Mutagenesis and Reversion Analysis of Residue Met-209 of the β-Subunit ofEscherichia coliATP Synthase

Residue β-Met-209 is conserved in all known F1- ATPase sequences, and the mutation βM209I inEscherichia colicauses profound inhibition of ATP synthesis and hydrolysis. Based on the properties of this mutant it had previously been proposed that residue β-209 lies close to the site of catalysis. Two approaches were used to study this residue further. First, revertants were sought. Only wild-type and β-Ser-209 were found; the Ser revertants involved a two-base change. Significantly, Ser is found at the equivalent position in the homologous vacuolar and archaebacterial ATPases. Second, all 20 natural amino acids were placed at position β-209 by mutagenesis, and catalytic properties of the mutants were analyzed. The results showed that only a limited set of residues supported significant growth or ATPase activity, and that many of the mutations impacted severely on catalysis. X-ray structure analysis of the bovine enzyme has revealed that residue β-Met-209 lies only 3.1 Å from residue β-Glu-181, which has been proposed to act as catalytic base. The results reported here emphasize that, in this discrete region of the catalytic site, specific stereochemical constraints on structure are critical for catalysis.