Single-molecule spectroscopy of LHCSR1 protein dynamics identifies two distinct states responsible for multi-timescale photosynthetic photoprotection (original) (raw)

In oxygenic photosynthesis, light harvesting is regulated to safely dissipate excess energy and prevent the formation of harmful photoproducts. Regulation is known to be necessary for fitness, but the molecular mechanisms are not understood. One challenge has been that ensemble experiments average over active and dissipative behaviours, preventing identification of distinct states. Here, we use single-molecule spectroscopy to uncover the photoprotective states and dynamics of the light-harvesting complex stress-related 1 (LHCSR1) protein, which is responsible for dissipation in green algae and moss. We discover the existence of two dissipative states. We find that one of these states is activated by pH and the other by carotenoid composition, and that distinct protein dynamics regulate these states. Together, these two states enable the organism to respond to two types of intermittency in solar intensity-step changes (clouds and shadows) and ramp changes (sunrise), respectively. Our findings reveal key control mechanisms underlying photoprotective dissipation, with implications for increasing biomass yields and developing robust solar energy devices. 1 P hotosynthetic light-harvesting complexes (LHCs) capture solar 2 energy and feed it to downstream molecular machinery 1. When 3 light absorption exceeds the capacity for utilization, the excess 4 energy can generate singlet oxygen, which causes cellular damage. 5 Thus, in oxygenic photosynthesis, LHCs have evolved a feedback 6 loop that triggers photoprotective energy dissipation 2-4. The 7 crucial importance of photoprotection for fitness has been demon-8 strated, as well as its impact on biomass yields 5. Recent efforts to 9 rewire photoprotection have demonstrated an impressive 20% 10 increase in biomass 6. However, the mechanisms of photoprotec-11 tion-from the fast photophysics of the pigments to the slow con-12 formational changes of proteins-have not yet been resolved. The 13 lack of mechanistic understanding is a major limitation in the 14 speed and efficacy of improving biomass yields. 15 Collectively, the photoprotective mechanisms are known as non-16 photochemical quenching (NPQ). NPQ involves changes to the 17 photophysics, conformation and organization of LHCs within 18 the membrane 2-4. The seconds to minutes component of NPQ is 19 the dissipation of excess sunlight within the LHCs. The LHCs 20 consist of pigments (chlorophyll and carotenoids) closely packed 21 within a protein matrix. The carotenoid composition is controlled 22 by light conditions via the xanthophyll cycle, in which violaxanthin 23 (Vio) is converted to zeaxanthin (Zea) under high light conditions. 24 Most LHCs are primarily responsible for light harvesting, but in 25 recent research, one of the LHCs, light-harvesting complex stress-26 related (LHCSR) protein, was identified as the key gene product 27 for the dissipation of excess sunlight in unicellular algae and 28 mosses 7-14. LHCSR consists of chlorophyll-a and carotenoids held 29 within a protein matrix 8,12,15. Activation of dissipation in LHCSR 30 occurs based on three functional parameters: (1) low pH 8,16-18 ,