Architecture of the membrane-bound cytochrome c heme lyase CcmF

The covalent attachment of one or multiple heme cofactors to their protein chain enables cytochromes c to be utilized in electron transfer and redox catalysis in extracytoplasmic environments. A dedicated heme maturation machinery, whose core component is a heme lyase, scans nascent peptides after Sec-dependent translocation for CX(n)CH binding motifs. Here we report the three-dimensional structure of the heme lyase CcmF from Thermus thermophilus, a 643 aa integral membrane protein. CcmF contains a heme b cofactor at the bottom of a large cavity that opens towards the extracellular side to receive heme groups for cytochrome maturation from the heme chaperone CcmE. A surface groove on CcmF may guide the extended apoprotein to heme attachment at or near a loop containing the functionally essential WXWD motif, situated above the putative cofactor binding pocket. The structure suggests heme delivery from within the membrane, redefining the role of the chaperone CcmE.