Metabolic-stress sensing protein kinase that phosphorylates the alpha subunit of eukaryotic translation initiation factor 2 eIF-2-alpha in response to low amino acid availability. Plays a role as an activator of the general amino acid control pathway required for adapatation to amino acid starvation. Converts phosphorylated eIF-2-alpha either to a competitive inhibitor of translation initiation, leading to a global protein synthesis repression, and thus to a reduced overall utilization of amino acids, or to a translational initiation activation of specific mRNAs, and hence allowing reprogramming of amino acid biosynthetic gene expression to alleviate nutrient depletion. Binds uncharged tRNAs. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] The kinase activity is stimulated upon binding to uncharged tRNAs. Homodimer; homodimerization is important for kinase activation by uncharged tRNAs. A number of isoforms are produced. According to EST sequences. Expressed in roots, leaves, stems, buds, flowers, siliques and seedlings. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. GCN2 subfamily. Although it is unknown whether it is a serine/threonine or a tyrosine protein kinase, it is strongly related to the serine/threonine-protein kinase family.