Lipid-binding SRPBCC domain of mammalian PITPNM1-2 and related proteins (Class IIA PITPs)
This subgroup includes an N-terminal SRPBCC (START/RHO_alpha_C/PITP/Bet_v1/CoxG/CalC) domain of mammalian Class II phosphatidylinositol transfer protein (PITPs), PITPNM1/PITPalphaI/Nir2 (PYK2 N-terminal domain-interacting receptor2) and PITPNM2/PITPalphaII/Nir3), Drosophila RdgB, and related proteins. These are membrane associated multidomain proteins belonging to the PITP family of lipid transfer proteins, and to the SRPBCC domain superfamily of proteins that bind hydrophobic ligands. SRPBCC domains have a deep hydrophobic ligand-binding pocket. In vitro, PITPs bind phosphatidylinositol (PtdIns), as well as phosphatidylcholine (PtdCho) but with a lower affinity. They transfer these lipids from one membrane compartment to another. The cellular roles of PITPs include inositol lipid signaling, PtdIns metabolism, and membrane trafficking. Ablation of the mouse gene encoding PITPNM1 results in early embryonic death. PITPNM1 is localized chiefly to the Golgi apparatus, and under certain conditions translocates to the lipid droplets. Targeting to the latter is dependent on a specific threonine residue within the SRPBCC domain. PITPNM1 plays a part in Golgi-mediated transport. It regulates diacylglycerol (DAG) production at the trans-Golgi network (TGN) via the CDP-choline pathway. Drosophila RdgB, the founding member of the PITP family, is implicated in the visual and olfactory transduction. RdgB is required for maintenance of ultra structure in photoreceptors and for sensory transduction. The mouse PITPNM1 gene rescues the phenotype of Drosophila rdgB mutant flies. In addition to the SRPBCC domain, PITPNM1 and -2 contain a Rho-inhibitory domain (Rid), six hydrophobic stretches, a DDHD calcium binding region, and a C-terminal tyrosine kinase Pyk2-binding / HAD-like phosphohydrolase domain. PITPNM1 has a role in regulating cell morphogenesis through its Rho inhibitory domain (Rid). This SRPBCC_PITPNM1-2_like domain model includes the first 52 residues of the 224 residues Rid (Rho-inhibitory domain).