Conceptus-derived interferon-tau (IFN-τ) induces bovine endometrial ubiquitin cross-reactive protein (UCRP) mRNA and protein on Days 15-21 of pregnancy. Bovine UCRP retains the Leu-Arg-Gly-Gly C-terminal sequence of ubiquitin that ligates to and directs degradation of cytosolic proteins. The objectives of the present experiments were to determine whether UCRP became conjugated to endometrial cytosolic proteins during early pregnancy and in response to recombinant bovine (rbo) IFN-τ. Ubiquitin (8 kDa), UCRP (17 kDa), and conjugates thereof (& or = 30 kDa) were quantitated using Western blotting and densitometry. Endometrial ubiquitin and its conjugates did not differ between Day 18 pregnant and nonpregnant cows, or between control and rboIFN-τ-treated (25 nM) explant cultures (Day 14; nonpregnant). Bovine UCRP was induced in endometrium from pregnant as compared with nonpregnant cows. Conjugation of endometrial proteins to UCRP was induced in pregnant as compared to nonpregnant cows. Recombinant boIFN-τ induced UCRP and its conjugates in cultured endometrial explants from nonpregnant cows. It is concluded that UCRP, in response to rboIFN-τ, becomes conjugated to endometrial cytosolic proteins during early pregnancy. The regulation of uterine proteins by UCRP may be integral to the maintenance of early pregnancy in ruminants.