Polyanion regulation of the plant-encoded double-stranded RNA- dependent protein kinase (pPKR)

The only known activators of the plant encoded, double- stranded RNA dependent protein kinase (pPKR) are dsRNA's and single-stranded RNA with extensive intramolecular base pairing such as viroid RNAs. DNA, DNA-RNA hybrids and most single stranded RNA's do not stimulate phosphorylation. However, the in vivo phosphorylation of pPKR in the cytoplasm of healthy cells suggests that alternate activators may be present. Here we show that select polyanions including heparin and dextran sulfate, stimulate pPKR phosphorylation in a concentration dependent fashion. Further, pPKR specifically binds to heparin-agarose indicating the presence of a polyanion binding domain. The functional significance of polyanion regulation of pPKR may be suggested by the inhibition of in vitro translation of brome mosaic virus RNA in wheat germ extracts in the presence of heparin. These studies further establish the analogy between pPKR and the mammalian interferon- induced, dsRNA dependent protein kinase, PKR, and present possibilities for the activation of pPKR in healthy cells.