Induces translational repression through 28S ribosomal RNA cleavage in response to ER stress. Pro-apoptotic. Appears to play no role in the unfolded-protein response, unlike closely related proteins. 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. Extended N-terminus.