Sucrose synthase (SS; EC 18.104.22.168) was radiolabeled in situ by incubating detached soybean nodules with 32 Pi. Phosphoamino acid analysis indicated that SS was phosphorylated on a serine residue(s). In-vitro phosphorylation of purified nodule SS by desalted nodule extracts was Ca 2+ - dependent. This SS-kinase was partially purified (~2200-fold) from nodules harvested from illuminated plants. The molecular mass of the SS-kinase was about 55000 on a Superdex 75 size-exclusion column or in a denaturing autophosphorylation gel. With either purified nodule SS or Syntide 2 as substrate, exogenous calmodulin and phosphatidylserine showed little or no effect on the in-vitro activity of this partially purified protein kinase. However, its activity was inhibited by W-7. The purified nodule SS-kinase (or CDPK) phosphorylated nodule PEP carboxylase (PEPC; EC 22.214.171.124) in the presence of Ca 2+ . In contrast, a partially purified nodule PEPC-kinase preparation was incapable of phosphorylating nodule SS. Unlike nodule PEPC [Zhang et al. (1995) Plant Physiol. 108, 1561-1568], the phosphorylation state of SS is not likely modulated in planta by photosynthate supply from the shoots.
Zhang, X. Q., & Chollet, R. (1997). Seryl-phosphorylation of soybean nodule sucrose synthase (nodulin-100) by a Ca 2+ -dependent protein kinase. FEBS Letters, 410(2–3), 126–130. https://doi.org/10.1016/S0014-5793(97)00537-1