A Cellular Non‐coding RNA Activator of Human 2′‐5′‐Oligoadenylate Synthetase 1

Human 2′–5′‐oligoadenylate synthetase 1 (OAS1) senses double‐stranded RNA (dsRNA) and is a key component of the innate immune response to viral infection. Binding of dsRNA activates OAS1 and results in synthesis of 2′–5′ linked oligadenylate second messengers that in turn activate ribonuclease L (RNaseL) and lead to cleavage of viral and cellular RNA. The recently discovered cellular non‐coding RNA 886 (nc886) is an endogenous inhibitor of another dsRNA‐sensing innate immune protein, the double‐stranded RNA activated protein kinase (PKR). We tested the hypothesis that nc886 might also act as a regulator of OAS1 and found, surprisingly, that nc886 potently activates OAS1, rather than inhibiting its activity. nc886 RNA adopts two structurally distinct conformers. OAS1 functional assays combined with RNA structure probing demonstrate that potent OAS1 activation is dependent on a complex RNA structure present within the apical region of only one nc886 conformer. Deletion or sequence modification of the apical stem‐loop or an adjacent internal asymmetric loop results in near complete loss of activity. In contrast, truncation of nc886 at its base‐paired termini results in minimal loss of activity. Furthermore, mutation of two known determinants of OAS1 activation, a consensus sequence and a 3′‐single‐stranded pyrimidine motif, present in nc886 result in minimal loss of activity. Our findings demonstrate that the degree of OAS1 activation is not solely dependent on dsRNA length; rather, potent OAS1 activity can be induced by small structured RNA, such as that present in nc886 RNA. Additionally, this is the first demonstration of a potential endogenous RNA regulator of OAS1 and suggests a mechanism for amplifying the immune response during infection.