Conformational Cycle of the Vitamin B12 ABC Importer in Liposomes Detected by Double Electron-Electron Resonance (DEER)

Background: Type II ABC importers transport diverse substrates into the cell. Results: EPR on BtuCD-F in liposomes shows the response of cytoplasmic gate II during nucleotide cycle in the presence of substrate. Conclusion: The cytoplasmic gate II closes with substrate and ATP as in the x-ray structure. Substrate can be released after hydrolysis. Significance: There is new insight into the mechanism of transport in membranes. Double electron-electron resonance is used here to investigate intermediates of the transport cycle of the Escherichia coli vitamin B-12 ATP-binding cassette importer BtuCD-F. Previously, we showed the ATP-induced opening of the cytoplasmic gate I in TM5 helices, later confirmed by the AMP-PNP-bound BtuCD-F crystal structure. Here, other key residues are analyzed in TM10 helices (positions 307 and 322) and in the cytoplasmic gate II, i.e. the loop between TM2 and TM3 (positions 82 and 85). Without BtuF, binding of ATP induces detectable changes at positions 307 and 85 in BtuCD in liposomes. Together with BtuF, ATP triggers the closure of the cytoplasmic gate II in liposomes (reported by both positions 82 and 85). This forms a sealed cavity in the translocation channel in agreement with the AMP-PNPBtuCD-F x-ray structure. When vitamin B-12 and AMP-PNP are simultaneously present, the extent of complex formation is reduced, but the short 82-82 interspin distance detected indicates that the substrate does not affect the closed conformation of this gate. The existence of the BtuCD-F complex under these conditions is verified with spectroscopically orthogonal nitroxide and Gd(III)-based labels. The cytoplasmic gate II remains closed also in the vanadate-trapped state, but it reopens in the ADP-bound state of the complex. Therefore, we suggest that the substrate likely trapped in ATPBtuCD-F can be released after ATP hydrolysis but before the occluded ADP-bound conformation is reached.