Activates SnRK1.1/KIN10 and SnRK1.2/KIN11 by phosphorylation of their activation-loop 'Thr-198' and 'Thr-176', respectively. Required for the regulation by SnRK1 kinases of the transcription of a large set of genes, the modification the activity of metabolic enzymes, and the control of various nutrient-responsive cellular developmental processes. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated when autophosphorylated at Thr-154 and inactivated when phosphorylated at Ser-261 by SnRK1.1/KIN10. Associates with the SNF1-related protein kinase (SnRK) complex (By similarity). Interacts with AL1, a geminivirus (TGMV) protein essential for viral replication. Restricted to the nucleus in cells infected by geminivirus. A number of isoforms are produced. According to EST sequences. Expressed in shoot apical meristem, leaf primordium and emerging petiole (at protein level). Highly expressed in young leaf and floral tissues but not expressed in mature tissues (at protein level). By geminivirus (TGMV, CaLCuV or BCTV) infection (at the protein level). Functionally able to complement the yeast elm1 sak1 tos3 triple mutant. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.