Cryo-EM structures of a prokaryotic heme transporter CydDC.

Dear Editor, Heme is an essential cofactor required across all kingdoms of life utilized in numerous biological processes,including cellular res-piration.CydDC is a prokaryotic ATP-binding cassette(ABC)trans-porter required for heme assembling in respiratory cytochrome bd oxidase(Georgiou et al.,1987;Poole et al.,1989),a promising target for drug discovery(Borisov et al.,2011).CydDC is thought to play a role in maintaining an optimum periplasmic redox poise that is required for the incorporation of heme cofactors(Yamashita et al.,2014).It also suggested that CydDC is involved in heme processing by mediating glutathione/cysteine translo-cation(Cook and Poole,2000;Cook et al.,2002),although a role in heme translocation seems unlikely(Yamashita et al.,2014).To date,CydDC has been shown to be important for disulfide bond formation,motility,respiration,and tolerance to nitric oxide and antibiotics(Poole et al.,2019).