69-kDa and 100-kDa isoforms of interferon-induced (2'-5')oligoadenylate synthetase exhibit differential catalytic parameters

How cells respond to interferons

Interferons play key roles in mediating antiviral and antigrowth responses and in modulating immune response. The main signaling pathways are rapid and direct. They involve tyrosine phosphorylation and activation of signal transducers and activators of transcription factors by Janus tyrosine kinases at the cell membrane, followed by release of signal transducers and activators of transcription and their migration to the nucleus, where they induce the expression of the many gene products that determine the responses. Ancillary pathways are also activated by the interferons, but their effects on cell physiology are less clear. The Janus kinases and signal transducers and activators of transcription, and many of the interferon-induced proteins, play important alternative roles in cells, raising interesting questions as to how the responses to the interferons intersect with more general aspects of cellular physiology and how the specificity of cytokine responses is maintained.

PACT, a protein activator of the interferon-induced protein kinase, PKR

PKR, a latent protein kinase, mediates the antiviral actions of interferon. It is also involved in cellular signal transduction, apoptosis, growth regulation and differentiation. Although in virus-infected cells, viral double-stranded (ds) RNA can serve as a PKR activator, cellular activators have remained obscure. Here, we report the cloning of PACT, a cellular protein activator of PKR. PACT heterodimerized with PKR and activated it in vitro in the absence of dsRNA. In mammalian cells, overexpression of PACT caused PKR activation and, in yeast, co-expression of PACT enhanced the anti-growth effect of PKR. Thus, PACT has the hallmarks of a direct activator of PKR.

Production, purification, and characterization of recombinant 2', 5'-oligoadenylate synthetases

2',5'-Oligoadenylate [2-5(A)] synthetases are a family of interferon-induced enzymes that polymerize ATP into 2'-5'-linked oligoadenylates in the presence of double-stranded RNA (dsRNA), their cofactor. The 2-5(A) molecules, in turn, activate the latent ribonuclease RNase L by promoting its dimerization. The 2-5(A) synthetase pathway has been implicated in interferon's antiviral and anticellular activities. In addition to their interesting cellular properties, these enzymes are also enzymologically interesting because they are the only known template and primer independent nucleotide (DNA or RNA)polymerases that synthesize 2'-5'-linked oligonucleotides. Moreover, their mode of activation by dsRNA remains unknown. In the past, biochemical and structure-function studies have been hampered by the lack of a convenient system for expressing recombinant 2-5(A) synthetases. These proteins are toxic to mammalian cells, probably because of RNase L activation, and proteins produced in bacteria do not have full enzymatic activity. To circumvent these problems, we have developed a baculovirus-insect cell system for high-yield expression of the small and medium isozymes. Here, methods are described for the production, purification, and characterization of the mouse small (9-2) (S. K. Ghosh, J. Kusari, S. K. Bandyopadhyay, H. Samanta, R. Kumar, and G. C. Sen, 1991, J. Biol. Chem. 266, 15293-15299) and human medium (P69) (I. Marie and A. G. Hovanessian, 1992, J. Biol. Chem. 267, 9933-9939) 2-5(A) synthetase isozymes and their mutants using the insect cell system. We also report methods for studying 2-5(A) synthetase-dsRNA interactions and protein-protein interactions among the subunits of the two isozymes.

Ultrastructural localization of interferon-inducible double-stranded RNA-activated enzymes in human cells

The protein kinase PKR and the 2',5'-oligoadenylate (2-5A) synthetase are two interferon-induced and double-stranded RNA-activated enzymes which are implicated in the mechanism of action of interferon. Their distribution was undertaken here at the ultrastructural level by the immunogold procedure, following the use of specific monoclonal antibodies directed against PKR and 69- and 100-kDa forms of the 2-5A synthetase. These enzymes were detected as a pool of nonaggregated proteins scattered throughout the cell and as aggregates often associated with electron-dense doughnut-like structures showing a similar aspect whatever their subcellular localization: the cytoplasm, the nuclear envelope, and the nucleus. In general, the 2-5A synthetases were present in much more lower amounts than the PKR, probably due to the difficulty of detecting traces of proteins by electron microscopy. To circumvent this, we used a human lymphoblastoid cell line overexpressing the 69-kDa form of the 2-5A synthetase. In such cells, the synthetase was then clearly observed in both the cytoplasm and the nucleus; isolated or small clusters of gold particles were numerous in the cell mainly over the RNP fibrils of the interchromatin space, nucleolus, and ribosomes. Interestingly, gold particles were also found to be associated with the membranes of nuclear envelope and rough endoplasmic reticulum probably due to the myristilated motif of this form of 2-5A synthetase. Finally, intensely labeled electron-opaque dots sometimes associated with the nuclear pore complexes were present in the nucleus and in the cytoplasm of cells which might suggest their transport from the nucleus to the cytoplasm or reciprocally through the nuclear pore complexes. These observations indicate the wider distribution of the dsRNA-activated enzymes in the cell, thus pointing out their potential implication in as yet undetermined physiological function(s) necessary for various cellular metabolic reactions.