Fig. 3

Absence of exchange observed with FRET. BtuCD_cys labelled with Alexa Fluor 488 was reconstituted in liposomes loaded with 50% BtuF labelled with Alexa Fluor 555 (acceptor, green), 50% BtuF labelled with Alexa Fluor 647 (acceptor, red), and 100 µM vitamin B₁₂. One example of an initially bound ‘green’ BtuF (middle panel) and one of an initially bound ‘red’ BtuF (right panel) are displayed. While ATP and Mg²⁺ were present, no increase in fluorescence intensity in the opposite channel (red or green respectively) was observed