In Campylobacter jejuni a new type of chaperone receives heme b from ferrochelatase

Intracellular heme formation and trafficking are fundamental processes in living organisms. Three biogenesis pathways are used by bacteria and archaea to produce iron protoporphyrin IX (heme b ) that diverge after the formation of the common intermediate uroporphyrinogen III (uro’gen III). In this work, we identify and provide a detailed characterization of the enzymes involved in the transformation of uro’gen III into heme. We show that in this organism operates the protoporphyrin-dependent pathway (PPD pathway), in which the last reaction is the incorporation of ferrous iron into the porphyrin ring by the ferrochelatase enzyme. In general, following this final reaction, little is known about how the formed heme b reaches the target proteins. In particular, the chaperons that are thought to be required to traffic heme for incorporation into hemeproteins to avoid the cytotoxicity associated to free heme, remain largely unidentified. We identified in C. jejuni a chaperon-like protein, named CgdH2, that binds heme with a dissociation constant of 4.9 ± 1.0 µM, a binding that is impaired upon mutation of residues histidine 45 and 133. We show that C. jejuni CgdH2 establishes protein-protein interactions with ferrochelatase, which should enable for the observed transfer of heme from ferrochelatase to CgdH2. Phylogenetic analysis revealed that C. jejuni CgdH2 is evolutionarily distinct from the currently known chaperones. Therefore, CgdH2 is a novel chaperone and the first protein identified as an acceptor of the intracellularly formed heme, thus enlarging our understanding of bacterial heme homeostasis. ### Competing Interest Statement The authors have declared no competing interest.