(a) tc-VLP activity. Plasmids of the tc-VLP system encoding wt or C40A L were transfected together with expression plasmids for ubiquitin, wt ISG15, the non-conjugatable ISG15 mutant ΔGG, or empty vector (CTRL). Top panel: activities in donor cells, bottom panel: activities in indicator cells. (b) Minireplicon activity. Plasmids with minireplicon system expressing wt or C40A L were cotransfected with a control plasmid (CTRL) or either ubiquitin (Ub) or ISG15 or ISG15_ΔGG. Mean values and standard deviations of 6 (a) and 3 (b) independent experiments are shown. Two-tailed, paired Student’s t tests with Bonferroni correction were used to compare the CTRL with the different other constructs. *, P < 0.05; n.s., not significant.