Catalytic subunit of the probable trimeric SNF1-related protein kinase (SnRK) complex, a central regulator of cellular energy homeostasis, which, in response to seemingly unrelated darkness, sugar and stress conditions, activates energy-producing pathways and inhibits energy-consuming processes. May also be involved in the regulation of fatty acid synthesis by phosphorylation of acetyl-CoA carboxylase and in assimilation of nitrogen by phosphorylating nitrate reductase. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by phosphorylation at Thr-175. Subunit of a probable heterotrimeric complex consisting of an alpha catalytic subunit, and a beta (KINB) and a gamma (KING or SNF4) non-catalytic regulatory subunits. Expressed at very low levels. The regulatory domain (RD) contains the auto-inhibitory domain (AID) that inhibits kinase activity of the protein kinase domain (KD). The PPI motif mediates the interaction with the ABI (abscisic acid-insensitive) phosphatases. Autophosphorylated. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. SNF1 subfamily.