Structural Characterization of Heme Uptake System in Corynebacteria

Abstract

Heme uptake machinery of Corynebacteria including Corynebacteria glutamicum and Corynebacterium diphtheriae consists of heme binding proteins, HtaA and HtaB, and the ABC-type heme transporter HmuTUV. In this work, we have studied the structural and functional relationships of HtaA, HtaB and HmuT in Corynebacterium glutamicum. Sequence analysis identified a conserved region (CR) of approximately 150 amino acids that is duplicated in HtaA and present in a single copy in HtaB [1]. HtaA consists of two homologous CRs in the N- and C-terminal regions. We have determined the crystal structures of the N-, and C-terminal CR of HtaA (HtaA-N and HtaA-C, respectively) and HtaB at the resolution of 2.0, 1.3, and 1.7 Å, respectively. HtaA-N consists of 11 β strands and two short α helices and accommodates one heme molecule with Tyr58 located in the first α helix as the heme axial ligand. Tyr58 forms a hydrogen bond with His111. A heme propionate forms hydrogen bonds with Ser54 and Tyr201. Heme is accommodated in an open pocket formed by hydrophobic amino acid residues. HtaA-C and HtaB show similar global structures to HtaA-N. The key residues for heme-binding and recognition including the axial ligand of heme and residues involved in the hydrogen bonding interactions with heme are conserved among HtaA-N, HtaA-C, and HtaB. We also determined the crystal structure of HmuT at the resolution of 1.4 Å [2]. HmuT consists of structurally similar two domains located in the N-terminal and C-terminal regions connected a long α helix. A single heme molecule is bound in the cleft between these domains. Heme iron is ligated by His141 and Tyr240, and Tyr240 forms a hydrogen bond with Arg242. Intriguingly, HmuT binds a heme with two different orientations.