The F1Fo-ATP synthase is a rotary molecular nanomotor. F-1 is a chemical motor driven by ATP hydrolysis while F-o is an electrical motor driven by the proton flow. The two stepping motors are mechanically coupled through a common rotary shaft. Up to now, the three available crystal structures of the F(1)c(10) subcomplex of the yeast F1Fo-ATP synthase were isomorphous and then named yF(1)c(10)(I). In this crystal form, significant interactions of the cm-ring with the F-1-head of neighboring molecules affected the overall conformation of the F-1-c-ring complex. The symmetry axis of the F-1-head and the inertia axis of the c-ring were tilted near the interface between the F-1-central stalk and the c-ring rotor, resulting in an unbalanced machine. We have solved a new crystal form of the F(1)c(10) complex, named yF(1)c(10)(II), inhibited by adenylyl-imidodiphosphate (AMP-PNP) and dicyclohexylcarbodiimide (DCCD), at 6.5 angstrom resolution in which the crystal packing has a weaker influence over the conformation of the F-1-c-ring complex. yF(1)c(10)(II) provides a model of a more efficient generator. yF(1)c(10)(II) and bovine bF(1)c(8) structures share a common rotor architecture with the inertia center of the F-1-stator close to the rotor axis. (C) 2011 Elsevier Inc. All rights reserved.