FIG. 5.
PKR is modulated by a number of cellular proteins. The PKR activator PACT, the melanoma differentiation factor MDA7, and the transcription factor E2F-1 are the only known proteins to induce PKR activation, although PACT is the best-characterized PKR activator. However, numerous cellular inhibitors of PKR have been described: p58IPK (the first cellular inhibitor reported) and the RNA-binding protein TRBP are both associated with influenza virus and HIV-1 infection. Other inhibitors of PKR action are the heat shock Hsp90/Hsp70 proteins, NPM, and the glycoprotein p67.