Characterization of Rhodobacter sphaeroides Cytochrome c2 Proteins with Altered Heme Attachment Sites

In c-type cytochromes, heme is attached to the polypeptide via thioether linkages between vinyl groups on the tetrapyrrole ring and cysteine thiols in a CX2CH motif. To study the role of the heme-binding site in c-type cytochrome assembly and function, we generated amino acid changes in this region of Rhodobacter sphaeroides cytochrome c2 (15Cys-Gln-Thr-Cys-His19). Amino acid substitutions at Cys15, Cys18, or His19 produced mutant proteins that did not support growth via photosynthesis where this electron carrier is required. Many of these changes appeared to slow signal peptide removal, suggesting that heme attachment is coupled to processing of the c-type cytochrome precursor protein. Inserting an alanine between the cysteine ligands (CycA–Ins17A) did not significantly alter the behavior of this protein in vivo and in vitro, suggesting that the existence of 2 residues between cysteine thiols is not essential for heme attachment to a Class I c-type cytochrome like cytochrome c2.