Interaction with Membranes of Cytochromec554 fromNitrosomonas europaea

Twoc-cytochromes extrinsically bound to the membranes ofNitrosomonas europaeahave been identified. One is the tetraheme cytochromec554, a protein previously described as soluble and periplasmic. Depending on the concentration of Fe and Cu in the growth medium, from 50 to 100% of the total cellular cytochromec554 is membrane-associated. The cytochromesc554 found in the soluble or membrane fractions are identical in the spectroscopic, chromatographic, or primary structural properties examined. The interaction of cytochromec554 with membranes is ionic in nature; it is disrupted by high concentrations of salt. Both membrane-derived and periplasmic forms of cytochromec554 rebind tightly to membranes which have been washed free of the cytochrome. Cytochromec554 binds to phospholipid vesicles, suggesting that phospholipids may play a role in the interaction of this cytochrome with the membrane. During the oxidation of NH2OH, the ability of the soluble hydroxylamine oxidoreductase (HAO) to transfer electrons to its natural electron acceptor, cytochromec554, is substantially impaired when the latter is bound to phospholipid vesicles. The secondc-cytochrome associated with membranes inN. europaeais identified as HAO based on its catalytic activity and the presence of a 464-nm ferrous absorption band. A small fraction of HAO is found to be membrane-bound and only in cells grown under low Fe/low Cu. This subpopulation of HAO can be released from the membranes without detergents.