Higher-plant plasma membrane cytochrome b561: 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 plasma membrane (PM) of higher plants. This cytochrome is named cytochrome b561 (cyt b561) according to the wavelength maximum of its alpha-band in the reduced form. More recent evidence suggests that this protein is homologous to a b-type cytochrome present in chromaffin granules of animal cells. The plant and animal cytochromes share a number of strikingly similar features, including the high redox potential, the ascorbate reducibility, and most importantly the capacity to transport electrons across the membrane they are located in. The PM cyt b561 is found in all plant species and in a variety of tissues tested so far. It thus appears to be a ubiquitous electron transport component of the PM. The cytochromes b561 probably constitute a novel class of transmembrane electron transport proteins present in a large variety of eukaryotic cells. Of particular interest is the recent discovery of a number of plant genes that show striking homologies to the genes coding for the mammalian cytochromes b561. A number of highly relevant structural features, including hydrophobic domains, heme ligation sites, and possible ascorbate and monodehydroascorbate binding sites are almost perfectly conserved in all these proteins. At the same time the plant gene products show interesting differences related to their specific location at the PM, such as potentially N-linked glycosylation sites. It is also clear that at least in several plants cyt b561 is represented by a multigene family. The current paper presents the first overview focusing exclusively on the plant PM cyt b561, compares it to the animal cyt b561, and discusses the possible physiological function of these proteins in plants.