Heat stress and light stress cooperatively damage the D1 protein in PS II

Abstract

Heat stress, as well as light stress, damages the D1 protein. When spinach thylakoids were incubated at 40ºC for 10 min in the dark, D1 cross-linked with D2 and a 23kDa D1N fragment was detected. In spinach leaf-discs, PS II activity monitored by Fv/Fm was dramatically decreased under illumination (600 µE m-2 s-1) at 40ºC, compared with 25ºC. The amount of the D1/D2 cross-linked products illuminated at 40ºC was 2-4 times larger than that at 25ºC. Obviously, the damage of the D1 protein by light stress was amplified by heat stress. Scavengers for reactive oxygen species suppressed the heat-induced cross-linking. However, these scavengers had no significant effect on the digestion of the D1 protein to the 23kDa D1N fragment. It was shown recently that phosphorylated proteins in PS II are dephosphorylated by heat (Rintamäki et al., 1996). We studied the relationship between dephosphorylation and degradation of the D1 protein under heat stress in vitro. It was observed that the D1 dephosphorylation does not affect the D1 degradation. Both the D1/D2 cross-linked products and the 23kDa D1N fragments generated by the heat stress were degraded by a stromal protease(s) at the non-appressed region of thylakoids.