Cross-phosphorylation between Arabidopsis thaliana Sucrose Nonfermenting 1-related Protein Kinase 1 (AtSnRK1) and Its Activating Kinase (AtSnAK) Determines Their Catalytic Activities

Arabidopsis thaliana sucrose nonfermenting 1-related protein kinase 1 complexes belong to the SNF1/AMPK/SnRK1 protein kinase family that shares an ancestral function as central regulators of metabolism. In A. thaliana, the products of AtSnAK1 and AtSnAK2, orthologous to yeast genes, have been shown to autophosphorylate and to phosphorylate/activate the AtSnRK1.1 catalytic subunit on Thr(175). The phosphorylation of these kinases has been investigated by site-directed mutagenesis and tandem mass spectrometry. The autophosphorylation site of AtSnAK2 was identified as Thr(154), and it was shown to be required for AtSnAK catalytic activity. Interestingly, activated AtSnRK1 exerted a negative feedback phosphorylation on AtSnAK2 at Ser(261) (Ser(260) of AtSnAK1) that was dependent on AtSnAK autophosphorylation. The dynamics of these reciprocal phosphorylation events on the different kinases was established, and structural modeling allowed clarification of the topography of the AtSnAK phosphorylation sites. A mechanism is proposed to explain the observed changes in the enzymatic properties of each kinase triggered by these phosphorylation events.