The 20 C-terminal amino acid residues of the chloroplast ATP synthase γ subunit are not essential for activity

Abstract

It has been suggested that the last seven to nine amino acid residues at the C terminus of the γ subunit of the ATP synthase act as a spindle for rotation of the γ subunit with respect to the αβ subunits during catalysis (Abrahams, J.P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628). To test this hypothesis we selectively deleted C- terminal residues from the chloroplast γ subunit, two at a time starting at the sixth residue from the end and finishing at the 20th residue from the end. The mutant γ genes were overexpressed in Escherichia coli and assembled with a native α3β3 complex. All the mutant forms of γ assembled as effectively as the wild-type γ. Deletion of the terminal 6 residues of γ resulted in a significant increase (>50%) in the Ca-dependent ATPase activity when compared with the wild-type assembly. The increased activity persisted even after deletion of the C-terminal 14 residues, well beyond the seven residues proposed to form the spindle. Further deletions resulted in a decreased activity to ~19% of that of the wild-type enzyme after deleting all 20 C-terminal residues. The results indicate that the tip of the γC terminus is not essential for catalysis and raise questions about the role of the C terminus as a spindle for rotation.