Conformational Coupling between Receptor and Kinase Binding Sites through a Conserved Salt Bridge in a Signaling Complex Scaffold Protein

Davi R Ortega,Jerome Baudry,Kwangwoon Lee,Frederick W Dahlquist,Guoya Mo,Igor B Zhulin,Hongjun Zhou,Christopher V Rao

doi:10.1371/journal.pcbi.1003337

Abstract

Bacterial chemotaxis is one of the best studied signal transduction pathways. CheW is a scaffold protein that mediates the association of the chemoreceptors and the CheA kinase in a ternary signaling complex. The effects of replacing conserved Arg62 of CheW with other residues suggested that the scaffold protein plays a more complex role than simply binding its partner proteins. Although R62A CheW had essentially the same affinity for chemoreceptors and CheA, cells expressing the mutant protein are impaired in chemotaxis. Using a combination of molecular dynamics simulations (MD), NMR spectroscopy, and circular dichroism (CD), we addressed the role of Arg62. Here we show that Arg62 forms a salt bridge with another highly conserved residue, Glu38. Although this interaction is unimportant for overall protein stability, it is essential to maintain the correct alignment of the chemoreceptor and kinase binding sites of CheW. Computational and experimental data suggest that the role of the salt bridge in maintaining the alignment of the two partner binding sites is fundamental to the function of the signaling complex but not to its assembly. We conclude that a key feature of CheW is to maintain the specific geometry between the two interaction sites required for its function as a scaffold.

Highlights

The Escherichia coli chemotaxis pathway employs dedicated chemoreceptors that are anchored in the membrane and detect signals from both outside and inside the cell [1]
CheW is thought to work as a scaffold protein that mediates the formation of the signaling complex with the CheA histidine kinase and the chemoreceptors
Using a series of molecular dynamics simulations, we found that the residue Arg62 can form a stable salt bridge with another highly conserved residue, Glu38

Summary

Introduction

The Escherichia coli chemotaxis pathway employs dedicated chemoreceptors that are anchored in the membrane and detect signals from both outside and inside the cell [1] Chemoreceptors relay this information to the CheA histidine kinase, which communicates the information to its cognate response regulator CheY. In a phosphorylated form, the CheY protein binds to flagellar motors to cause a change in the direction of its rotation, converting the initial signal detected by chemoreceptors into a behavioral response – a change in the swimming direction. This pathway employs the receptor-modifying enzymes CheB and CheR as well as the CheZ phosphatase, which acts on CheY [2]. In addition to the chemoreceptors and the CheA kinase, this complex contains the CheW protein, which is interchangeably referred to as a docking, scaffold, coupling, or adaptor protein [10,11,12]

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Discussion

Conclusion