Interferon-stimulated gene (ISG) 15 mediates antiviral responses and also is upregulated within the endometrium in response to the developing embryo during early pregnancy. Structurally, ISG15 resembles two ubiquitin domains (30% identical) that are separated by a hinge region. Recombinant (r) bovISG15 is not stable in solution. It was hypothesized that the hinge region contributed to the instability of rbovISG15. Within 24 h of dialysis, rbovISG15 formed complexes as detected by reducing and denaturing SDS-PAGE. However, chemical perturbations of cysteine prevented formation of rbovISG15 complexes over time. Furthermore, a site-directed mutant of rbovISG15 (Cys80Ser) was isomeric and more stable than rbovISG15. Neither wild-type nor mutant rbovISG15 was able to interact with the ISG15 E1 initiating enzyme, UBE1L, in an in vitro pull-down assay. Ovine (ov) ISG15 has three additional amino acids within the hinge region that were hypothesized to increase stability and the degree of interaction with UBE1L because of increased separation of the ubiquitin-like domains. Over time in solution, rovISG15 the level of rovISG15 secondary structure was diminished, whereas the Cys80Ser rovISG15 structure did not change. A GST-Cys80Ser rovISG15 fusion protein had increased structural stability and enhanced protein-protein interaction with UBE1L after dialysis for 48 h, when compared to the GST-rovISG15 fusion protein or rbovISG15. Models of bovISG15, Cys80Ser bovISG15, and ovISG15 were constructed, which confirmed that the hinge region between the two ubiquitin domains destabilizes rbovISG15 in solution.