This is the largely alpha-helical, C-terminal half of 2'-5'-oligoadenylate synthetase 1, being described as domain 2 of the enzyme and homologous to a tandem ubiquitin repeat. It carries the region of enzymic activity between 320 and 344 at the extreme C-terminal end [1]. Oligoadenylate synthetases are antiviral enzymes that counteract vial attack by degrading viral RNA. The enzyme uses ATP in 2'-specific nucleotidyl transfer reactions to synthesise 2'.5'-oligoadenylates, which activate latent ribonuclease, resulting in degradation of viral RNA and inhibition of virus replication [2]. This domain is often associated with NTP_transf_2 Pfam:PF01909.

1: Crystal structure of the 2'-specific and double-stranded RNA-activated interferon-induced antiviral protein 2'-5'-oligoadenylate synthetase. Hartmann R, Justesen J, Sarkar SN, Sen GC, Yee VC; Mol Cell. 2003;12:1173-1185. PMID:14636576

2: Cloning, sequencing, and expression of two murine 2'-5'-oligoadenylate synthetases. Structure-function relationships. Ghosh SK, Kusari J, Bandyopadhyay SK, Samanta H, Kumar R, Sen GC; J Biol Chem. 1991;266:15293-15299. PMID:1651324