Because IFN-ts and their homologues, the IFN-cos, have six amino acid extensions at their carboxyl termini, it has been tempting to speculate that this structural feature might contribute to the function of these IFNs. It could, for example, allow them either to recruit additional polypeptide components to the receptor complex or to interact with the receptor in a unique manner. Although such theories were not tested directly in the present studies, it was clear that loss of the hexapeptide tail did not influence either the antiviral potency of ovIFN-т (as tested on bovine cells) or its ability to extend CL life span in nonpregnant ewes. buy asthma inhaler
The absence of an effect on antiviral activity was not unexpected because many IFN-as, which are, in general, 166 amino acids long, are at least equally as potent as IFN-ts in protecting MDBK cells from vesicular stomatitis virus. In addition, replacement of Leu 169 and Leu 172 of ovIFN-т with tyrosines by site-directed mutagenesis did not affect either the antiviral activity or the affinity of IFN-т for endometrial receptors. However, a further truncation of the carboxyl terminus to produce an ovIFN-т of only 161 amino acids provided a protein with severely reduced antiviral and antiproliferative activities, and completely ablated its ability to extend CL life span when administered i.m. to nonpregnant sheep. Interestingly, such a truncation reduced receptor binding affinity only slightly on both bovine endometrial cell membranes and MDBK cells, suggesting that the eleven residue carboxyl terminus is not involved in primary receptor interaction but is required for biological activity.