PDBsum entry 2cn4

Transport protein

PDB id: 2cn4

Contents

Protein chains

173 a.a.

Ligands

HEM ×2

PO4 ×2

Waters ×149

References listed in PDB file

Key reference

Title

The crystal structure of the secreted dimeric form of the hemophore hasa reveals a domain swapping with an exchanged heme ligand.

Authors

M.Czjzek, S.Létoffé, C.Wandersman, M.Delepierre, A.Lecroisey, N.Izadi-Pruneyre.

Ref.

J Mol Biol, 2007, 365, 1176-1186. [DOI no: 10.1016/j.jmb.2006.10.063]

PubMed id

17113104

Abstract

To satisfy their iron needs, several Gram-negative bacteria use a heme uptake system involving an extracellular heme-binding protein called hemophore. The function of the hemophore is to acquire free or hemoprotein-bound heme and to transfer it to HasR, its specific outer membrane receptor, by protein-protein interaction. The hemophore HasA secreted by Serratia marcescens, an opportunistic pathogen, was the first to be identified and is now very well characterized. HasA is a monomer that binds one b heme with strong affinity. The heme in HasA is highly exposed to solvent and coordinated by an unusual pair of ligands, a histidine and a tyrosine. Here, we report the identification, the characterization and the X-ray structure of a dimeric form of HasA from S. marcescens: DHasA. We show that both monomeric and dimeric forms are secreted in iron deficient conditions by S. marcescens. The crystal structure of DHasA reveals that it is a domain swapped dimer. The overall structure of each monomeric subunit of DHasA is very similar to that of HasA but formed by parts coming from the two different polypeptide chains, involving one of the heme ligands. Consequently DHasA binds two heme molecules by residues coming from both polypeptide chains. We show here that, while DHasA can bind two heme molecules, it is not able to deliver them to the receptor HasR. However, DHasA can efficiently transfer its heme to the monomeric form that, in turn, delivers it to HasR. We assume that DHasA can function as a heme reservoir in the hemophore system.

Figure 1.
Figure 1. Gel filtration on a HiPrep 16/60 Sephacryl S-100 High resolution column equilibrated with 50 mM sodium phosphate buffer (pH 7). The flow rate was 30 ml/h. The volume of each fraction was 1.5 ml.

Figure 3.
Figure 3. Superimposition of 600 MHz ^1H-^15N HSQC spectra of HasA (in black) and DHasA (in red) (1.5 mM in 20 mM sodium phosphate buffer, pH 5.6, 30 °C). Both spectra were obtained using the same acquisition and processing parameters.

The above figures are reprinted by permission from Elsevier: J Mol Biol (2007, 365, 1176-1186) copyright 2007.