Post-translational modification of chloroplast proteins and the regulation of protein turnover

Summary

Our efforts are centred around the regulatory mechanisms controlling the assembly, function and turnover of the photosystem II (PSII) reaction centre. The PSII reaction centre consists of D1, D2, the α and β subunits of cytochrome b₅₅₉ and the psbl gene product. Of these proteins, the metabolism of the D1 protein is relatively well understood. Because D1 is synthesised and rapidly degraded in the light, it is well suited for in vivo pulse–chase analysis. The D1 protein undergoes at least five post-translational modifications during its life cycle: C-terminal processing, removal of the initiating methionine residue, N-acetylation of the resulting N-terminal threonine residue, covalent palmitoylation, and O-phosphorylation of the N-terminal threonine of the mature protein. Processing of D1 occurs on stroma-exposed membranes while palmitoylation and phosphorylation occur in spatially distinct grana membranes. The palmitoylation is light-stimulated, inhibited by the herbicides atrazine and DCMU, and is apparently transient in nature. This modification might be important in the assembly of the protein in the PSII reaction centre and/or in its translocation from stroma membranes to grana. D1 phosphorylation increases with increasing light intensity, is inhibited by atrazine and DCMU and is redox regulated in vitro. In vivo, phosphorylated D1 undergoes light-dependent turnover as a function of light intensity; this process occurs under blue, green, and red illuminations and is inhibited by sodium fluoride but not by the PSII herbicides. We are investigating the role of phosphorylation–dephosphorylation in the function and dynamics of PSII.