Maturation of Plastid c -type Cytochromes

Cytochromes are hemoproteins, with the prosthetic group covalently linked to the apoprotein, which function as electron carriers. A class of cytochromes is defined by a C CH heme-binding motif where the cysteines form thioether bonds with the vinyl groups of heme. Plastids are known to contain up to three cytochromes . The membrane-bound cytochrome and soluble cytochrome operate in photosynthesis while the activity of soluble cytochrome remains unknown. Conversion of apo- to holocytochrome occurs in the thylakoid lumen and requires the independent transport of apocytochrome and heme across the thylakoid membrane followed by the stereospecific attachment of ferroheme via thioether linkages. Attachment of heme to apoforms of plastid cytochromes is dependent upon the products of the (for ytochrome ynthesis) genes, first uncovered via genetic analysis of photosynthetic deficient mutants in the green alga . The CCS pathway also occurs in cyanobacteria and several bacteria. CcsA and CCS1, the signature components of the CCS pathway are polytopic membrane proteins proposed to operate in the delivery of heme from the stroma to the lumen, and also in the catalysis of the heme ligation reaction. CCDA, CCS4, and CCS5 are components of trans-thylakoid pathways that deliver reducing equivalents in order to maintain the heme-binding cysteines in a reduced form prior to thioether bond formation. While only four CCS components are needed in bacteria, at least eight components are required for plastid cytochrome assembly, suggesting the biochemistry of thioether formation is more nuanced in the plastid system.