Interaction between the photoprotective protein LHCSR3 and C 2 S 2 Photosystem II supercomplex in Chlamydomonas reinhardtii.

Photosynthetic organisms can thermally dissipate excess of absorbed energy in high-light conditions in a process known as non-photochemical quenching (NPQ). In the green alga Chlamydomonas reinhardtii this process depends on the presence of the light-harvesting protein LHCSR3, which is only expressed in high light. LHCSR3 has been shown to act as a quencher when associated with the Photosystem II supercomplex and to respond to pH changes, but the mechanism of quenching has not been elucidated yet. In this work we have studied the interaction between LHCSR3 and Photosystem II C2 S2 supercomplexes by single particle electron microscopy. It was found that LHCSR3 predominantly binds at three different positions and that the CP26 subunit and the LHCII trimer of C2 S2 supercomplexes are involved in binding, while we could not find evidences for a direct association of LHCSR3 with the PSII core. At all three locations LHCSR3 is present almost exclusively as a dimer.