A SINGLE-POINT MUTATION (E166Q) PREVENTS DICYCLOHEXYLCARBODIIMIDE BINDING TO THE PHOTOSYSTEM-II SUBUNIT CP29

Energy-dependent quenching of chlorophyll fluorescence (qE) reflects t he action of a powerful mechanism of protection from photoinhibition i n which the low pH in the chloroplast lumen induces dissipation of exc ess excitation energy. Dicyclohexylcarbodiimide (DCCD), a protein-modi fying agent, is a powerful inhibitor of qE and has been shown to react with acidic residues, in a hydrophobic environment, involved in proto n translocation. The CP29 subunit of photosystem II has been proposed to be the site of qE quenching and shown to bind DCCD. We have hypothe sised, on the basis of the CP29 protein sequence and of the structure of light-harvesting complex II protein, that glutamic acid 166 is the DCCD binding site. In this study, we have produced recombinant protein s either with wild-type sequence or carrying a mutation on the 166 pos ition. We show that the mutant protein does not bind DCCD. This identi fies E166 as the site whose protonation may lead to a conformational c hange triggering qE.