down-regulation of components of the epidermal growth factor (EGF) signalling pathway, such as HER3 and HER4 EGF receptors, and ACK. The fibroblast growth factor receptor 1 (FGFR1) undergoes NEDD4 mediated ubiquitination and down-regulation, and contains a novel site (VL***PSR) that binds the C2 and WW3 domain of NEDD4. There is a role for NEDD4 in viral budding via ubiquitination of viral matrix proteins for a number of viruses, and NEDD4 also interacts with components of the endocytic machinery required for budding. NEDD4 can also function independently of its ubiquitin ligase activity. NEDD4 interacts with VEGFR2, leading to the degradation of VEGFR2 irrespective of whether the HECT domain is catalytically active. NEDD4 can bind and ubiquitinate the epithelial sodium channel (ENaC), leading to down-regulation of sodium channel activity. However, in vivo studies have implicated the NEDD4 family member NEDD4-2 as the main ligase responsible for ENaC regulation. Regulation NEDD4 activity can be regulated by auto-inhibition, whereby the C2 domain binds to the HECT domain to create an inhibitory conformation of the protein. This auto-inhibitory conformation can be disrupted by the presence of calcium, by proteins that bind to NEDD4 to prevent this conformation, or by phosphorylation of NEDD4 at specific tyrosine residues to activate NEDD4 ubiquitin ligase activity. The NDFIP1 and NDFIP2 proteins function as adaptor proteins that can facilitate NEDD4 binding to substrates that lack PY motifs, as well as a role in binding NEDD4 to abrogate auto-inhibition. NDFIP1 may also regulate NEDD4 recruitment to exosomes for secretion. Oxidative stress induces the activation of NEDD4 transcription via the FOXM1B transcription factor. Ras signalling also up-regulates NEDD4 transcription. Physiological significance In vivo, NEDD4 is involved in the regulation of insulin and insulin-like growth factor (IGF-1) signalling by regulating the amount of insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF-1R) on the cell surface. The deletion of NEDD4 in mice leads to a reduced number of effector T-cells, and a slower T-cell response to antigen, suggesting that NEDD4 may function to convert naïve T-cells into activated T-cells. NEDD4 plays an important role in neuronal development, and is responsible for the formation and arborisation of dendrites in neurons by forming a signalling complex with TINK and Rap2A. It is also required for proper formation and function of neuromuscular junctions, and normal numbers of cranial neural crest cells, motor neurons and axons. NEDD4 has been shown to interact with and ubiquitinate the