Substantiation of CcmB as heme extruder exploitable for microbial production of secretory heme

Heme, a redox-active cofactor for vital processes across all domains of life, has widespread applications in healthcare and food supplement industries. However, heme in excess not only inhibits its own synthesis but also is highly cytotoxic, becoming a hindrance for microbial production of heme. In order to pin down heme exporters exploitable for production of secretory heme in bacteria, in this study, we carried out a comprehensive comparative study of heme exporters known to date, CydDC, HrtAB, and CcmABC(D). While CydDC proteins are ubiquitous, homologs of HrtAB and CcmABC(D) are restricted to Gram⁺ and proteobacteria respectively. We substantiate that CcmB, rather than CcmC or CcmCD, are able to extrude heme out of the membrane, a mechanism shared by HrtB and HrtAB but not CydDC. By expressing CcmB in engineered high-performance heme-producing strains of Escherichia coli, secretory heme production is substantially improved. Our findings offer a solution for high-yield microbial bioproduction of secretory heme with industrial and medical applications.