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During mitochondrial ATP synthesis, F 1 -ATPase-the portion of the ATP synthase that contains the catalytic and regulatory nucleotide binding sites-undergoes a series of concerted conformational changes that couple proton translocation to the synthesis of the high levels of ATP required for cellular function. In the structure of the rat liver F 1 -ATPase, determined to 2.8-Å resolution in the presence of physiological concentrations of nucleotides, all three ␤ subunits contain bound nucleotide and adopt similar conformations. This structure provides the missing configuration of F 1 necessary to define all intermediates in the reaction pathway. Incorporation of this structure suggests a mechanism of ATP synthesis͞hydrolysis in which configurations of the enzyme with three bound nucleotides play an essential role.