The surface protein Shr of Streptococcus pyogenes binds heme and transfers it to the streptococcal heme-binding protein Shp

Hui Zhu,Benfang Lei,Mengyao Liu

doi:10.1186/1471-2180-8-15

Abstract

BackgroundThe heme acquisition machinery in Streptococcus pyogenes is believed to consist of the surface proteins, Shr and Shp, and heme-specific ATP-binding cassette transporter HtsABC. Shp has been shown to rapidly transfer its heme to the lipoprotein component, HtsA, of HtsABC. The function of Shr and the heme source of Shp have not been established.ResultsThe objective of this study was to determine whether Shr binds heme and is a heme source of Shp. To achieve the objective, recombinant Shr protein was prepared. The purified Shr displays a spectrum typical of hemoproteins, indicating that Shr binds heme and acquires heme from Escherichia coli hemoproteins in vivo. Spectral analysis of Shr and Shp isolated from a mixture of Shr and heme-free Shp (apoShp) indicates that Shr and apoShp lost and gained heme, respectively; whereas Shr did not efficiently lose its heme in incubation with apoHtsA under the identical conditions. These results suggest that Shr directly transfers its heme to Shp. In addition, the rates of heme transfer from human hemoglobin to apoShp are close to those of simple ferric heme dissociation from hemoglobin, suggesting that methemoglobin does not directly transfer its heme to apoShp.ConclusionWe have demonstrated that recombinant Shr can acquire heme from E. coli hemoproteins in vivo and appears to directly transfer its heme to Shp and that Shp appears not to directly acquire heme from human methemoglobin. These results suggest the possibility that Shr is a source of heme for Shp and that the Shr-to-Shp heme transfer is a step of the heme acquisition process in S. pyogenes. Further characterization of the Shr/Shp/HtsA system would advance our understanding of the mechanism of heme acquisition in S. pyogenes.

Highlights

The heme acquisition machinery in Streptococcus pyogenes is believed to consist of the surface proteins, Shr and Shp, and heme-specific ATP-binding cassette transporter HtsABC
The substrate-binding component of the ABC transporters binds free Fe3+, ferric siderophore complex, or heme, which is transported across the cytoplasmic membrane by the permease component using the energy from the hydrolysis of ATP catalyzed by the ATPase component [1]
We have proposed that Shp functions to relay heme from host proteins or another S. pyogenes heme-binding protein to HtsABC [18]

Summary

Introduction

The heme acquisition machinery in Streptococcus pyogenes is believed to consist of the surface proteins, Shr and Shp, and heme-specific ATP-binding cassette transporter HtsABC. Besides hemespecific ABC transporters, cell surface heme-binding proteins are required for heme acquisition in Gram-positive pathogens. These proteins have been identified in S. pyogenes [12], Staphylococcus aureus [13], and Streptococcus equi [14]. Heme acquisition machinery in Gram-positive bacteria may have to have evolved mechanisms to overcome these obstacles in heme acquisition, i.e. inability of host hemoproteins to reach the ABC transporters and the extremely high affinity of host proteins for heme. The cell surface heme-binding proteins are believed to have evolved to overcome these obstacles in heme acquisition in Grampositive pathogens. How these proteins are involved in heme acquisition is largely unknown

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