Structural and functional analysis reveals that human OASL binds dsRNA to enhance RIG-I signaling.

The oligoadenylate synthetase (OAS) enzymes are cytoplasmic dsRNA sensors belonging to the antiviral innate immune system. Upon binding to viral dsRNA, the OAS enzymes synthesize 2'-5' linked oligoadenylates (2-5As) that initiate an RNA decay pathway to impair viral replication. The human OAS-like (OASL) protein, however, does not harbor the catalytic activity required for synthesizing 2-5As and differs from the other human OAS family members by having two C-terminal ubiquitin-like domains. In spite of its lack of enzymatic activity, human OASL possesses antiviral activity. It was recently demonstrated that the ubiquitin-like domains of OASL could substitute for K63-linked poly-ubiquitin and interact with the CARDs of RIG-I and thereby enhance RIG-I signaling. However, the role of the OAS-like domain of OASL remains unclear. Here we present the crystal structure of the OAS-like domain, which shows a striking similarity with activated OAS1. Furthermore, the structure of the OAS-like domain shows that OASL has a dsRNA binding groove. We demonstrate that the OAS-like domain can bind dsRNA and that mutating key residues in the dsRNA binding site is detrimental to the RIG-I signaling enhancement. Hence, binding to dsRNA is an important feature of OASL that is required for enhancing RIG-I signaling.

Ibsen MS ，Gad HH ，Andersen LL ，Hornung V ，Julkunen I ，Sarkar SN ，Hartmann R ... -  《-》

Distinct domain organization and diversity of 2'-5'-oligoadenylate synthetases.

Koul A ，Hui LT ，Lubna N ，McKenna SA ... -  《-》

Length dependent activation of OAS proteins by dsRNA.

The oligoadenylate synthetase (OAS) family of enzymes are interferon-inducible antiviral proteins, which synthesize the secondary messenger 2'-5'-linked oligoadenosine (2-5A) in response to viral infection. The production of 2-5As induces RNA decay within the infected cells, thereby effectively preventing further viral replication. OAS shares structural similarity as well as the enzymatic mechanism with a different antiviral protein, cyclic GMP-AMP synthase (cGAS), but OAS is activated by dsRNA whereas cGAS is activated by dsDNA. Here, we have studied the structural requirement for the dsRNA activating OAS1 and OAS3, and compared it to recent studies on cGAS. We find that both OAS1 and OAS3, like cGAS, achieve their maximum activity with dsRNA molecules that are substantial longer than what one monomer of the enzyme can interact with. One molecule of OAS1 can cover approximately 18-20 base pairs of dsRNA, which is just short of two turns of a helix. However, RNAs of this length gave a very limited activity and the length dependency was even more pronounced for OAS3. Our data suggest that the OAS enzymes evolved to recognize long dsRNA as virally derived PAMPs, and that the length of the dsRNA is an important factor in discriminating self from non-self. Several structures of OAS1 bound to short dsRNAs exist, but our data show that OAS can only achieve minimal activity with these short activators (approximately 7-8% of maximal activity) and it is thus possible that these structures do not reveal the fully activated state of the OAS enzymes.

Wang Y ，Holleufer A ，Gad HH ，Hartmann R ... -  《-》

Structural mechanism of sensing long dsRNA via a noncatalytic domain in human oligoadenylate synthetase 3.

Donovan J ，Whitney G ，Rath S ，Korennykh A ... -  《-》

Molecular Mechanisms for the Adaptive Switching Between the OAS/RNase L and OASL/RIG-I Pathways in Birds and Mammals.

Rong E ，Wang X ，Chen H ，Yang C ，Hu J ，Liu W ，Wang Z ，Chen X ，Zheng H ，Pu J ，Sun H ，Smith J ，Burt DW ，Liu J ，Li N ，Huang Y ... -  《Frontiers in Immunology》