Localization of an antiviral site on the pregnancy recognition hormone, ovine trophoblast protein

Ovine trophoblast protein 1 (oTP-1) is the interferon α (IFN-α) variant with potent antiviral activity and low toxicity that is responsible for maternal recognition of pregnancy in sheep. To examine the structure/function basis for the potent antiviral activity of oTP-1, we have exploited the direct approach of synthetic peptide competition with oTP-1 for receptor, using N-terminal oTP-1-(1-37) and C-terminal oTP-1-(139-172) peptides. These peptides possess structures similar to those predicted for the intact molecule on the basis of circular dichroism. oTP-1-(1-37) at 1.5 mM specifically blocked oTP-1 antiviral activity without affecting the antiviral activity of natural ovine IFN-α, recombinant bovine IFN-α, and recombinant human IFN-α. At concentrations as low as 0.15 mM, oTP-1-(139-172) blocked the antiviral activity of oTP-1, as well as that of natural ovine IFN-α, recombinant bovine IFN-α, and recombinant human IFN-α, but not recombinant bovine interferon γ. Further, binding of radiolabeled oTP-1 to endometrial membrane preparations could be effectively inhibited by polyclonal anti-C-terminal and anti-N-terminal antisera, with the anti-C-terminal antiserum being the more effective inhibitor. Consistent with peptide and antiserum functional data, oTP-1 and recombinant bovine IFN-α are predicted to possess similar C-terminal structure but different N-terminal structure by composite surface profile predictions. The findings suggest that the C-terminal regions of IFN-αs bind to a common site on the IFN-α receptor while the N-terminal region binds to a site unique for the particular IFN-α.