A distinct mechanism for the ABC transporter BtuCD-F revealed by the dynamics of complex formation

ATP binding cassette (ABC) transporters are integral membrane proteins that translocate a diverse array of substrates across cell membranes. We present here the dynamics of complex formation of three structurally characterized ABC transporters: the BtuCD vitamin B(12) importer and MetNI D/L-methionine importer from Escherichia coli, and the Haemophilus influenzae Hi1470/1 metal-chelate importer, with their cognate binding proteins. Similar to other ABC importers, MetNI interacts with its binding protein with low affinity (K(d) ~ 10(-4) M). In contrast, BtuCD-F and Hi1470/1-2 form stable, high affinity complexes (K(d) ~ 10(-13) and 10(-9) M, respectively). In BtuCD-F, vitamin B(12) accelerates complex dissociation rate ~10(7)-fold, with ATP having an additional destabilizing effect. The findings presented here highlight substantial mechanistic differences between BtuCD-F, and likely Hi1470/1-2, and the better characterized maltose and related ABC transport systems, indicating considerable mechanistic diversity within this large protein super-family.