Higher-plant plasma membrane cytochromeb 561: A protein in search of a function

Abstract

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.

Auxin-Responsive Genes AIR12 Code for a New Family of Plasma Membrane b-Type Cytochromes Specific to Flowering Plants

“…On the other hand, PM preparations from different plant species were invariably found to contain ascorbate-reducible cytochromes b displaying a symmetric a-band around 561 nm and typically responding to high redox potentials, usually in the +50-to +300-mV range at neutral pH, with some differences among species and tissues (Asard et al, 1989;Askerlund et al, 1989). Ascorbate-reducible cytochromes b may represent up to 80% of the total cytochrome content in purified PM preparations (Asard et al, 2001).…”

“…The reaction was thought to be catalyzed by a cytochrome b561 of the PM (Asard et al, 2001) in analogy with the system for ascorbate regeneration in chromaffin vesicles of mammals (Kelley and Njus, 1986). This concept needs revision, however, because true cytochrome b561 (CYBASC1) is localized in vacuolar membranes (tonoplast) in both Arabidopsis (Griesen et al, 2004) and bean (Preger et al, 2005), and chromaffin membranes are certainly closer to plant vacuolar membranes than PM.…”

“…It was suggested (Asard et al, 2001) that the trans-PM electron transfer from cytosolic ascorbate to apoplastic MDA may be effected by a cytochrome b561, in analogy to the electron transfer of animal chromaffin vesicles (Kelley and Njus, 1986). Cytochromes b561 are high-potential, transmembrane redox proteins of about 25 kD made of six membrane-spanning a-helices, which bind two hemes b.…”

Human Erythrocyte Membranes Contain a Cytochrome b561 That May Be Involved in Extracellular Ascorbate Recycling

“…An Asc-reducible Cyt b in Stripped Human Erythrocyte Membranes-Asc is generally accepted as the physiological electron donor for the Cyts b 561 (20). It has been observed that recombinant and native plant and mammalian Cyts b 561 have characteristic Asc-reducible spectra (30,33,(41)(42)(43)(44).…”

“…Cyts b 561 are transmembrane proteins that mediate Ascdriven transmembrane electron transport (20,21). One member of this protein family, the chromaffin granule Cyt b 561 (CGCytb), has been shown to reduce MDHA, at least in vitro, thereby regenerating Asc (22,23).…”