Fig. 1.

Conservation of ubiquitin and ISG-15 across vertebrates. Sequence alignment of ubiquitin from zebrafish, mice, and humans indicates perfect conservation of ubiquitin. The ISG15 protein contains two ubiquitin-like domains (UBLs). Although the 3D structure of each of the ISG15 domains resembles the 3D fold of Ub, a comparison of each domain of human ISG15 to the Ub sequence results in roughly 33 % sequence identity by BLAST [129]. In comparison to ubiquitin, ISG15 is less conserved across vertebrates, dropping to 66 % and 40 % identity between humans and mice or zebrafish, respectively.