Cryo-EM reveals unique structural features of the FhuCDB Escherichia coli ferrichrome importer

Wenxin Hu,Hongjin Zheng

doi:10.1038/s42003-021-02916-2

Wenxin Hu, Hongjin Zheng

Open Access

https://doi.org/10.1038/s42003-021-02916-2

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Abstract

As one of the most elegant biological processes developed in bacteria, the siderophore-mediated iron uptake demands the action of specific ATP-binding cassette (ABC) importers. Although extensive studies have been done on various ABC importers, the molecular basis of these iron-chelated-siderophore importers are still not fully understood. Here, we report the structure of a ferrichrome importer FhuCDB from Escherichia coli at 3.4 Å resolution determined by cryo electron microscopy. The structure revealed a monomeric membrane subunit of FhuB with a substrate translocation pathway in the middle. In the pathway, there were unique arrangements of residues, especially layers of methionines. Important residues found in the structure were interrogated by mutagenesis and functional studies. Surprisingly, the importer’s ATPase activity was decreased upon FhuD binding, which deviated from the current understanding about bacterial ABC importers. In summary, to the best of our knowledge, these studies not only reveal a new structural twist in the type II ABC importer subfamily, but also provide biological insights in the transport of iron-chelated siderophores.

Highlights

As one of the most elegant biological processes developed in bacteria, the siderophoremediated iron uptake demands the action of specific ATP-binding cassette (ABC) importers
All of siderophore importers belong to the superfamily of ATP-binding cassette (ABC) transporters, the architecture of the four systems is not the same: YbtPQ resembles the fold of type IV exporter[14], while the other three seem to be bacterial type II importers
The cooperativity of the ATP binding was weakened, the ATP binding affinity seemed to be at the same level as indicated by similar Km values. These results suggest that the molecular mechanism of FhuCDB might deviate from the standard model of type II ABC importers derived from

Summary

Introduction

As one of the most elegant biological processes developed in bacteria, the siderophoremediated iron uptake demands the action of specific ATP-binding cassette (ABC) importers. To the best of our knowledge, these studies reveal a new structural twist in the type II ABC importer subfamily, and provide biological insights in the transport of iron-chelated siderophores. The importance of siderophore molecules is further highlighted by the increasing reports of their biological functions other than Fe3+ chelating[2] These functions include binding and transporting a variety of metal ions as metallophores[3,4], acting as signaling molecules for gene regulation[5], protecting bacteria from oxidative stress[6,7], as well as providing antibacterial activity as sideromycins[8,9]. We functionally characterized the ferrichrome, a hydroxamate-type siderophore molecule, importer FhuCDB complex from E. coli and determined its structure at 3.4 Å resolution by single-particle cryo-electron microscopy (cryo-EM). With the FhuCDB structure and related functional experiments, our study provides important mechanical insights in the import of iron-chelated-siderophores

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