CHARACTERIZATION OF AN INTERFERON-INDUCED 48-KD PROTEIN IMMUNOLOGICALLY RELATED TO THE DOUBLE-STRANDED RNA-ACTIVATED PROTEIN-KINASE PKR

Polyclonal antibodies raised against purified and urea-denatured doubl e-stranded protein kinase (PKR) from human origin cross-reacted by imm unoblotting with a 48-kD protein (p48) induced by the three types of i nterferon (IFN), alpha, beta, and gamma. The induction of p48 is IFN d ose dependent and its accumulation occurs a few hours after the additi on of IFN. The induction of p48 is blocked by actinomycin D. Analysis by two-dimensional gel isoelectric-focusing, revealed p48 as a single spot with an isoelectric point (pI) of 6.8. In the same experiment the PKR was revealed as several subspecies with pI values in the pH range of 7.4-8.0. Cell fractionation experiments indicated that PKR and p48 have different subcellular localizations: PKR was found to be associa ted with the microsomal pellet as shown previously whereas p48 was rec overed in the microsomal supernatant fraction. In addition to these di fferences, PKR and p48 were found to be differentially expressed in so me human cells treated with the three types of IFN. For example, in He La cells, IFN-alpha or IFN-beta induced similarly both PKR and p48 whe reas IFN-gamma induced mainly p48. In U937 cells in which PKR was not expressed with or without IFN treatment, p48 was strongly induced by a ll three types of IFN. These results suggest different mechanisms for the induction of PKR and p48. In view of its presence in different typ es of human cells and its induction by different types of IFN, it is p ossible to suggest that p48 might play an important role in mediating some of the action of IFN.