The trans-membrane cytochrome b561 proteins: structural information and biological function.

Cytochrome b561 (CYB561) proteins are ascorbate reducible, trans-membrane proteins consisting of 200-300 amino acids, about half of which are hydrophobic. The first identified CYB561 protein was discovered more than 40 years ago, and is localized in the chromaffin granule membrane of the mammalian adrenal glands. Proteins with similar structural elements and biophysical and biochemical properties were identified in a wide range of animal and plant phyla in the past 15 years. CYB561 proteins have six trans-membrane helices and two b-type hemes, one on each side of the membrane. The two heme-b centers are coordinated by two pairs of His residues localized in the central four trans-membrane domains, probably very close to the membrane interface. The midpoint redox potentials of the two hemes are above 0 mV and about 100 mV apart from each other. These proteins are different in many respects from the well-known two heme-bcontaining, trans-membrane b-type cytochromes localized in the inner membrane of mitochondria, in the chloroplast thylakoids or in the cell membrane. The atomic-level structure of only one CYB561 protein is available to date. In this paper we discuss in detail the biophysical and biochemical properties of the CYB561 proteins and provide a short overview of their known or putative biological functions and significance.