Role of the DELSEED Loop in Torque Transmission of F1-ATPase

F-1-ATPase is an ATP-driven rotary motor that generates torque at the interface between the catalytic beta-subunits and the rotor gamma-subunit. The beta-subunit inwardly rotates the C-terminal domain upon nucleotide binding/dissociation; hence, the region of the C-terminal domain that is in direct contact with gamma-termed the DELSEED loop-is thought to play a critical role in torque transmission. We substituted all the DELSEED loop residues with alanine to diminish specific DELSEED loop-gamma interactions and with glycine to disrupt the loop structure. All the mutants rotated unidirectionally with kinetic parameters comparable to those of the wild-type F-1, suggesting that the specific interactions between DELSEED loop and gamma is not involved in cooperative interplays between the catalytic beta-subunits. Glycine substitution mutants generated half the torque of the wild-type F-1, whereas the alanine mutant generated comparable torque. Fluctuation analyses of the glycine/alanine. mutants revealed that the gamma-subunit was less tightly held in the alpha(3)beta(3)-stator ring of the glycine mutant than in the wild-type F-1 and the alanine mutant. Molecular dynamics simulation showed that the DELSEED loop was disordered by the glycine substitution, whereas it formed an alpha-helix in the alanine mutant. Our results emphasize the importance of loop rigidity for efficient torque transmissions.