Crystal Structure of Escherichia coli CusC, the Outer Membrane Component of a Heavy Metal Efflux Pump

Rithika Kulathila,Ragini Kulathila,Bert Van Den Berg,Mridhu Indic,Hendrik W Van Veen

doi:10.1371/journal.pone.0015610

Abstract

BackgroundWhile copper has essential functions as an enzymatic co-factor, excess copper ions are toxic for cells, necessitating mechanisms for regulating its levels. The cusCBFA operon of E. coli encodes a four-component efflux pump dedicated to the extrusion of Cu(I) and Ag(I) ions.Methodology/Principal FindingsWe have solved the X-ray crystal structure of CusC, the outer membrane component of the Cus heavy metal efflux pump, to 2.3 Å resolution. The structure has the largest extracellular opening of any outer membrane factor (OMF) protein and suggests, for the first time, the presence of a tri-acylated N-terminal lipid anchor.Conclusions/SignificanceThe CusC protein does not have any obvious features that would make it specific for metal ions, suggesting that the narrow substrate specificity of the pump is provided by other components of the pump, most likely by the inner membrane component CusA.

Highlights

Copper, while being required for the function of a number of enzymes in Escherichia coli, is extremely toxic, requiring mechanisms to regulate its levels inside the cell
CusB is a membrane fusion/adaptor protein (MFP) and CusC is an outer membrane (OM) protein belonging to the outer membrane factor (OMF family) [4]
Co-crystal structures reveal several binding sites for Cu(I) and Ag(I) ions that look similar to those observed in CusF [6], with a methionine and an aromatic residue involved in metal coordination

Summary

Introduction

While being required for the function of a number of enzymes in Escherichia coli, is extremely toxic, requiring mechanisms to regulate its levels inside the cell. Conclusions/Significance: The CusC protein does not have any obvious features that would make it specific for metal ions, suggesting that the narrow substrate specificity of the pump is provided by other components of the pump, most likely by the inner membrane component CusA. Co-crystal structures reveal several binding sites for Cu(I) and Ag(I) ions that look similar to those observed in CusF [6], with a methionine and an aromatic residue involved in metal coordination.

Results

Conclusion