During Cytochrome c Maturation CcmI chaperones Class I Apocytochromes until the Formation of their b-type Cytochrome Intermediates [Microbiology]

May 15th, 2015 by Verissimo, A. F., Shroff, N. P., Daldal, F.

The c-type cytochromes are electron transfer proteins involved in energy transduction. They have heme-binding (CXXCH) site(s) that ligate covalently heme b via thioether bonds, and are classified into different classes based on their protein folds, locations and properties of their cofactors. Rhodobacter capsulatus produces various c-type cytochromes using the Cytochrome c maturation (Ccm)-System I, formed of the CcmABCDEFGHI proteins. CcmI is a component of the heme ligation complex CcmFHI, interacts with the heme-handling protein CcmE, and chaperones apocytochrome c2 by binding its C-terminal helix. Whether CcmI also chaperones other c-type apocytochromes, and the effects of heme on these interactions, were unknown hitherto. Here, we purified different classes of soluble and membrane-bound c-type apocytochromes (class I c2 and c1 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 μM KD values). When present, heme induced conformational changes in class I apocytochromes (e.g., c2), and decreased significantly their high affinity for CcmI. Knowing that CcmI does not interact with mature cytochrome c2, and that heme converts apocytochrome c2 into its b-type derivative, these findings indicate that CcmI holds tightly the class I apocytochromes (e.g., c2) until their non-covalent 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.
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