The expression of both domains of the 69/71 kDa 2 ',5 ' oligoadenylate synthetase generates a catalytically active enzyme and mediates an anti-viral response

The 2',5' oligoadenylate synthetase (OAS) represents a family of interferon -induced proteins which, when activated by double-stranded (ds) RNA, polyme rizes ATP into 2',5'-linked oligomers with the general formula pppA(2'p5'A) (n), where n greater than or equal to 1. The 69-kDa form of human OAS has t wo isoforms (p69 and p71) that are identical for their first 683 amino acid s and consist of two homologous and adjacent domains, each homologous to th e small 40-kDa GAS. Here, we demonstrate that mRNA species specific for the isoforms p69 and p71 are enhanced in interferon-treated cells, with the p6 9 mRNA being more abundant than that of p71. In transfected cells, both iso forms could be expressed independently to generate enzymes with similar cat alytic activity, typical of the natural 69-kDa OAS from interferon-treated cells. On the other hand, deletion mutants expressing either the N- or C-te rminal domain common in p69 and p71 were greatly unstable and were found to be devoid of catalytic activity, in spite of the capacity of the C-termina l domain to bind dsRNA. Finally, we show that murine cell lines stably expr essing either p69 or p71 isoforms partially resist infection by the encepha lomyocarditis virus. These results indicate that both isoforms of the 69-kD a form of 2',5' OAS are expressed in interferon-treated cells, and that eac h isoform could be implicated in the mechanism of the anti-viral action of interferon.