A heme periplasmic-binding protein hHBP mediates heme transport in Haemophilus ducreyi

Abstract

Haemophilus ducreyi, a Gram-negative and heme-dependent bacterium, is the causative agent of chancroid, a genital ulcer sexually transmitted infection. Although the precise molecular mechanism of heme acquisition in H. ducreyi is unclear, heme uptake likely proceeds via a receptor mediated process. The initial event involves binding to either of two outer membrane receptors, TdhA and HgbA. Once heme is deposited into the periplasmic space, a heme permease is postulated to transport heme across the periplasmic space to the inner membrane. In prior experiments, using protein expression profiling of the H. ducreyi periplasmic proteome, we identified a periplasmic-binding protein hHBP that we propose is a component of a heme trafficking operon. Biochemical and genetic approaches were used to functionally characterize hHBP. First, purified hHBP was incubated with increasing concentrations of heme and the mixtures were resolved by non-denaturing polyacylamide gel electrophoresis. Separated proteins were transferred onto PVDF membranes and heme-protein complexes were detected by enhanced chemiluminescence (ECL). Second, the hhbp gene was cloned in the E. coli recombinant mutant E. coli FB827 dppA::Km mppA::Cm (pAM238-HasR) which expresses the Serratia marcescens HasR heme receptor allowing heme translocaton into the periplasm, but denies heme entry into the cytoplasm because of the presence of the double mutation (dppA::Km mppA::Cm) resulting in a mutant lacking the two periplasmic proteins DppA and MppA. We found that heme binding to hHBP was saturable as determined by ECL. Genetic complementation in trans with hhbp repaired the growth defect of the mutant E. coli for heme utilization as an iron source. The growth restoration was comparable to that seen with the E. coli mutant complemented with the intact Dpp permease. Additionally, growth of the mutant was not rescued with the empty plasmid vector. We concluded that H. ducreyi hHBP functionally binds heme. Complementation of the E. coli mutant for heme competency supports the proposal that hHBP participates in the transit of heme in H. ducreyi.