This entry corresponds to the MUN domain [1] found in Munc13 proteins. These constitute a family of three highly homologous molecules (Munc13-1, Munc13-2 and Munc13-3) with homology to Caenorhabditis elegans unc-13p. Munc13 proteins contain a phorbol ester-binding C1 domain and two C2 domains, which are Ca2+/phospholipid binding domains. Sequence analyses have uncovered two regions called Munc13 homology domains 1 (MHD1) and 2 (MHD2) that are arranged between two flanking C2 domains. MHD1 and MHD2 domains are present in a wide variety of proteins from Arabidopsis thaliana, C. elegans, Drosophila melanogaster, mouse, rat and human, some of which may function in a Munc13-like manner to regulate membrane trafficking. Structural studies have defined MHD1 and MHD2 to be part of the larger MUN domain which forms an elongated structure composed of any pairs of alpha helices.

1: Mechanistic insights into neurotransmitter release and presynaptic plasticity from the crystal structure of Munc13-1 C1C2BMUN. Xu J, Camacho M, Xu Y, Esser V, Liu X, Trimbuch T, Pan YZ, Ma C, Tomchick DR, Rosenmund C, Rizo J; Elife. 2017; [Epub ahead of print] PMID:28177287