Translation Inhibition in Apoptosis

Abstract

The protein kinase PKR is a major player in the cel- lular antiviral response, acting mainly by phosphoryla- tion of the -subunit of the eukaryotic translation initi- ation factor 2 (eIF2-) to block de novo protein synthesis. PKR activation requires binding of double-stranded RNA or PACT/RAX proteins to its regulatory domain. Since several reports have demonstrated that transla- tion is inhibited in apoptosis, we investigated whether PKR and eIF2- phosphorylation contribute to this process. We show that PKR is proteolysed and that eIF2-is phosphorylated at the early stages of apoptosis induced by various stimuli. Both events coincide with the onset of caspase activity and are prevented by caspase inhibitors. Using site-directed mutagenesis we show that PKR is specifically proteolysed at Asp251 dur- ing cellular apoptosis. This site is cleaved in vitro by recombinant caspase-3, caspase-7, and caspase-8 and not by the proinflammatory caspase-1 and caspase-11. The released kinase domain efficiently phosphorylates eIF2-at the cognate Ser51 residue, and its overexpres- sion in mammalian cells impairs the translation of its own mRNA and of reporter mRNAs. Our results demon- strate a new and caspase-dependent activation mode for PKR, leading to eIF2-phosphorylation and translation inhibition in apoptosis.