Substrate-Induced Conformational Changes of the Perplasmic N-Terminus of an Outer-Membrane Transporter by Site-Directed Spin Labeling

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Abstract
The structure and dynamics of the N-terminal and core regions of BtuB, an outer membrane vitamin B12 transporter from Escherichia coli, were investigated by site-directed spin labeling. Cysteine mutants were generated by site-directed mutagenesis to place spin labels in the N-terminal region (residues 1−17), the core region (residues 25−30), and double labels into the Ton box (residues 6−12). BtuB mutants were expressed, spin labeled, purified, and reconstituted into phosphatidylcholine. In the presence of substrate (vitamin B12), EPR spectroscopy demonstrates that there is a conformational change in the Ton box similar to that seen previously for BtuB in intact outer membranes. The Ton box is positioned within the β-barrel of BtuB in the absence of substrate (docked configuration) but becomes unfolded and increases its aqueous exposure upon substrate binding (undocked configuration). This conformational change and the similarity in the EPR spectra between reconstituted and native membranes indicate that BtuB is correctly folded and functional in the reconstituted system. The protein segment on the N-terminal side of the Ton box is highly mobile, and it becomes more mobile in the presence of substrate. Side chains in the region C-terminal to the Ton box also show increases in mobility with substrate addition, but position 16 appears to define a hinge point for this conformation change. EPR line shapes and relaxation data indicate that residues 25−30 form a β-strand structure, which is analogous to the first β-strand in the cores of the homologous iron transporters. When substrate binds to BtuB, this first β-strand remains folded. The EPR spectra of double-nitroxide labels within the Ton box are broadened because of dipolar and collisional exchange interactions. The broadening pattern indicates that the Ton box is not helical but is in an extended or β-strand structure.