Asymmetry in the Structure of the ABC Transporter-Binding Protein Complex BtuCD-BtuF

Abstract

BtuCD is an adenosine triphosphate-binding cassette (ABC) transporter that translocates vitamin B12 from the periplasmic binding protein BtuF into the cytoplasm of Escherichia coli. The 2.6 angstrom crystal structure of a complex BtuCD-F reveals substantial conformational changes as compared with the previously reported structures of BtuCD and BtuF. The lobes of BtuF are spread apart, and B12 is displaced from the binding pocket. The transmembrane BtuC subunits reveal two distinct conformations, and the translocation pathway is closed to both sides of the membrane. Electron paramagnetic resonance spectra of spin-labeled cysteine mutants reconstituted in proteoliposomes are consistent with the conformation of BtuCD-F that was observed in the crystal structure. A comparison with BtuCD and the homologous HI1470/71 protein suggests that the structure of BtuCD-F may reflect a posttranslocation intermediate.

Structural and mechanistic insights into prokaryotic energy-coupling factor transporters

“…In contrast to the ATPases, the structures of the integral membrane subunits are not universally conserved among ABC transporters. Four different structural classes have been discovered, which probably have different mechanisms of substrate translocation (see the figure): type I importers (which are exemplified by the maltose transporter MalEFGK 2 from Escherichia coli [57][58][59][60][61] ; Protein Data Bank (PDB) accession 2R6G); type II importers (exemplified by vitamin B 12 transporter BtuC 2 D 2 F from E. coli [62][63][64] ; PDB accession 4FI3); exporters (such as multidrug and peptide transporters [65][66][67] , exemplified by the drug exporter TM287/288) from Thermotoga maritima; PDB accession 3QF4); and ECF transporters (such as ECF-FolT from Lactobacillus brevis for folate transport 21 ; PDB accession 4HUQ). Type I and II importers and energy-coupling factor (ECF) transporters are only found in prokaryotes, whereas exporters are found in all kingdoms of life 28 .…”

Dynamics and Structural Changes Induced by ATP Binding in SAV1866, a Bacterial ABC Exporter

“…All but one correspond to bacterial transporters. Five are ABC importers (BtuCD, ModBC-A, HI1470/1, MalFG/ K, MetNI), [9][10][11][12][13][14][15] and three are drug exporters (SAV1866, MsbA, and mouse P-glycoprotein). [16][17][18][19] High-resolution structures and NMR, EPR, and electron microscopy data strongly suggest that ABC exporters undergo large movements during substrate efflux, in particular upon ATP binding and/or hydrolysis.…”

Structure and mechanism of ATP-binding cassette transporters

“…Since then, two more crystal structures have been determined, one of the homologous HIF protein (HI1470/71) from Haemophilus influenzae (Pinkett et al 2007), and that of the BtuCD protein in complex with the binding protein BtuF (Hvorup et al 2007; figure 5b). The structures of BtuCD, HIF and BtuCD-F demonstrated that a subset of TM helices (3-5a) can adopt one of two conformations (figure 5c).…”