Activity of membrane-associated sucrose synthase is regulated by its phosphorylation status in cultured cells of sycamore (Acer pseudoplatanus)

Changes in sucrose synthase (SuSy) activity, protein level and degree of phosphorylation were investigated in plasmalemma and tonoplast of sycamore cells cultured either in the presence of sucrose or after 24 h of starvation. SuSy activity was shown to be higher in the plasmalemma than in the tonoplast of cells cultured in the presence of sucrose. In clear contrast, SuSy was shown to be more active in the tonoplast than in the plasmalemma of starved cells. Western blot analyses on both membrane types did not show noticeable differences in SuSy protein levels under the two different regimes. However, phosphorylation state at the serine moieties of the enzyme was shown to be different in the presence or in the absence of sucrose. Plasmalemma-associated SuSy is not phosphorylated in the presence of sucrose, whereas tonoplast-associated SuSy is phosphorylated under similar conditions. Starvation brought about a reverse in phosphorylation state of membrane-bound SuSy. Whereas plasmalemma-associated SuSy became phosphorylated, tonoplast-associated SuSy was completely de-phosphorylated. Together, the data demonstrate that SuSy is simultaneously present in various cell membranes and also demonstrate a lack of direct relationship between membrane type location, and degree of phosphorylation, but substantiate the relevance of phosphorylation to enzymatic activity.