Identification of proton-active residues in a higher plant light-harvesting complex

The thermal dissipation of absorbed light energy by the light-harvesting apparatus of higher plants is important in protecting the photosynthetic machinery from the effects of excess illumination. A major mechanism for such photoprotection, known as trans-thylakoid ΔpH-dependent chlorophyll fluorescence quenching (qE), is induced by acidification of the lumen, is correlated with the interconversion of xanthophyll pigments, and is manifested as quenching of chlorophyll fluorescence. The mechanistic basis for qE remains unknown. The reagent N,N′-dicyclohexylcarbodiimide (DCCD) specifically inhibits qE and covalently binds to two minor light-harvesting pigment–protein complexes (LHCII), LHCIIa and LHCIIc. It is shown that DCCD treatment of isolated LHCIIc complexes reverses acid-induced chlorophyll fluorescence quenching in an in vitro system. Fingerprinting of [(14)C]DCCD-labeled LHCIIc demonstrates that there are two DCCD-sensitive amino acid residues on this complex, and these are shown to be glutamate residues, each of which is located near the lumen. In view of the effects of DCCD on the pattern of proton release from photosystem II during photosynthesis, we propose a model for the mechanism of the induction of qE—that these residues form part of a proton pathway, the lumen pH being sensed via its effects on the rate of proton release. One possibility is that the resulting changes in the protonation state of these carboxyl side chains may modulate the structural and energetic organization of LHCII.