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 B12 across the Escherichia coli inner membrane. A previously reported X-ray structure of BtuCD in complex with the periplasmic vitamin B12-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 interactionsBtuF, BtuD and BtuCphysically interact by X-ray crystallography (View interaction)

► We have solved a structure of hydrolysis-deficient mutant of BtuCDF transporter.
► The structure showed inverted conformation of BtuF, unlike that observed previously.
► SeMet labeling of BtuF confirmed presence of both conformations in BtuCDF crystals.
► Thus, BtuF does not impose or discriminate between the asymmetric states of BtuCD.