Projection structure of the bacterial oxalate transporter OxlT at 3.4A resolution

OxlT is a bacterial transporter protein with 12 transmembrane segments that belongs to the Major Facilitator Superfamily of transporters. It facilitates the exchange of oxalate and formate across the membrane of the Gram-negative bacterium Oxalobacter formigenes. From an electron crystallographic analysis of two-dimensional, tube-like crystals of OxlT, we have previously determined the three-dimensional structure of this transporter at 6.5 A resolution. Here, we report conditions to obtain crystalline, two-dimensional sheets of OxlT with diameters exceeding 2 microm. Images of the crystalline sheets were recorded at liquid nitrogen temperatures on a transmission electron microscope equipped with a field-emission gun, operated at 300 kV. Computed optical diffraction patterns from the best images display measurable reflections to about 3.4A, and electron diffraction patterns show spots to about 3.2 A resolution in the best cases. As in the case of the tube-like crystals, the new crystalline sheets also belong to the p22(1)2(1) symmetry group. However, the unit cell dimensions of 102.7A x 67.3 A are significantly smaller in one direction than those previously observed with the tube-like crystals that display unit cell dimensions of 100.3A x 79.0 A. Different regions of OxlT are involved in intermolecular contacts in the two types of crystals, and the improved resolution of the sheet crystals appears to be mainly attributable to this tighter packing of the monomers within the unit cell.