AUTHOR=Cline Sara G. , Laughbaum Isaac A. , Hamel Patrice P. 

TITLE=CCS2, an Octatricopeptide-Repeat Protein, Is Required for Plastid Cytochrome c Assembly in the Green Alga Chlamydomonas reinhardtii

JOURNAL=Frontiers in Plant Science

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01306

DOI=10.3389/fpls.2017.01306

ISSN=1664-462X

ABSTRACT=<p>In bacteria and energy generating organelles, <italic>c</italic>-type cytochromes are a class of universal electron carriers with a heme cofactor covalently linked via one or two thioether bonds to a heme binding site. The covalent attachment of heme to apocytochromes is a catalyzed process, taking place via three evolutionarily distinct assembly pathways (Systems I, II, III). System II was discovered in the green alga <italic>Chlamydomonas reinhardtii</italic> through the genetic analysis of the <italic>ccs</italic> mutants (<bold><underline>c</underline></bold>ytochrome <bold><italic><underline>c</underline></italic><underline>s</underline></bold>ynthesis), which display a block in the apo- to holo- form conversion of cytochrome <italic>f</italic> and <italic>c</italic><sub>6</sub>, the thylakoid lumen resident <italic>c</italic>-type cytochromes functioning in photosynthesis. Here we show that the gene corresponding to the <italic>CCS2</italic> locus encodes a 1,719 amino acid polypeptide and identify the molecular lesions in the <italic>ccs2-1</italic> to <italic>ccs2-5</italic> alleles. The CCS2 protein displays seven degenerate amino acid repeats, which are variations of the <bold><underline>o</underline></bold>ctatrico<bold><underline>p</underline></bold>eptide-<bold><underline>r</underline></bold>epeat motif (OPR) recently recognized in several nuclear-encoded proteins controlling the maturation, stability, or translation of chloroplast transcripts. A plastid site of action for CCS2 is inferred from the finding that GFP fused to the first 100 amino acids of the algal protein localizes to chloroplasts in <italic>Nicotiana benthamiana</italic>. We discuss the possible functions of CCS2 in the heme attachment reaction.</p>