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 double-stranded protein kinase (PKR) from human origin cross-reacted by immunoblotting with a 48-kD protein (p48) induced by the three types of interferon (IFN), alpha, beta, and gamma. The induction of p48 is IFN dose dependent and its accumulation occurs a few hours after the addition 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 associated with the microsomal pellet as shown previously whereas p48 was recovered in the microsomal supernatant fraction. In addition to these differences, PKR and p48 were found to be differentially expressed in some human cells treated with the three types of IFN. For example, in HeLa cells, IFN-alpha or IFN-beta induced similarly both PKR and p48 whereas IFN-gamma induced mainly p48. In U937 cells in which PKR was not expressed with or without IFN treatment, p48 was strongly induced by all three types of IFN. These results suggest different mechanisms for the induction of PKR and p48. In view of its presence in different types of human cells and its induction by different types of IFN, it is possible to suggest that p48 might play an important role in mediating some of the action of IFN.