heme iron utilization protein ChuX and similar proteins
This family contains proteins similar to ChuX, a member of the conserved heme utilization operon from pathogenic E. coli, and includes ChuS, HutX, HuvX, HugX, and ShuX in proteobacteria, among others. It forms a dimer which displays a very similar fold and organization to the monomeric structure of other heme utilization proteins such as HemS, ChuS, HmuS, PhuS; these latter occurring as duplicated domains. They all bind heme via a key conserved histidine. The genes encoded within these heme utilization operons enable the effective uptake and utilization of heme as an iron source in pathogenic microorganisms to enable multiplication and survival within hosts they invade. ChuX, a member of the conserved heme utilization operon from pathogenic E. coli O157:H7, forms a dimer with a very similar fold to the monomer structure of two other heme utilization proteins, ChuS and HemS, despite low sequence homology. ChuX has been shown to bind heme in a 1:1 manner, inferring that the ChuX homodimer could coordinate two heme molecules in contrast to only one heme molecule bound in ChuS and HemS. Similarly, cytoplasmic heme-binding protein HutX in Vibrio cholera, an intracellular heme transport protein for the heme-degrading enzyme HutZ, forms a dimer, each domain binding heme that is transferred from HutX to HutZ via a specific protein-protein interaction. This family also includes AGR_C_4470p from Agrobacterium tumefaciens and found to be a dimer, with each subunit having strong structural homology and organization to the heme utilization protein ChuS from Escherichia coli and HemS from Yersinia enterocolitica. However, the heme binding site is not conserved in AGR_C_4470p, suggesting a possible different function.