Cryo-EM of CcsBA reveals the basis for cytochrome c biogenesis and heme transport

Although the individual structures and respiratory functions of cytochromes are well studied, the structural basis for their assembly, including transport of heme for attachment, are unknown. We describe cryo-EM structures of CcsBA, a bifunctional heme transporter and cytochrome c synthase. Models built from the cryo-EM densities show that CcsBA is trapped with heme in two conformations, herein termed the closed and open states. The closed state has heme located solely at a transmembrane (TM) site, with a large periplasmic domain oriented such that access of heme to the cytochrome acceptor is denied. The open conformation contains two heme moieties, one in the TM-heme site and another in external site (P-heme site). The presence of heme in the periplasmic site, at the base of a chamber, induces a large conformational shift which exposes the heme for reaction with apocytochrome c. Consistent with these structures, in vivo and in vitro cytochrome c synthase studies suggest a mechanism for transfer of the periplasmic heme to cytochrome.