LHCSR1 induces a fast and reversible pH-dependent fluorescence quenching in LHCII in Chlamydomonas reinhardtii cells

Emine Dinca*, Lijin Tiana*, Laura M. Roya, Robyn Rothb, Ursula Goodenoughc, Roberta Crocea
aBiophysics of Photosynthesis, Department of Physics and Astronomy, Faculty of Sciences, VU University Amsterdam and LaserLab Amsterdam. De Boelelaan, 1081, 1081 HV, Amsterdam, The Netherlands; bDepartment of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri, USA; cDepartment of Biology, Washington University, St. Louis, Missouri, USA

To avoid photodamage, photosynthetic organisms are able to thermally dissipate the energy absorbed in excess in a process known as non-photochemical quenching (NPQ). Although NPQ has been studied extensively, the major players and the mechanism of quenching remain debated. This is due to the difficulty in extracting molecular information from in vivo experiments and the absence of a validation system for in vitro experiments. Here, we have created a minimal cell of the green alga Chlamydomonas reinhardtii able to undergo NPQ. We show that LHCII, the main-light harvesting complex of algae, cannot switch to a quenched conformation in response to pH changes by itself. Instead, a small amount of the protein LHCSR1 is able to induce a large, fast and reversible pH-dependent quenching in an LHCII-containing membrane. These results strongly suggest that LHCSR1 acts as pH sensor and it modulates the excited state lifetimes of a large array of LHCII, also explaining the NPQ observed in the LHCSR3-less mutant. The possible quenching mechanisms are discussed.