C2 domains found in Dedicator Of CytoKinesis (Dock) class B proteinsDock-B is one of 4 classes of Dock family proteins. The members here include: Dock3/MOCA (modifier of cell adhesion) and Dock4. Most of these members have been shown to be GEFs specific for Rac, although Dock4 has also been shown to interact indirectly with the Ras family GTPase Rap1, probably through Rap regulatory proteins. In addition to the C2 domain (AKA Dock homology region (DHR)-1, CED-5, Dock180, MBC-zizimin homology (CZH) 1) and the DHR-2 (AKA CZH2, or Docker), which all Dock180-related proteins have, Dock-B members contain a SH3 domain upstream of the C2 domain and a proline-rich region downstream. DHR-2 has the catalytic activity for Rac and/or Cdc42, but is structurally unrelated to the DH domain. The C2/DHR-1 domains of Dock180 and Dock4 have been shown to bind phosphatidylinositol-3, 4, 5-triphosphate (PtdIns(3,4,5)P3). The C2 domain was first identified in PKC. 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.