LHCSR

LHCSR stands for Light-Harvesting Complex Stress-Related proteins, a family of chlorophyll-binding proteins found in the thylakoid membranes of photosynthetic organisms, notably green algae such as Chlamydomonas reinhardtii. LHCSR proteins are part of the extended light-harvesting complex (LHC) superfamily and are distinct from the light-harvesting complexes of higher plants as well as from PSBS, another component of non-photochemical quenching.

Functionally, LHCSR proteins participate in non-photochemical quenching (NPQ), a protective mechanism that dissipates excess absorbed light energy as heat to prevent photodamage to photosystem II under high light or rapidly changing light conditions. They are believed to respond to the thylakoid lumen pH and pigment–protein interactions to switch between energy harvesting and dissipative states. In many algae, LHCSR activity provides rapid, thylakoid-mpH–dependent NPQ that complements or substitutes for PSBS-based protection found in land plants.

Genetically, organisms such as Chlamydomonas reinhardtii express LHCSR genes (for example LHCSR1 and LHCSR3). LHCSR3 is often a major contributor to fast NPQ, while LHCSR1 can participate under different light regimes or stress conditions. Regulation of these genes is linked to light intensity and other environmental stresses, enabling dynamic photoprotection.

Structure and localization, LHCSR proteins are membrane-associated, binding pigments such as chlorophylls and carotenoids, and localizing to PSII-containing regions of the thylakoid membrane. They share sequence and structural features with other LHC proteins but have adaptations suited to their photoprotective function.

Distribution and evolution: LHCSR is widely found in green algae and some bryophytes, whereas higher plants rely more on PSBS for NPQ. Ongoing research investigates the precise mechanisms and potential applications for improving photosynthetic efficiency in algae.