Prevalence of Slam-dependent hemophores in Gram-negative bacteria

Iron acquisition systems are crucial for pathogen growth and survival in iron-limiting host environments. To overcome nutritional immunity, bacterial pathogens evolved to use diverse mechanisms to acquire iron. Here, we examined a heme acquisition system driven by hemophores called HphAs from several Gram-negative bacteria. Structural determination of HphAs revealed a N-terminal clamp-like domain that binds heme and a C-terminal eight-stranded {beta}-barrel domain that shares the same architecture as the Slam-dependent Neisserial surface lipoproteins. The structure of these HphAs is strikingly similar to a novel hemophore discovered by Latham et al. (2019), named hemophilin1. The genetic organization of HphAs consist of genes encoding a Slam homolog and a TonB-dependent receptor (TBDR). We investigated the Slam-HphA system in the native organism or the reconstituted system in E. coli cells and found that the efficient secretion of HphA is dependent on Slam. The TBDR also played an important role for heme uptake and conferred specificity for its cognate HphA. Furthermore, bioinformatic analysis of HphA homologs revealed that HphAs are conserved in the alpha, beta, and gammaproteobacteria Together, these results show that HphA presents a new class of hemophores in Gram-negative bacteria and further expands the role of Slams in transporting soluble proteins supporting it role as a type 11 secretion system.