Two-domain structure of cytochrome b5 in deoxycholate solution

The membrane protein cytochrome b5 and the polar and hydrophobic fragments into which it is cleaved by trypsin have been investigated, with major emphasis on the deoxycholate-solubilized form of the protein. Molecular weight measurements show that both the intact protein and the fragments are in a monomeric state in deoxycholate and that a small peptide of perhaps 15 residues is excised when the fragments are formed. Measurements of Stokes radius show that the major fragments are globular, but that intact cytochrome b5 has an asymmetric shape, consistent with a structure composed of two globular domains joined by a link region that may be as long as 30 to 40 A. Circular dichroism measurements were made in the far-ultraviolet and in the Soret region, and they add to previously existing data to make it virtually certain that the polar heme-containing domain is unaffected by proteolysis or by removal of deoxycholate. A significant change in the ultraviolet circular dichroism is, however, observed when proteolysis occurs and it is likely that it arises from the link between the domains, which appears to be highly structured (perhaps helical) in the intact protein, but randomly coiled after it is excised. The binding studies reported previously from this laboratory suggest that these inferences about the structure of cytochrome b5 in deoxycholate solution apply also to the protein as solubilized by detergent micelles, by phospholipid vesicles, or by the microsomal membrane.