DIFFERENCES IN ACTIVITY BETWEEN ALPHA-TYPE-I AND BETA-TYPE-I INTERFERONS EXPLORED BY MUTATIONAL ANALYSIS

Type I interferon (IFN) subtypes alpha and beta share a common multico mponent, cell surface receptor and elicit a similar range of biologica l responses, including antiviral, antiproliferative, and immunomodulat ory activities. However, alpha and beta IFNs exhibit key differences i n several biological properties. For example, IFN-beta, but not IFN-al pha, induces the association of tyrosine-phosphorylated receptor compo nents ifnar1 and ifnar2, and has activity in cells lacking the IFN rec eptor-associated, Janus kinase tyk2. To define the structural basis fo r these functional differences we produced human IFN-beta with point m utations and compared them to wild-type IFN-beta in assays that distin guish alpha and beta IFN subtypes. IFN-beta mutants with charged resid ues (N86K, N86E, or Y92D) introduced at two positions in the C helix l ost the ability to induce the association of tyrosine-phosphorylated r eceptor chains and had reduced activity on tyk2-deficient cells. The c ombination of negatively charged residues N86E and Y92D (homologous wi th IFN-alpha 8) increased the cross-species activity of the mutant IFN -beta s on bovine cells to a level comparable to that of human IFN-alp ha s. In contrast, point mutations in the AB loop and D helix had no s ignificant effect on these subtype-specific activities. A subset of th ese latter mutations did, however, reduce activity in a manner analogo us to IFN-alpha mutations, The effects of these mutations on IFN-beta activity are discussed in the context of a family of related ligands a cting through a common receptor and signaling pathway.