On the Lateral Distribution of Thylakoid Phosphoproteins

Abstract

Several protein component of the thylakoid membrane can be phosphorylated by a membrane bound kinase (c.f. Bennett, 1983). The influence of this phosphorylation on the thylakoid function is not known, except in the case of the light-harvesting complex. Upon phosphorylation of LHCP its energy supply to PS2 decreases while the supply to PS1 increases. How can phosphorylation bring about such a redistribution of excitation energy from PS2 to PS1? Any explanation has to take into account the spatial separation of the two photosystems. PS1 is mainly excluded from the appressed thylakoid regions and concentrated in the non-appressed ones, while most of the PS2 complex and LHCP are concentrated in the appressed regions (Andersson, Anderson, 1980). It has been suggested that phosphorylation would lead to a lateral migration of LHCP from the appressed thylakoids to the non-appressed ones rich in PS1 (Barber, 1982, Haworth et al, 1982). In this report, we have subfractionated phosphorylated thylakoids into appressed and non-appressed thylakoids in order to obtain information about the lateral distribution of thylakoid phosphoproteins. The result showed that the incorporation of phosphate into LHCP was much more pronounced in the non-appressed thylakoid fraction than in the appressed ones. Moreover, there was an increased proportion of LHCP in stroma lamellae of phosphorylated thylakoid compared to unphosphorylated ones. These observations are in support of a lateral migration of LHCP from a PS2 to a PS1 region upon phosphorylation. In contrast, phosphoproteins at 9, 34 and 45 kDa were markedly enriched in the appressed thylakoid fraction, showing a close association with PS2 even under phosphorylated conditions.