C2 domain found in 5' repressor element under dual repression binding protein-1 (Freud-1)Freud-1 is a novel calcium-regulated repressor that negatively regulates basal 5-HT1A receptor expression in neurons. It may also play a role in the altered regulation of 5-HT1A receptors associated with anxiety or major depression. Freud-1 contains two DM-14 basic repeats, a helix-loop-helix DNA binding domain, and a C2 domain. The Freud-1 C2 domain is thought to be calcium insensitive and it lacks several acidic residues that mediate calcium binding of the PKC C2 domain. In addition, it contains a poly-basic insert that is not present in calcium-dependent C2 domains and may function as a nuclear localization signal. 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. This cd contains the first C2 repeat, C2A, and has a type-II topology.