A distinct mechanism for the ABC transporter BtuCD-BtuF 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 Hi1470/1 metal-chelate importer from Haemophilus influenzae-in complex with their cognate binding proteins. Similarly to other ABC importers, MetNI interacts with its binding protein with low affinity (K-d similar to 10(-4) M). In contrast, BtuCD-BtuF and Hi1470/1-Hi1472 form stable, high-affinity complexes (K-d similar to 10(-13) and 10(-9) M, respectively). In BtuCD-BtuF, vitamin B-12 accelerates the complex dissociation rate similar to 10(7)-fold, with ATP having an additional destabilizing effect. The findings presented here highlight substantial mechanistic differences between BtuCD-BtuF, and likely Hi1470/1-Hi1472, and the better-characterized maltose and related ABC transport systems, indicating that there is considerable mechanistic diversity within this large protein super-family.