Structural and molecular determinants for the interaction of ExbB from Serratia marcescens and HasB, a TonB paralog

ExbB and ExbD are cytoplasmic membrane proteins that associate with TonB to convey the energy of the proton-motive force to outer membrane receptors in Gram-negative bacteria for iron uptake. The opportunistic pathogen Serratia marcescens (Sm) possesses both TonB and a heme-specific TonB paralog, HasB. ExbB; Sm; has a long periplasmic extension absent in other bacteria such as E. coli (Ec). Long ExbB's are found in several genera of Alphaproteobacteria, most often in correlation with a hasB gene. We investigated specificity determinants of ExbB; Sm; and HasB. We determined the cryo-EM structures of ExbB; Sm; and of the ExbB-ExbD; Sm; complex from S. marcescens. ExbB; Sm; alone is a stable pentamer, and its complex includes two ExbD monomers. We showed that ExbB; Sm; extension interacts with HasB and is involved in heme acquisition and we identified key residues in the membrane domain of ExbB; Sm; and ExbB; Ec; , essential for function and likely involved in the interaction with TonB/HasB. Our results shed light on the class of inner membrane energy machinery formed by ExbB, ExbD and HasB.