Heat- and light-induced detachment of the light harvesting complex from isolated photosystem I supercomplexes

In a previous study, using photosystem I enriched stroma thylakoid membrane vesicles, we have shown that the light harvesting complexes of this photosystem are prone to heat- and light-induced, thermo-optically driven detachment from the supercomplex [43]. We have also shown that the splitting of the supercomplex occurs in a gradual and specific manner, selectively affecting the different constituents of the antenna complexes. Here we further analyse these heat- and light-induced processes in isolated Photosystem I supercomplex using circular dichroism and 77K fluorescence emission spectroscopy and immuno blotting, and obtain further details on the sequence of events of the dissociation process as well as on the thermal stability of the different components. Our absorption and circular dichroism spectroscopy and immuno blotting data show that the dissociation of LHCI from PSI-LHCI supercomplex starts above 50°C. Also, the low temperature fluorescence emission spectra depicts decrease of maximum fluorescence emission at 730nm and an increase of the intensity at 685nm, and about 10nm blue-shifts, from 730 to 720nm and from 685 to 676nm, respectively, indicating the heat (50°C) induced detachment of LHCI from PSI core complexes. The reaction centre proteins are highly stable even at high temperatures. Lhca2 is more heat stable than the other light harvesting protein complexes of PSI, whereas Lhca4 and Lhca3 are rather labile. Combined heat and light treatments significantly enhances the disorganization of PSI-LHCI supercomplexes, indicating a thermo-optic mechanism, which might have significant role under combined heat and light stress conditions.