An Induced Fit Conformational Change Underlies the Binding Mechanism of the Heme Transport Proteobacteria-Protein HemS

Sabine Schneider,Katherine H Sharp,Paul D Barker,Max Paoli

doi:10.1074/jbc.m607516200

Sabine Schneider, Katherine H Sharp + Show 2 more

Open Access

https://doi.org/10.1074/jbc.m607516200

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Abstract

Bacteria rely on their environment and/or host to acquire iron and have evolved specialized systems to sequester and transport heme. The heme uptake system HemRSTUV is common to proteobacteria, and a major challenge is to understand the molecular mechanism of heme binding and transfer between the protein molecules that underlie this heme transport relay process. In the Gram-negative pathogen Yersinia enterocolitica, the HemRSTUV system culminates with the cytoplasmic recipient HemS, which stores and delivers heme for cellular needs. HemS belongs to a family of proteins essential and unique to proteobacteria. Here we report on the binding mechanism of HemS based on structural data from its apo- and ligand-loaded forms. This heme carrier protein associates with its cargo through a novel, partly preformed binding pocket, formed between a large beta-sheet dome and a three-helix subdomain. In addition to a histidine interacting with the iron, the complex is stabilized by a distal non-coordinating arginine that packs along the porphyrin plane and extensive electrostatic contacts that firmly anchor the heme propionate groups within the protein. Comparison of apo- and ligand-bound HemS crystal structures reveals striking conformational changes that underlie a "heme-induced fit" binding mechanism. Local shifts in amino acid positions combine with global, rigid body-like domain movements, and together, these bring about a switch from an open, apo-form to a closed, bound state. This is the first report in which both liganded and unliganded forms of a heme transport protein are described, thus providing penetrating insights into its mechanism of heme binding and release.

Highlights

In the host, free iron and iron in the form of heme is usually sequestered by high affinity binding proteins such as lactoferrin, transferrin, hemopexin, or serum albumin
The names of homologous proteins in distinct species differ, their functions are likely to be the same given the relatively high levels of sequence conservation [3, 8]. Genetic studies on this heme transport system in Y. enterocolitica and other species revealed the common organization of the components in a single operon, whose expression is regulated through the abundance of iron and heme (9 –16)
The determination of the apostructure of the homologous protein ChuS from Escherichia coli revealed a novel fold composed of two similar domains likely to have evolved through gene duplication and fusion [18]

Summary

Introduction

In the host, free iron and iron in the form of heme is usually sequestered by high affinity binding proteins such as lactoferrin, transferrin, hemopexin, or serum albumin. The structure of the heme-HemS complex was solved by molecular replacement using PHASER [23] with the apo-ChuS structure, Protein Data Bank code 1U9T [18]. Structure of the Heme-HemS Complex—The heme-HemS complex was solved by molecular replacement using the atomic coordinates of apo-ChuS (1U9T) [18] and refined against data to 1.7 Å spacing.

Results

Conclusion