#200-84 PCK1 is phosphorylated by BIN2 to mediate PEPCK activity and hypocotyl elongation in Arabidopsis thaliana

Body of Abstract: Phosphoenolpyruvate Carboxykinase 1 (PCK1) catalyzes the reversible conversion of oxaloacetate (OAA) to phosphoenolpyruvate (PEP) through the gluconeogenic pathway to transform lipids and some amino acids into sugars in germinating seeds. The seedlings lacking PCK1 are compromised in the ability to use both storage lipid and storage protein by gluconeogenesis to produce soluble sugars, resulting in reduced soluble sugar levels and seedling establishment. The regulatory mechanisms of PCK1 activity are not fully explored. In this study, we found Arabidopsis PCK1 interacts with GSK3-like kinase BIN2 in vitro and in vivo. PCK1 is phosphorylated by BIN2 at two conserved residues, Ser-62 and Thr-66, leading to reduced PEPCK activity. Consistently, phosphorylated PCK1 accumulated in bin2-1 and wild-type seedlings exposed to propiconazole (PPZ) that showed significantly reduced PEPCK activity. Expressing PCK1(S62A/T66A) over-rescues the hypocotyl elongation and PEPCK activity of pck1 mutant grown in the dark, while expressing the PCK1(S62D/T66E) fails to fully rescue the hypocotyl elongation and activity of pck1 mutant. Recombinant PCK1 protein forms hexamer, while over one third of the recombinant PCK1(S62D/T66E) forms dodecamer which shows little activity. Our results suggest that BR signaling regulates PEPCK activity and hypocotyl elongation through BIN2-mediated phosphorylation of PCK1.