The human 2 ',5 '-oligoadenylate synthetase family: Interferon-induced proteins with unique enzymatic properties

2',5'-Oligoadenylate synthetase (2',5'-OAS) was discovered and characterize d as an interferon (IFN)-induced enzyme that in the presence of double-stra nded (ds) RNA converts ATP into 2',5'-linked oligomers of adenosine with th e general formula pppA(2'p'A)(n), n greater than or equal to 1. The product is pppG2'p5'G when GTP is used as a substrate. Now, 20 years later, this a ctivity is attributed to several well-characterized, homologous, and IFN-in duced proteins in human cells. Three distinct forms of 2',5'-OAS exist, sma ll, medium, and large, which contain 1, 2, and 3 OAS units, respectively, a nd are encoded by distinct genes clustered on the 2',5'-OAS locus on human chromosome 12, Recently, other IFN-induced proteins homologous to the OAS u nit but devoid of the typical 2',5'-OAS catalytic activity have been descri bed, These GAS-related proteins are encoded by a gene located at the proxim ity of the 2',5'-OAS locus. These findings illustrate the apparent structur al and functional complexity of the human 2',5'-OAS family.