Heat- and light-induced detachment of the light-harvesting antenna complexes of photosystem I in isolated stroma thylakoid membranes

The multisubunit pigment–protein complex of photosystem I (PSI) consists of a core and peripheral light-harvesting antenna (LHCI). PSI is thought to be a rather rigid system and very little is known about its structural and functional flexibility. Recent data, however, suggest LHCI detachment from the PSI supercomplex upon heat and light treatments. Furthermore, it was suggested that the splitting off of LHCI acts as a safety valve for PSI core upon photoinhibition (Alboresi et al., 2009). s work we analyzed the heat- and light-induced reorganizations in isolated PSI vesicles (stroma membrane vesicles enriched in PSI). Using differential scanning calorimetry we revealed a stepwise disassembly of PSI supercomplex above 50 °C. Circular dichroism, sucrose gradient centrifugation and 77 K fluorescence experiments identified the sequence of events of PSI destabilization: 3 min heating at 60 °C or 40 min white light illumination at 25 °C resulted in pronounced Lhca1/4 detachment from the PSI supercomplex, which is then followed by the degradation of Lhca2/3. milarity of the main structural effects due to heat and light treatments supports the notion that thermo-optic mechanism, structural changes induced by ultrafast local thermal transients, which has earlier been shown to be responsible for structural changes in the antenna system of photosystem II, can also regulate the assembly and functioning of PSI antenna.