Flexibility in the ABC transporter MsbA: Alternating access with a twist

doi:10.1073/pnas.0709388104

Flexibility in the ABC transporter MsbA: Alternating access with a twist

Departments of *Cell Biology and
^†Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, CB-105, La Jolla, CA 92037

Communicated by K. Barry Sharpless, The Scripps Research Institute, La Jolla, CA, October 3, 2007 (received for review August 6, 2007)

Abstract

ATP-binding cassette (ABC) transporters are integral membrane proteins that translocate a wide variety of substrates across cellular membranes and are conserved from bacteria to humans. Here we compare four x-ray structures of the bacterial ABC lipid flippase, MsbA, trapped in different conformations, two nucleotide-bound structures and two in the absence of nucleotide. Comparison of the nucleotide-free conformations of MsbA reveals a flexible hinge formed by extracellular loops 2 and 3. This hinge allows the nucleotide-binding domains to disassociate while the ATP-binding half sites remain facing each other. The binding of the nucleotide causes a packing rearrangement of the transmembrane helices and changes the accessibility of the transporter from cytoplasmic (inward) facing to extracellular (outward) facing. The inward and outward openings are mediated by two different sets of transmembrane helix interactions. Altogether, the conformational changes between these structures suggest that large ranges of motion may be required for substrate transport.

Footnotes

^‡To whom correspondence should be addressed. E-mail: gchang{at}scripps.edu
Author contributions: A.W. and C.L.R. contributed equally to this work; A.W., C.L.R., and G.C. designed research; A.W., C.L.R., J.Y., C.B.R., and G.C. performed research; G.C. contributed new reagents/analytic tools; A.W., C.L.R., J.Y., and G.C. analyzed data; and A.W., C.L.R., J.Y., C.B.R., and G.C. wrote the paper.
The authors declare no conflict of interest.
Data deposition: The coordinates and structure factors have been deposited in the Protein Data Bank, www.pdb.org (PDB ID codes 3B5W, 3B5X, 3B5Y, 3B5Z, and 3B60).
This article contains supporting information online at www.pnas.org/cgi/content/full/0709388104/DC1.