Heme handling in the system II heme lyase CcsBA from Bacteroides thetaiotaomicron.

Cytochrome c biogenesis in bacteria relies on complex membrane-bound machineries that facilitate the covalent attachment of heme to apocytochrome c. System II, mediated by the bifunctional protein CcsBA, is prevalent in Gram-positive and some Gram-negative bacteria, as well as chloroplasts. Here, we report a 2.9 Å resolution cryo-EM structure of a W703C mutant of CcsBA from Bacteroides thetaiotaomicron, revealing its conformation in the heme-loaded, closed state. Comparative analysis with the previously published Helicobacter hepaticus CcsBA structure shows a conserved membrane architecture and WxWD domain organization, but notable differences in the arrangement of the periplasmic domain and the active site configuration. The W703C mutation allowed heme occupancy in the active site without inducing the open conformation, implicating the native W703 in regulating structural transitions critical for heme attachment. Our findings suggest that while the open conformation facilitates heme ligation, it is not essential for heme translocation. This work expands the understanding of structure-function relationships in System II cytochrome c maturation and highlights the potential regulatory role of the periplasmic domain conformational dynamics.