Nedd4 and Nedd4-2 are ubiquitin-protein ligases that can regulate cell surface levels of membrane proteins by binding C-terminal PY motifs leading to ubiquitination, internalization and lysosomal/proteosomal degradation of the target proteins. Since Nedd4/4-2 are expressed at high levels in neurons, ion channels involved in neuronal excitability are potential targets for this regulation. We examined the regulation of cloned voltage-dependent Na⁺ and K⁺ channels expressed in Xenopus oocytes by Nedd4/4-2. An ovarian lobe was removed from X. laevis frogs under anaesthesia. 2-3 days after cRNA injection of defolliculated oocytes, depolarization-activated membrane currents were recorded using the two-electrode voltage clamp technique. Six of the nine mammalian voltage-gated Na⁺ channels (VGSCs) share the PY-motif PPXY. We have recently shown, using in vitro ubiquitination and Far Western techniques, that Nedd4/4-2 can bind to and ubiquitinate the neuronal VGSCs Na_v1.2, Na_v1.7 and Na_v1.8. In addition, co-expression experiments in X. laevis oocytes demonstrated that both Nedd4 and Nedd4-2 interact with Na_v1.2 and Na_v1.7, while only Nedd4-2 acts on Na_v1.8 (Fotia et al. 2004). The co-expression study has now been extended to include Na_v1.3, which was found to be down-regulated by both Nedd4 and Nedd4-2 to 31 ± 4 % (n = 23, P < 0.001) and 11 ± 2% (n = 24, P < 0.0001) of control, respectively. The muscarine-sensitive K⁺ current (M-current), mediated by a heteromeric channel consisting of KCNQ2 and KCNQ3 subunits, plays a key role in regulating neuronal excitability by stabilizing the membrane potential (Peters et al. 2005). The intracellular C-terminal tail of KCNQ3 contains the motif PPXPPY, similar to the PY motif in ClC-5 which is regulated by Nedd4-2 (Hryciw et al. 2004). KCNQ2/3 was co-expressed with Nedd4-2 in X. laevis oocytes, resulting in a pronounced reduction in the K⁺ current amplitude (60 ± 5% of control, n = 42; P < 0.001), whereas a ligase deficient Nedd4-2 mutant had no effect. When KCNQ2 was expressed alone, Nedd4-2 did not affect K⁺ current amplitude, suggesting that the current through KCNQ2/3 can be regulated by Nedd4-2 limiting the availability of the KCNQ3 subunit to the complex.