Asymmetric states of vitamin B12 transporter BtuCD are not discriminated by its cognate substrate binding protein BtuF

BtuCD is an ABC transporter catalyzing the uptake of vitamin B-12 across the Escherichia coli inner membrane. A previously reported X-ray structure of BtuCD in complex with the periplasmic vitamin B-12-binding protein BtuF revealed asymmetry of the transmembrane BtuC subunits. The functional relevance of this asymmetry has remained uncertain. Here we report the X-ray structure of a catalytically impaired BtuCD mutant in complex with BtuF, where the BtuC subunits adopt a distinct asymmetric conformation. The structure suggests that BtuF does not discriminate between, or impose, asymmetric conformations of BtuCD. It also explains the conformational disorder observed in BtuCDF crystals. Structured summary of protein interactions: BtuF, BtuD and BtuC physically interact by X-ray crystallography (View interaction) (C) 2012 Federation of European Biochemical Societies. Published by Elsevier B. V. All rights reserved.