Inhibition of mRNA binding to ribosomes by localized activation of dsRNA-dependent protein kinase

Abstract

The initiation of protein synthesis can be regulated in mammalian cells by protein kinases which phosphorylate the α subunit of initiation factor eIF-2 (ref. 1). This phosphorylation results in a block in the recycling of eIF-2 (refs 2, 3) and in the inhibition of messenger RNA binding to 80S initiation complexes^4,5. After eIF-2α is phosphorylated, the mRNA becomes associated with 48S complexes consisting of a 40S ribosomal subunit⁴, eIF-2(αP), GDP and Met-tRNA_f (ref. 5). One of the eIF-2α kinases is activated by low concentrations of double-stranded RNA (dsRNA)⁶. This kinase (PK_ds) is present at a basal level in all mammalian cells investigated⁷ and its synthesis is induced in cells treated with Interferon⁸. The PK_ds may be involved in the inhibition of translation of viral mRNA in interferon-treated cells infected with RNA viruses, as it is activated by viral replicative complexes. It is not known, however, if the activated PK_ds preferentially inhibits the translation of viral mRNA when cellular protein synthesis proceeds at a normal rate in infected cells⁹. We now report that mRNA covalently linked to dsRNA is preferentially inhibited from binding to 80S complexes by a localized activation of PK_ds. This suggests that in interferon-treated cells the binding of some nascent viral mRNAs to functional initiation complexes may be preferentially inhibited by a similar mechanism.