Domain assembly of NAADP-gated two-pore channels

TPCs (two-pore channels) have recently been identified as targets for the Ca2+-mobilizing messenger NAADP (nicotinic acid–adenine dinucleotide phosphate). TPCs have a unique structure consisting of cytosolic termini, two hydrophobic domains (I and II) each comprising six transmembrane regions and a pore, and a connecting cytosolic loop; however, little is known concerning how these channels are assembled. In the present paper, we report that both domain I and II of human TPCs are capable of independent insertion into membranes, whereas the loop linking the domains fails to insert. Pairs of transmembrane regions within domain I of TPC1 are also capable of insertion, consistent with sequential translational integration of hydrophobic regions. Insertion of the first two transmembrane regions, however, was inefficient, indicating possible interaction between transmembrane regions during translation. Both domains, and each pair of transmembrane regions within domain I, were capable of forming oligomers, highlighting marked redundancy in the molecular determinants driving oligomer formation. Each hydrophobic domain formed dimers upon cross-linking. The first four transmembrane regions of TPC1 also formed dimers, whereas transmembrane regions 5 and 6, encompassing the pore loop, formed both dimers and tetramers. TPCs thus probably assemble as dimers through differential interactions between transmembrane regions. The present study provides new molecular insight into the membrane insertion and oligomerization of TPCs

Molecular Characterization of a Novel Intracellular ADP-Ribosyl Cyclase

Background. ADP-ribosyl cyclases are remarkable enzymes capable of catalyzing multiple reactions including the synthesis of the novel and potent intracellular calcium mobilizing messengers, cyclic ADP-ribose and NAADP. Not all ADP-ribosyl cyclases however have been characterized at the molecular level. Moreover, those that have are located predominately at the outer cell surface and thus away from their cytosolic substrates. Methodology/Principal Findings. Here we report the molecular cloning of a novel expanded family of ADP-ribosyl cyclases from the sea urchin, an extensively used model organism for the study of inositol trisphosphate-independent calcium mobilization. We provide evidence that one of the isoforms (SpARC1) is a soluble protein that is targeted exclusively to the endoplasmic reticulum lumen when heterologously expressed. Catalytic activity of the recombinant protein was readily demonstrable in crude cell homogenates, even under conditions where luminal continuity was maintained. Conclusions/Significance. Our data reveal a new intracellular location for ADP-ribosyl cyclases and suggest that production of calcium mobilizing messengers may be compartmentalized

Determination of cellular nicotinic acid-adenine dinucleotide phosphate (NAADP) levels.

Nicotinic acid-adenine dinucleotide phosphate (NAADP) is fast emerging as a new intracellular Ca2+-mobilizing messenger. In sea urchin egg homogenates, binding of NAADP to its receptor is not readily reversible; hence, prior incubation with low concentrations of NAADP is more effective in inhibiting subsequent binding of radiolabelled NAADP than incubating the preparation with the two ligands simultaneously [Patel, Churchill and Galione (2000) Biochem. J. 352, 725-729]. We extend this finding to show that NAADP is more effective still in inhibiting the subsequent radioligand binding at lower homogenate concentrations, an effect again quite probably due to the non-reversible nature of the receptor-ligand interaction. Enhanced sensitivity of the preparation to NAADP afforded by simple manipulation of the experimental conditions has been applied to determine low levels of NAADP in acid extracts from human red blood cells, rat hepatocytes and Escherichia coli without interference from NADP breakdown. Our improved method for the quantification of NAADP should prove useful in the further assessment of its signalling role within cells

Time sensing by NAADP receptors

NAADP (nicotinic acid–adenine dinucleotide phosphate) is a newly described intracellular messenger molecule that mediates Ca(2+) increases in a variety of cells. However, little is known of the mechanism whereby ligand binding regulates the target protein. We report in the present paper that NAADP receptors from sea urchin eggs undergo an unusual stabilization process that appears to be dependent upon the time during which receptors are exposed to their ligand. We demonstrate that receptors ‘tagged’ with NAADP for short periods were more readily dissociated following subsequent delipidation than those labelled for longer. Stabilization of NAADP receptors by their ligand was delayed relative to ligand association taking on the order of minutes to develop at picomolar concentrations. The stabilizing effects of NAADP did not require cytosolic factors or the continued presence of NAADP and persisted upon solubilization. NAADP receptors, however, failed to stabilize at reduced temperature. We conclude that NAADP receptors possess a simple molecular memory endowing them with the remarkable ability to detect the duration of their activation