Transferrin binding protein B and Transferrin binding protein A2 expand the transferrin recognition range of Histophilus somni

Abstract

The bacterial bipartite transferrin receptor is an iron acquisition system that is required for survival by several key human and animal pathogens. It consists of the TonB-dependent transporter Transferrin binding protein A (TbpA) and the surface lipoprotein Transferrin binding protein B (TbpB). Curiously, the Tbps are only found in host specific pathogens, and are themselves host specific, meaning that they will bind to the transferrin of their host species, but not to those of other animal species. While this phenomenon has long been established, neither the steps in the evolutionary process that led to this exquisite adaptation for the host, nor the steps that could alter it, are known. We sought to gain insight into these processes by studying Tbp specificity in Histophilus somni, a major pathogen of cattle. A past study showed that whole cells of H. somni specifically bind bovine transferrin, but not transferrin from sheep and goats, two bovids whose transferrins share 93% amino acid sequence identity with bovine transferrin. To our surprise, we found that H. somni can use sheep and goat transferrins as iron sources for growth, and that HsTbpB, but not HsTbpA, has detectable affinity for sheep and goat transferrins. Furthermore, a third transferrin binding protein, HsTbpA2, also showed affinity for sheep and goat transferrins. Our results show that H. somni TbpB and TbpA2 act to broaden the host transferrin recognition range of H. somni.\n\nImportanceHost restricted pathogens infect a single host species or a narrow range of host species. Histophilus somni, a pathogen that incurs severe economic losses for the cattle industry, infects cattle, sheep, and goats, but not other mammals. The transferrin binding proteins, TbpA and TbpB, are thought to be a key iron acquisition system in H. somni, however, surprisingly, they were also shown to be cattle transferrin-specific. In our study we find that H. somni TbpB, and another little-studied Tbp, TbpA2, bind sheep and goat transferrins as well as bovine transferrin. Our results suggest that TbpA2 may have allowed for host range expansion, and provide a mechanism for how host specificity in Tbp containing pathogens can be altered.