PsbS

PsbS is a small thylakoid membrane protein encoded by the nuclear genome in higher plants and green algae. It is imported into chloroplasts and integrated into the thylakoid membrane, typically as a four-transmembrane helix protein that forms dimers.

PsbS is a key component of non-photochemical quenching (NPQ), particularly the rapidly reversible energy-dependent component qE. It acts as a pH sensor in the thylakoid lumen: under high light, lumen acidifies, triggering protonation of specific lumen-exposed acidic residues in PsbS, which induces conformational changes. These changes promote interactions with light-harvesting complexes, facilitating energy dissipation as heat and protecting photosystem II from excess excitation.

PsbS does not bind pigments itself, but modulates excitation energy distribution by interacting with LHCII and related antenna proteins. The exact mechanism remains under study, but the protonation of key residues and dimerization are essential for activity.

Genetic and physiological data show that loss of PsbS severely reduces NPQ capacity and lowers photoprotection under fluctuating light, whereas overexpression can enhance NPQ. PsbS is widespread in higher plants and green algae but is not universal in cyanobacteria, which rely on different NPQ pathways.