Submitted to: Archives Of Biochemistry and Biophysics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/1995
Publication Date: N/A
Citation: Interpretive Summary: Sucrose phosphate synthase (SPS) is one of the key enzymes involved in regulation of photosynthesis in plants. As such it is of great importance to understand how this regulation takes place. By varying the structure of model fragments of the spinach enzyme regulatory site, it was possible to determine the way in which another enzyme, which adds phosphate to this site to reduce the enzyme activity, binds to SPS during this process. Comparison of the regulatory site structure of the spinach enzyme with those reported for corn and potato suggests that the same mechanism applies to these other plants as well. This information will benefit other scientists studying the mechanisms of photosynthesis and plant sugar metabolism.
Technical Abstract: Sucrose-phosphate synthase (SPS; EC 18.104.22.168) is regulated by reversible protein phosphorylation. When the enzyme is phosphorylated it is inactivated and can be reactivated by removal of phosphate. The major regulatory phosphorylation site is known to be Ser158 in the spinach-leaf enzyme, and two protein kinase activities have been resolved chromatographically which phosphorylate SPS at this site in vitro. In this report, we use a set of synthetic peptide substrate analogs based on the phosphorylation site sequence, and a set of Esherichia coli expressed 26-kDa fragments of spinach SPS which contain the site, to identify the recognition elements that target the two protein kinases to Ser158. The major recognition element consists of basic residues at P-3 and P-6 relative to the phosphorylated serine. Comparison of the spinach enzyme amino-acid sequence with two other plant species show conservation of these amino acids and implies that these signals are also conserved. We also present evidence that glucose-6-phosphate is not only an allosteric activator of SPS but also an inhibitor of SPS-protein kinase per se, thereby allowing it to act at both levels of SPS regulation.