Thr90 is a key residue of the bacteriorhodopsin proton pumping mechanism

Mutation of Thr90 to Ala has a profound effect on bacteriorhodopsin propert ies. T90A shows about 20% of the proton pumping efficiency of wild type, on ce reconstituted into liposomes. Mutation of Thr90 influences greatly the S chiff base/ Asp85 environment, as demonstrated by altered lambda (max) of 5 55 nm and pK(a) of Asp85 (about 1.3 pH units higher than wild type). Hydrox ylamine accessibility is increased in both dark and light and differential scanning calorimetry and visible spectrophotometry show decreased thermal s tability. These results suggest that Thr90 has an important structural role in both the unphotolysed bacteriorhodopsin and in the proton pumping mecha nism. (C) 2001 Published by Elsevier Science B.V. on behalf of the Federati on of European Biochemical Societies.