POLYANION REGULATION OF THE PLANT-ENCODED DOUBLE-STRANDED RNA-DEPENDENT PROTEIN-KINASE (PPKR)

The only known activators of the plant encoded, double-stranded RNA de pendent protein kinase (pPKR) are dsRNA's and single-stranded RNA with extensive intramolecular base pairing such as viroid RNAs. DNA, DNA-R NA hybrids and most single stranded RNA's do not stimulate phosphoryla tion. 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 pre sence of a polyanion binding domain. The functional significance of po lyanion regulation of pPKR may be suggested by the inhibition of in vi tro translation of brome mosaic virus RNA in wheat germ extracts in th e presence of heparin. These studies further establish the analogy bet ween pPKR and the mammalian interferon-induced, dsRNA dependent protei n kinase, PKR, and present possibilities for the activation of pPKR in healthy cells.