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

It has been suggested that the last seven to nine amino acid residues at the C terminus of the gamma subunit of the ATP synthase act as a spindle for relation of the gamma subunit with respect to the c alpha beta 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 gamma subunit, two at a time starting at the sixth residue from the end and finishing at the 20th residue from. the end. The mutant gamma genes were overexpressed in Escherichia coli and assembled with a native alpha(3)beta(3) complex, All the mutant forms of gamma assembled as effectively as the wild-type gamma. Deletion of the terminal 6 residues of gamma resulted in a significant increase (>50%) in the Ca-dependent ATPase activity when compared with the wildtype 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 similar to 19% of that of the wild-type enzyme after deleting all 20 C-terminal residues. The results indicate that the tip of the gamma C terminus is not essential for catalysis and raise questions about the role of the C terminus as a spindle for rotation.