Real-time observation of vitamin B12 transport by BtuCD-F at the single-molecule level

ATP-binding cassette (ABC) transporters use the power of ATP binding and hydrolysis to deliver a wide variety of molecules across membrane bilayers. Crystal structures in the past two decades have provided snapshots of these transporters under various conditions, which revealed conformational changes of transporters upon substrate and ATP binding along the transport pathway. However, understanding of kinetics of substate translocation and the knowledge of transient intermediates along the transport pathway remain primitive, especially for Type II ABC importers. In this thesis work, I employed fluorescence resonance energy transfer (FRET) to study the transport mechanism of BtuCD, a model type II ABC importer that transports vitamin B12. I also exploited the photophysical property of the transport substrate, B12, that quenches the fluorescence of certain fluorophores nearby through FRET, which enables the monitoring of B12 binding and release. Our ensemble FRET data showed the binding of B12 and nucleotide both weaken the interaction between BtuCD and BtuF. Our single-molecule FRET (smFRET) experiments not only revealed stepwise movement of substrate molecule through the transporter in real-time and at the single-molecule level, but also yielded the rates of transition between individual conformational states, which had not been previously characterized in any other transporters. The results showed that ATP hydrolysis, instead of ATP binding, drives the power-stroke for the transport cycle. They also showed two sequential ATP hydrolysis events are required to complete a transport cycle, with the first ATP hydrolysis event delivering B12 into the cavity of BtuCD and the second resetting the BtuCD-F complex for a new cycle of transport.