SummaryF1-ATPase (F1) is an ATP-driven rotary motor protein ubiquitously found in many species as the catalytic portion of FoF1-ATP synthase. Despite the highly conserved amino acid sequence of the catalytic core subunits: α and β, F1shows diversity in the maximum catalytic turnover rateVmaxand the number of rotary steps per turn. To study the design principle of F1, we prepared eight hybrid F1s composed of subunits from two of three genuine F1s: thermophilicBacillusPS3 (TF1), bovine mitochondria (bMF1), andParacoccus denitrificans(PdF1), differing in theVmaxand the number of rotary steps. TheVmaxof the hybrids can be well fitted by a quadratic model highlighting the dominant roles of β and the couplings between α-β. Although there exist no simple rules on which subunit dominantly determines the number of steps, our findings show that the stepping behavior is characterized by the combination of all subunits.