FIGURE 3.

Normalized X-band EPR spectra of purified reconstituted BtuB labeled in the Ton box at position 10 (V10R1): (a) in POPC without substrate, and (b) in POPC with substrate (vitamin B₁₂) added. The addition of substrate induces an unfolding in the Ton box. (c) In the soaking solution used for protein crystallization with substrate added. Each spectrum is a composite arising from a highly mobile R1 population (downward-facing arrow), corresponding to the unfolded state and a motionally restricted R1 population, corresponding to the folded or docked state (upward-facing arrow). Spectra are 100 gauss scans.