Production and characterization of a soluble, active form of Tva, the subgroup A avian sarcoma and leukosis virus receptor

The receptor for the subgroup A avian sarcoma and leukosis viruses [ASLV(A) ] is the cellular glycoprotein Tva, A soluble form of Tva, sTva, was produc ed and purified with a baculovirus expression system. Using this system, 7 to 10 mg of purified sTva per liter of cultured Sf9 cells was obtained. Cha racterization of the carbohydrate modification of sTva revealed that the th ree N glycosylation sites in sTva were differentially utilized; however, th e O glycosylation common to Tva produced in mammalian and avian cells was n ot observed. Purified sTva demonstrates significant biological activity, sp ecifically blocking infection of avian cells by ASLV(A) with a 90% inhibito ry concentration of similar to 25 pM. A quantitative enzyme-linked immunoso rbent assay, developed to assess the binding of sTva to ASLV envelope glyco protein, demonstrates that sTva has a high affinity fur EnvA, with an appar ent dissociation constant of approximately 0.3 nM. Once they are bound, a v ery stable complex is formed between EnvA and sTva, with an estimated compl ex half-life of 6 h, The soluble receptor protein described here represents a valuable tool for analysis of the receptor-envelope glycoprotein interac tion and for structural analysis of Tva.