C2 domain present in the Human neural precursor cell-expressed, developmentally down-regulated 4 (NEDD4) and NEDD4-like (NEDD4L/NEDD42)
Nedd4 and Nedd4-2 are two of the nine members of the Human Nedd4 family. All vertebrates appear to have both Nedd4 and Nedd4-2 genes. They are thought to participate in the regulation of epithelial Na+ channel (ENaC) activity. They also have identical specificity for ubiquitin conjugating enzymes (E2). Nedd4 and Nedd4-2 are composed of a C2 domain, 2-4 WW domains, and a ubiquitin ligase Hect domain. Their WW domains can bind PPxY (PY) or LPSY motifs, and in vitro studies suggest that WW3 and WW4 of both proteins bind PY motifs in the key substrates, with WW3 generally exhibiting higher affinity. Most Nedd4 family members, especially Nedd4-2, also have multiple splice variants, which might play different roles in regulating their substrates. C2 domains fold into an 8-standed beta-sandwich that can adopt 2 structural arrangements: Type I and Type II, distinguished by a circular permutation involving their N- and C-terminal beta strands. Many C2 domains are Ca2+-dependent membrane-targeting modules that bind a wide variety of substances including bind phospholipids, inositol polyphosphates, and intracellular proteins. Most C2 domain proteins are either signal transduction enzymes that contain a single C2 domain, such as protein kinase C, or membrane trafficking proteins which contain at least two C2 domains, such as synaptotagmin 1. However, there are a few exceptions to this including RIM isoforms and some splice variants of piccolo/aczonin and intersectin which only have a single C2 domain. C2 domains with a calcium binding region have negatively charged residues, primarily aspartates, that serve as ligands for calcium ions.