Caspase-mediated cleavage of eukaryotic translation initiation factor subunit 2 alpha

Eukaryotic translation initiation factor 2 alpha (eIF-2 alpha), a target mo lecule of the interferon-inducible double-stranded-RNA-dependent protein ki nase (PKR), was cleaved in apoptotic Saos-2 cells on treatment with poly(I) .poly(C) or tumour necrosis factor alpha. This cleavage occurred with a tim e course similar to that of poly(ADP-ribose) polymerase, a well-known caspa se substrate. In addition, eIF-2 alpha was cleaved by recombinant active ca spase-3 in vitro. By site-directed mutagenesis, the cleavage site was mappe d to an Ala-Glu-Val-Asp(300) down arrow Gly(301) sequence located in the C- terminal portion of eIF-2 alpha. PKR phosphorylates eIF-2 alpha on Ser(51), resulting in the suppression of protein synthesis. PKR-mediated translatio nal suppression was repressed when the C-terminally cleaved product of eIF- 2 alpha was overexpressed in Saos-2 cells, even though PKR can phosphorylat e this cleaved product. These results suggest that caspase-3 or related pro tease(s) can modulate the efficiency of protein synthesis by cleaving the a subunit of eIF-2, a key component in the initiation of translation.