Energy-Dependent Non-Photochemical Quenching: PsbS, LhcSR, and Other Players

The photosynthetic apparatus of plants is capable of capturing even weak fluxes of light energy. Hence, strong and rapid increase in irradiance should be dangerous for plants. To solve the problems caused by fluctuations of incident radiation (up to excessive), plants have developed a number of protective mechanisms, including non-photochemical quenching (NPQ) of excited chlorophyll states. NPQ is a set of mechanisms that shorten the lifetime of excited chlorophyll states in the photosynthetic antenna, thereby reducing dangerous effects of light. The most rapid mechanism of NPQ is energy-dependent quenching (qE) triggered by the proton potential formation on the thylakoid transmembrane. The main molecular players of qE are xanthophylls (oxygen-containing carotenoids) and proteins of the thylakoid membrane: antenna component LhcSR in algae and mosses and photosystem II component PsbS in higher plants and some groups of "green lineage" alga. This review discusses molecular mechanisms of qE, with a special focus on the PsbS-dependent quenching. The discovery that PsbS does not bind pigments has led to the hypothesis of PsbS-dependent indirect activation of quenching, in which PsbS acts as a relay switching on the quenching sites in the major (LHCII) and/or minor photosynthetic antennae. The suggested mechanisms include the effect of PsbS on carotenoid conformation and/or pK_a values of amino acid residues in PSII antennae. PsbS can also act as a membrane "lubricant" that ensures migration of the major antenna LHCII in the thylakoid membrane and its aggregation followed by transition to the quenched state.