H+-ATPase/synthases are membrane-bound rotary nanomotors that are essential for energy conversion in nearly all life forms. A member of the family of the vacuolar-type ATPases (V-ATPases) from Thermus thermophilus, sometimes also termed A-type ATPase, was purified to homogeneity and subjected to two-dimensional (2D) crystallization trials, A novel approach to the 2D crystallization of unstable complexes yielded densely packed sheets or V-ATPase, exhibiting crystalline arrays, Aggregation of the V-ATPase under acidic conditions during reconstitution circumvented the continuous dissociation of the whole complex into the I-1' and I-o' domains. The resulting three-dimensional aggregates were converted into 2D sheets by the use ora basic buffer, and after it short annealing cycle, ordered arrays of up to 1.5 mu m diameter appeared. Fourier transforms calculated from micrographs taken from the negatively stained sample showed diffraction spots to a resolution of 23 angstrom. The Fourier transforms of the untilted images revealed unit-cell dimensions of a = 232 angstrom, b = 132 angstrom, and gamma = 90 degrees, and a projection map was calculated by merging 111 images. The most probable molecular packing suggests p22(1)2(1) symmetry of the crystals and dimer contacts between the V-1 domains. (C) 2005 Elsevier Inc. All rights reserved.