Lysine 63-linked Polyubiquitination of the Dopamine Transporter Requires WW3 and WW4 Domains of Nedd4-2 and UBE2D Ubiquitin-conjugating Enzymes

RNA interference screen previously revealed that a HECT-domain E3 ubiquitin ligase, neuronal precursor cell expressed, developmentally down-regulated 4-2 (Nedd4-2), is necessary for ubiquitination and endocytosis of the dopamine transporter (DAT) induced by the activation of protein kinase C (PKC). To further confirm the role of Nedd4-2 in DAT ubiquitination and endocytosis, we demonstrated that the depletion of Nedd4-2 by two different small interfering RNA (siRNA) duplexes suppressed PKC-dependent ubiquitination and endocytosis of DAT in human and porcine cells, whereas knock-down of a highly homologous E3 ligase, Nedd4-1, had no effect on DAT. The abolished DAT ubiquitination in Nedd4-2-depleted cells was rescued by expression of recombinant Nedd4-2. Moreover, overexpression of Nedd4-2 resulted in increased PKC-dependent ubiquitination of DAT. Mutational inactivation of the HECT domain of Nedd4-2 inhibited DAT ubiquitination and endocytosis. Structure-function analysis of Nedd4-2-mediated DAT ubiquitination revealed that the intact WW4 domain and to a lesser extent WW3 domain are necessary for PKC-dependent DAT ubiquitination. Moreover, a fragment of the Nedd4-2 molecule containing WW3, WW4, and HECT domains was sufficient for fully potentiating PKC-dependent ubiquitination of DAT. Analysis of DAT ubiquitination using polyubiquitin chain-specific antibodies showed that DAT is mainly conjugated with Lys(63)-linked ubiquitin chains. siRNA analysis demonstrated that this polyubiquitination is mediated by Nedd4-2 cooperation with UBE2D and UBE2L3 E2 ubiquitin-conjugating enzymes. The model is proposed whereby each ubiquitinated DAT molecule is modified by a single four-ubiquitin Lys(63)-linked chain that can be conjugated to various lysine residues of DAT.