Role in expression of the DDIT3 transcription factor, required for the unfolded-protein response, growth arrest and apoptosis. Has no effect on 28S ribosomal RNA cleavage, unlike the corresponding human protein. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] The kinase domain is activated by trans-autophosphorylation. Kinase activity is required for activation of the endoribonuclease domain (By similarity). Autophosphorylated. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.