Most commensal or pathogenic bacteria have heme acquisition systems for utilizing host heme as an iron source.  The heme acquisition systems consist of cell surface localized heme binding proteins and heme transport proteins.  In several Gram-positive bacteria such as Staphylococcus aureus and Bacillus anthracis, the iron regulated surface determinant (Isd) proteins capture and transfer heme to ABC transporter-type heme transport proteins.  Though a similar ABC-type heme transporter consisting of HmuT, HmuU, and HmuV proteins is used by Corynebacteria glutamicum and Corynebacterium diphtheriae, they adopt different heme binding proteins, HtaA and HtaB, instead of Isd proteins.  The ABC-type heme transporter system is widely used for heme acquisition, which consists of an ATP-binding protein, heme permease, and heme-binding protein (substrate-binding protein). In this work, we have studied the structural and functional relationships of two heme-binding proteins, HtaA and HmuT in Corynebacterium glutamicum. HtaA and HmuT act as a heme-binding protein in the heme acquisition system of HtaAB-HmuTUV for Corynebacterium glutamicum. Heme captured in HtaA is transferred into cytoplasm by the ABC-type HmuUV heme transporter, but the detailed molecular mechanisms of heme transport in these systems remain to be elucidated. In this work, the molecular mechanisms of heme acquisition in Corynebacterium glutamicum were elucidated based on the structural analyses of HtaA and HmuT.

We determined the crystal structures of an N-terminal domain of HtaA (HtaA-N) and HmuT (CgHmuT) from C. glutamicum.  Although the amino acid sequence of HtaA-N is not similar to those of Isd proteins, HtaA-N has a similar β-sandwich structure to the NEAT-domains of Isd proteins.  In HtaA-N, a single heme molecule is bound in a hydrophobic pocket with Tyr58 as the axial ligand. 

CgHmuT consists of structurally similar two domains located in the N-terminal and C-terminal regions.  A single heme molecule is bound in the cleft between these  domains.  Heme iron is ligated by His141 and Tyr240.  The details of heme environmental structure of heme-bound HmuT revealed by resonance Raman spectroscopy will be discussed.