Characterization of HasB, a Serratia marcescens TonB-like protein specifically involved in the haemophore-dependent haem acquisition system

In Gram-negative bacteria, the TonB-ExbB-ExbD inner membrane multiprotein complex is required for active transport of diverse molecules through the outer membrane. We present evidence that Serratia marcescens, like several other Gram-negative bacteria, has two TonB proteins: the previously characterized TonBSM, and also HasB, a newly identified component of the has operon that encodes a haemophore-dependent haem acquisition system. This system involves a soluble extracellular protein (the HasA haemophore) that acquires free or haemoprotein-bound haem and presents it to a specific outer membrane haemophore receptor (HasR). TonBSM and HasB are significantly similar and can replace each other for haem acquisition. However, TonBSM, but not HasB, mediates iron acquisition from iron sources other than haem and haemoproteins, showing that HasB and TonBSM only display partial redundancy. The reconstitution in Escherichia coli of the S. marcescens Has system demonstrated that haem uptake is dependent on the E. coli ExbB, ExbD and TonB proteins and that HasB is non-functional in E. coli. Nevertheless, a mutation in the HasB transmembrane anchor domain allows it to replace TonBEC for haem acquisition. As the change affects a domain involved in specific TonBEC-ExbBEC interactions, HasB may be unable to interact with ExbBEC, and the HasB mutation may allow this interaction. In E. coli, the HasB mutant protein was functional for haem uptake but could not complement the other TonBEC-dependent functions, such as iron siderophore acquisition, and phage DNA and colicin uptake. Our findings support the emerging hypothesis that TonB homologues are widespread in bacteria, where they may have specific functions in receptor-ligand uptake systems.