Complex Binding of the Embryonic Interferon,Ovine Trophoblast Protein-1,to Endometrial Receptors

Ovine embryos produce an interferon (IFN)-α_II in significant quantities during early pregnancy. This IFN, previously termed ovine trophoblast protein-1 (oTP-1), is a 172-amino-acid polypeptide which has been suggested to be the causal agent in maternal recognition of pregnancy in the ewe. Here we report the binding of oTP-1 and a recombinant bovine IFN-α_I1 (rBoIFN-α_I1; 165–166 amino acids long) to membrane preparations from ovine uterine endometrium. Both oTP-1 and rBoIFN-α_I1 competed with each other for receptor binding. Based on Scatchard analysis, [¹²⁵I]oTP-1 binding was determined to be complex and resolvable into a high-affinity (K _d = 3.8 × 10⁻¹¹ M, 30 fmoles/mg protein) and a low affinity (K _d = 1.7 × 10⁻¹⁰ M; 96 fmoles/mg protein) component. Conversely [¹²⁵I]rBoIFN-α_I1 bound to only a single high-affinity receptor (K _d = 6.1 × 10⁻¹¹ M; 174 fmoles/mg protein). Cross-linking experiments using disuccinimidyl suberate revealed that [¹²⁵I]oTP-1 associated with membrane polypeptides of two molecular weight classes (M _r 100,000 and 70,000), and could be displaced from both with rBoIFN-α_I1. In contrast, [¹²⁵I]rBoIFN-α_I1 cross-linked to only the 100,000 M _r membrane polypeptide. These data provide evidence that the binding parameters of oTP-1 and rBoIFN-α_I1 to endometrial receptors are different.