Cytosolic glutamine synthetase and not nitrate reductase from the green alga Chlamydomonas reinhardtii is phosphorylated and binds 14-3-3 proteins

The nitrate reductase activity from Chlamydomonas reinhardtii was not alter ed when extracts were incubated with yeast 14-3-3 proteins in the presence of Mg-ATP. However, the C. reinhardtii extracts contained 14-3-3 proteins c apable of inhibiting the spinach nitrate reductase, raising the question of their physiological substrates. Two C. reinhardtii proteins of about 48 an d 35 kDa were eluted from 14-3-3 affinity chromatography columns and bound to 14-3-3s in overlay assays. The 48-kDa protein corresponded to the cytoso lic isoform of glutamine synthetase (GSI). The C;SI was phosphorylated by a Ca2+ and calmodulin-dependent protein kinase partially purified from the a lga. How-ever, neither phosphorylation nor 14-3-3 binding seemed to change GS catalytic activity.