eIF-2 is a kinase enzyme that phosphorylates eIF-2. There are four forms in mammals: EIF2AK1: heme-regulated inhibitor kinase (HRI) EIF2AK2: the double-stranded RNA-dependent kinase (PKR) EIF2AK3: PEK/PERK EIF2AK4: GCN2 These are all responsible for the phosphorylation of the alpha subunit of eIF-2 at serine 51, one of the best-characterized mechanisms for down-regulating protein synthesis in eukaryotes in response to various cellular stress response's. References EC 2.7.11