is partially Asc- reducible, displays asymmetric -band with maximum at 561 nm and a shoulder at
558 nm, it can be oxidized by both MDA and ferric-EDTA, it is DEPC-sensitive, it is not glycosylated
and its apparent MW is 24 kDa, compatible with a highly hydrophobic protein of about 250 amino
acids. All these features clearly suggest that peak II is a plant ACYB. Interestingly, the distribution of
peak II in sucrose gradients is compatible with a location on the tonoplast.
Therefore we conclude that plant ACYB are tonoplast cytochromes with similar features to animal
ACYB of chromaffine granules (invoved in Asc regeneration) and of the PM of duodenal cells
(involved in iron reduction). Whether the role of ACYB proteins in plants is related to Asc metabolism
or to iron uptake in vacuoles it remains to be understood and will be the aim of future efforts.