Characterization of a mutant pancreatic eIF-2 alpha kinase, PEK, and co-localization with somatostatin in islet delta cells

Phosphorylation of eukaryotic translation initiation factor-2 alpha (eIF-2 alpha) is one of the key steps where protein synthesis is regulated in resp onse to changes in environmental conditions. The phosphorylation is carried out in part by three distinct eIF-2 alpha kinases including mammalian doub le-stranded RNA-dependent eIF-2 alpha: kinase (PKR) and heme-regulated inhi bitor kinase (HRI), and yeast GCN2. We report the identification and charac terization of a related kinase, PEK, which shares common features with othe r eIF-2 alpha kinases including phosphorylation of eIF-2 alpha in vitro, We show that human PEK is regulated by different mechanisms than PKR or HRI, In contrast to PKR or HRI, which are dependent on autophosphorylation for t heir kinase activity, a point mutation that replaced the conserved Lys-614 with an alanine completely abolished the eIF-2 alpha: kinase activity, wher eas the mutant PEK was still autophosphorylated when expressed in Sf-9 cell s. Northern blot analysis indicates that PEK mRNA was predominantly express ed in pancreas, though low expression was also present in several tissues. Consistent with the high levels of mRNA in pancreas, the PEK protein was on ly detected in human pancreatic islets, and the kinase co-localized with so matostatin, a pancreatic delta cell-specific hormone. Thus PEK is believed to play an important role in regulating protein synthesis in the pancreatic islet, especially in islet delta cells.