Thr90 is a key residue of the bacteriorhodopsin proton pumping mechanism

Abstract

Mutation of Thr90 to Ala has a profound effect on bacteriorhodopsin properties. T90A shows about 20% of the proton pumping efficiency of wild type, once reconstituted into liposomes. Mutation of Thr90 influences greatly the Schiff base/Asp85 environment, as demonstrated by altered λ max of 555 nm and p K a of Asp85 (about 1.3 pH units higher than wild type). Hydroxylamine accessibility is increased in both dark and light and differential scanning calorimetry and visible spectrophotometry show decreased thermal stability. These results suggest that Thr90 has an important structural role in both the unphotolysed bacteriorhodopsin and in the proton pumping mechanism.