The C-Terminal Flexible Domain of the Heme Chaperone CcmE Is Important but Not Essential for Its Function

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Abstract
CcmE is a heme chaperone active in the cytochrome c maturation pathway of Escherichia coli . It first binds heme covalently to strictly conserved histidine H130 and subsequently delivers it to apo-cytochrome c . The recently solved structure of soluble CcmE revealed a compact core consisting of a β-barrel and a flexible C-terminal domain with a short α-helical turn. In order to elucidate the function of this poorly conserved domain, CcmE was truncated stepwise from the C terminus. Removal of all 29 amino acids up to crucial histidine 130 did not abolish heme binding completely. For detectable transfer of heme to type c cytochromes, only one additional residue, D131, was required, and for efficient cytochrome c maturation, the seven-residue sequence 131 DENYTPP 137 was required. When soluble forms of CcmE were expressed in the periplasm, the C-terminal domain had to be slightly longer to allow detection of holo-CcmE. Soluble full-length CcmE had low activity in cytochrome c maturation, indicating the importance of the N-terminal membrane anchor for the in vivo function of CcmE.