Higher-plant plasma membrane cytochrome b(561): a protein in search of a function

During the past twenty years evidence has accumulated on the presence of a specific high-potential, ascorbate-reducible b-type cytochrome in the plasm a membrane (PM) of higher plants. This cytochrome is named cytochrome b(561 ) (cyt b(561)) according to the wavelength maximum of its cc-band in the re duced form. More recent evidence suggests that this protein is homologous t o a b-type cytochrome present in chromaffin granules of animal cells. The p lant and animal cytochromes share a number of strikingly similar features, including the high redox potential, the ascorbate reducibility, and most im portantly the capacity to transport electrons across the membrane they are located in. The PM cyt b(561) is found in all plant species and in a variet y of tissues tested so far. It thus appears to be a ubiquitous electron tra nsport component of the PM. The cytochromes b(561) probably constitute a no vel class of transmembrane electron transport proteins present in a large v ariety of eukaryotic cells. Of particular interest is the recent discovery of a number of plant genes that show striking homologies to the genes codin g for the mammalian cytochromes b(561) A number of highly relevant structur al features, including hydrophobic domains, heme ligation sites, and possib le ascorbate and monodehydroascorbate binding sites are almost perfectly co nserved in all these proteins. At the same time the plant gene products sho w interesting differences related to their specific location at the PM, suc h as potentially N-linked glycosylation sites. It is also clear that at lea st in several plants cyt b(561) is represented by a multigene family. The c urrent paper presents the first overview focusing exclusively on the plant PM cyt b(561), compares it to the animal cyt b(561), and discusses the poss ible physiological function of these proteins in plants.