Regulation of Plant Metabolism by Protein Phosphorylation. Possible Regulation of Sucrose-Phosphate Synthase by 14-3-3 Proteins

Abstract

Reversible protein phosphorylation is known to be an important regulatory mechanism for several plant enzymes involved in primary carbon and nitrogen metabolism (1,2). Our laboratory has been studying two enzymes of sucrose metabolism, sucrose-phosphate synthase (SPS) and sucrose synthase (SuSy), and an enzyme of N-metabolism, NADH:nitrate reductase (NR), which are known to be phosphorylated in vivo. Interestingly, the effect of phosphorylation is somewhat different for the three enzymes. With SuSy, phosphorylation of the single, major phosphorylation site (Ser-15; ref. 3) has little effect on activity but appears to be one of the factors that controls its association with the plasma membrane (4) and actin cytoskeleton (5). Thus, phosphorylation affects intracellular localization of the enzyme. With NR, phosphorylation of Ser-543 has no direct effect on activity but completes the motif required for binding of a 14-3-3 inhibitor protein (6,7). Thus, phosphorylation affects enzyme activity indirectly by affecting protein:protein interactions. With SPS, phosphorylation appears to directly affect enzyme activity, at least in some cases. Current work suggests that SPS may have three regulatory phosphorylation sites: i) Ser-158, which is largely (or solely) responsible for light/dark modulation (8); ii) Ser-424, which is thought to be responsible for osmotic stress activation of SPS (9), and iii) Ser-229, which may constitute a binding site for a 14-3-3 inhibitor protein (10).