Molecular cloning and characterization of two related and interferon-induced 56-kDa and 30-kDa proteins highly similar to 2'-5' oligoadenylate synthetase.

The 2'-5' oligoadenylate synthetase (OAS) represents a family of interferon-induced proteins which when activated by double-stranded (ds) RNA polymerizes ATP into 2'-5' linked oligomers with the general formula pppA(2'p5'A)n, n> or =1. Three forms of human OASs have been described corresponding to proteins of 40/46, 69/71 and 100 kDa (p40/p46, p69/p71 and p100). Polyclonal antibodies raised against p100 cross reacted with the other forms p40/p46, p69/p71 but also with an interferon-induced 56-kDa protein (p56). By screening a cDNA expression library, these polyclonal antibodies selected a cDNA encoding p56. Further studies by the RACE procedure using primers corresponding to this cDNA, a p56-related protein of 30 kDa (p30) was isolated. Both p56 and p30 mRNA are expressed in interferon-treated cells as transcripts of 2 kb and 1.8 kb, respectively. The 1.8-kb mRNA is homologous to the 2-kb mRNA but with a 243-nucleotide deletion at position 1011, which results in a frameshift. Consequently, the p56 and p30 have their first 219 amino acid residues identical but differ at their C-termini. In vitro transcription-translation of p56 and p30 cDNAs generated proteins of 56 and 30 kDa, respectively. The deduced amino acid sequence of p56 kDa shares strong similarity with the previously cloned OASs, and contains the subdomains conserved in p40/p46 and p69/p71 forms. Transient expression in HeLa cells indicated that p30 has a cytoplasmic localization, whereas p56 has cytoplasmic and nucleolar localizations. The p56 isolated from transfected cells was shown to bind dsRNA and DNA, but it was devoid of 2'-5'OAS activity typical of the three known forms of this enzyme. Thus, p56 and p30 are two related and interferon-induced proteins outside the family of 2'-5'OAS, which might have as yet unidentified catalytic activities or functions.