During Cytochrome c Maturation CcmI Chaperones the Class I Apocytochromes until the Formation of Their b-Type Cytochrome Intermediates

作者：Verissimo Andreia F; Shroff Namita P; Daldal Fevzi^*

来源：JOURNAL OF BIOLOGICAL CHEMISTRY, 2015, 290(27): 16989-17003.

DOI：10.1074/jbc.M115.652818

The c-type cytochromes are electron transfer proteins involved in energy transduction. They have heme-binding (CXXCH) sites that covalently ligate heme b via thioether bonds and are classified into different classes based on their protein folds and the locations and properties of their cofactors. Rhodobacter capsulatus produces various c-type cytochromes using the cytochrome c maturation (Ccm) System I, formed from the CcmABCDEFGHI proteins. CcmI, a component of the heme ligation complex CcmFHI, interacts with the heme-handling protein CcmE and chaperones apocytochrome c(2) by binding its C-terminal helix. Whether CcmI also chaperones other c-type apocytochromes, and the effects of heme on these interactions were unknown previously. Here, we purified different classes of soluble and membrane-bound c-type apocytochromes (class I, c(2) and c(1), and class II c') and investigated their interactions with CcmI and apoCcmE. We report that, in the absence of heme, CcmI and apoCcmE recognized different classes of c-type apocytochromes with different affinities (nM to mu M K-D values). When present, heme induced conformational changes in class I apocytochromes (e.g. c(2)) and decreased significantly their high affinity for CcmI. Knowing that CcmI does not interact with mature cytochrome c(2) and that heme converts apocytochrome c(2) into its b-type derivative, these findings indicate that CcmI holds the class I apocytochromes (e.g. c(2)) tightly until their noncovalent heme-containing b-type cytochrome-like intermediates are formed. We propose that these intermediates are subsequently converted into mature cytochromes following the covalent ligation of heme via the remaining components of the Ccm complex.

• 出版日期2015-7-3