Mutation of the catalytic site of Nedd4-2 (Nedd4-2CS) renders the ligase ineffective in modulating Kv1.3 current amplitude. A and B: same voltage paradigm, I–V plot, and statistical design as in Fig. 1, A and B, except substitution of a Nedd4-2 ligase with a cysteine to serine point mutation (C938SNedd4-2) known to disrupt all catalytic activity of the ligase. ■, Kv1.3; ○, + Nedd4-2; half-filled circle, +Nedd4-2CS. C: bar graph of the mean (±SE) current amplitude for the −90- to +40-mV step for a number of patches as noted. Solid bar, channel (Kv1.3); open bar, channel plus wild-type ligase (+Nedd4-2); hatched bar, channel plus mutant ligase (+Nedd42C-S). ***P ≤ 0.001, one-way ANOVA, Student-Newman-Keuls (SNK) post hoc test.