c-Type Cytochrome Assembly in Saccharomyces cerevisiae: A Key Residue for Apocytochrome c1/Lyase Interaction

Abstract

Abstract The electron transport chains in the membranes of bacteria and organelles generate proton-motive force essential for ATP production. The c-type cytochromes, defined by the covalent attachment of heme to a CXXCH motif, are key electron carriers in these energy-transducing membranes. In mitochondria, cytochromes c and c1 are assembled by the cytochrome c heme lyases (CCHL and CC1HL) and by Cyc2p, a putative redox protein. A cytochrome c1 mutant with a CAPCH heme-binding site instead of the wild-type CAACH is strictly dependent upon Cyc2p for assembly. In this context, we found that overexpression of CC1HL, as well as mutations of the proline in the CAPCH site to H, L, S, or T residues, can bypass the absence of Cyc2p. The P mutation was postulated to shift the CXXCH motif to an oxidized form, which must be reduced in a Cyc2p-dependent reaction before heme ligation. However, measurement of the redox midpoint potential of apocytochrome c1 indicates that neither the P nor the T residues impact the thermodynamic propensity of the CXXCH motif to occur in a disulfide vs. dithiol form. We show instead that the identity of the second intervening residue in the CXXCH motif is key in determining the CCHL-dependent vs. CC1HL-dependent assembly of holocytochrome c1. We also provide evidence that Cyc2p is dedicated to the CCHL pathway and is not required for the CC1HL-dependent assembly of cytochrome c1.