ATP-binding cassette (ABC) transporters, also known as traffic ATPases, form a large family of integral membrane proteins responsible for the translocation of a variety of chemically diverse substrates across the lipid bilayers of cellular membranes of both prokaryotes and eukaryotes by the hydrolysis of ATP. The ATP-binding subunit of an ABC transporter fromGeobacillus kaustophilus, a homodimeric enzyme, was overexpressed inEscherichia coliand purified. Crystals were obtained using the microbatch-under-oil method at 291 K. X-ray diffraction data to 1.6 Å resolution were collected on SPring-8 beamline BL26B1. The crystals belonged to the orthorhombic space groupI222, with unit-cell parametersa= 54.94,b = 78.63,c= 112.96 Å. Assuming the presence of a dimer in the asymmetric unit gave a crystal volume per protein weight (VM) of 2.32 Å3 Da−1and a solvent content of 47%; this was consistent with the results of a dynamic light-scattering experiment, which showed a dimeric state of the protein in solution. Molecular-replacement trials using the crystal structure of HisP from theSalmonella typhimuriumATP-binding subunit of an ABC transporter as a search model did not provide a satisfactory solution, indicating that the two ATP-binding subunits of ABC transporters have substantially different structures.
X-Ray Structure of a Heterodimeric ABC Transporter Crystallized in its Inward-Facing Conformation
