A CONSERVED CARBOXYLIC-ACID GROUP MEDIATES LIGHT-DEPENDENT PROTON UPTAKE AND SIGNALING BY RHODOPSIN

A carboxylic acid residue is conserved at the cytoplasmic border of th e third transmembrane segment among nearly all G protein-coupled recep tors. In the visual receptor rhodopsin, replacement of the conserved G lu(134) by a neutral glutamine results in enhanced transducin activati on. Here we show that a key event in forming the active state of rhodo psin is proton uptake by Glu(134) in the metarhodopsin II (MII) photop roduct. Site-directed mutants E134D and E134Q were studied by flash ph otolysis, where formation rates of their photoproducts and rates of pH change could be monitored simultaneously. Both mutants showed normal MII formation rates. However, E134D displayed a slowed rate of proton uptake and E134Q displayed a loss of light-induced uptake of two proto ns from the aqueous phase. Thus, Glu(134) mediates light-dependent pro ton uptake by MII. We propose that receptor activation requires a ligh t-induced conformational change that allows protonation of Glu(134) an d subsequent protonation of a second group. The strong conservation of Glu(134) in G protein-coupled receptors implies a general requirement for a proton acceptor group at this position to allow light- or ligan d-dependent receptor activation.