Double-stranded RNA-dependent protein kinase (PKR) is regulated by reovirus structural proteins

Reovirus sigma 3 is a virion outer shell protein that also binds dsRNA and stimulates translation by blocking activation of the dsRNA-dependent protein kinase, PKR Purified sigma 3 was shown by gel shift assay to bind specifically to RNA duplexes of minimal length 32-45 base pairs. PKR binding to dsRNA was prevented by sigma 3, and translation inhibition of luciferase reporter by PKR expression in transfected cells was reversed by sigma 3. Association of sigma 3 with its outer capsid partner mu 1/mu 1C eliminated dsRNA binding and prevented restoration of protein synthesis, Analyses of sigma 3 mutants demonstrated a direct correlation between dsRNA binding and reversal of the down-regulation of translation by PKR. In infected cells, sigma 3 was stable but dsRNA binding decreased, presumably due to mu 1/mu 1C complex formation. The results suggest a functional transition from early inhibition of PKR activation by sigma 3 to its association with mu 1/mu 1C in capsid structures. (C) 1997 Academic Press.