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FEBS Lett. 1997 Jun 30;410(2-3):126-30.

Seryl-phosphorylation of soybean nodule sucrose synthase (nodulin-100) by a Ca2+-dependent protein kinase.

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Department of Biochemistry, University of Nebraska-Lincoln, George W. Beadle Center, 68588-0664, USA.


Sucrose synthase (SS; EC was radiolabeled in situ by incubating detached soybean nodules with 32Pi. 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 Ca2+-dependent. This SS-kinase was partially purified (approximately 2200-fold) from nodules harvested from illuminated plants. The molecular mass of the SS-kinase was about 55,000 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 in the presence of Ca2+. 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.

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