C2 domain second repeat present in Synaptotagmin-like proteins 1-4
All Slp members basically share an N-terminal Slp homology domain (SHD) and C-terminal tandem C2 domains (named the C2A domain and the C2B domain) with the SHD and C2 domains being separated by a linker sequence of various length. Slp1/JFC1 and Slp2/exophilin 4 promote granule docking to the plasma membrane. Additionally, their C2A domains are both Ca2+ independent, unlike the case in Slp3 and Slp4/granuphilin in which their C2A domains are Ca2+ dependent. It is thought that SHD (except for the Slp4-SHD) functions as a specific Rab27A/B-binding domain. In addition to Slps, rabphilin, Noc2, and Munc13-4 also function as Rab27-binding proteins. It has been demonstrated that Slp3 and Slp4/granuphilin promote dense-core vesicle exocytosis. 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 second C2 repeat, C2B, and has a type-I topology.