Chloroplast Thylakoid Protein Phosphatase: This project will continue the characterization of two protein phosphatases associated with chloroplast thylakoid membranes isolated from wheat (Triticum aestivum). Characterization will include kinetic analysis of the activities, studies on substrate specificity using endogenous and synthetic peptides, and biochemical analysis of tertiary and quaternary structures. Antibodies recognizing the phosphatases will be prepared and used in studies on the location of the phosphatase enzymes within the photosynthetic membrane network. If antibodies inhibit the enzymatic activity, they will be utilized to substantiate that phosphatase activity is involved in the reversal of State 1/State 2 transition, which regulates photosynthetic energy production. Antibodies will also be used to study the amphiphilic properties of the enzyme, i.e., the potential to partition between the thylakoid surface and stromal phase, and the potential of this process to regulate protein dephosphorylation, thereby complementing known mechanisms of modulating thylakoid protein kinase activity. Chloroplast thylakoid membrane protein phosphorylation plays an important role in the regulation of steady-state energy production by photosynthesis. The key elements are a protein kinase whose activity is stimulated when the plastoquinone pool is reduced, and the light-harvesting complex, thereby differentially partitions between the two photosystems. This system is thought to govern the photon flux density between the two photosystems. The protein kinases present in the thylakoid membrane have been characterized, but the protein phosphatases, which reverse the light-harvesting complex phosphorylation, are little understood. The results are essential to a complete understanding of how photosynthetic organisms are able to optimize their efficiency. Such information may also be useful to engineering or breeding desirable molecular response traits into commercially useful plants.
