P2X receptors are cation channels gated by extracellular ATP. The seven known P2X isoforms possess no sequence homology with other proteins. Here we studied the quaternary structure of P2X receptors by chemical cross-linking and blue native PAGE. P2X 1 and P2X 3 were N-terminally tagged with six histidine residues to allow for non-denaturing receptor isolation from cRNA-injected, [ 35 S]methionine-labeled oocytes. The His-tag did not change the electrophysiological properties of the P2X 1 receptor. His-P2X 1 was found to carry four N-glycans per polypeptide chain, only one of which acquired Endo H resistance en route to the plasma membrane. 3,3Ј-Dithiobis(sulfosuccinimidylpropionate) (DTSSP) and two of three bifunctional analogues of the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2Ј,4Ј-disulfonic acid (PPADS) cross-linked digitonin-solubilized His-P2X 1 and His-P2X 3 quantitatively to homo-trimers. Likewise, when analyzed by blue native PAGE, P2X receptors purified in digitonin or dodecyl-β-D-maltoside migrated entirely as non-covalently linked homo-trimers, whereas the α 2 βγδ nicotinic acetylcholine receptor (used as a positive control) migrated as the expected pentamer. P2X monomers remained undetected soon after synthesis, indicating that trimerization occurred in the endoplasmic reticulum. The plasma membrane form of His-P2X 1 was also identified as a homo-trimer. If n-octylglucoside was used for P2X receptor solubilization, homo-hexamers were observed, suggesting that trimers can aggregate to form larger complexes. We conclude that trimers represent an essential element of P2X receptor structure.

P2X7 is the largest member of the P2X subfamily of purinergic receptors. A typical feature is the carboxyl tail, which allows formation of a large pore. Recently a naturally occurring truncated P2X7 splice variant, isoform B (P2X7B), has been identified. Here we show that P2X7B expression in HEK293 cells, a cell type lacking endogenous P2X receptors, mediated ATP-stimulated channel activity but not plasma membrane permeabilization, raised endoplasmic reticulum Ca(2+) content, activated the transcription factor NFATc1, increased the cellular ATP content, and stimulated growth. In addition, P2X7B-transfected HEK293 cells (HEK293-P2X7B), like most tumor cells, showed strong soft agar-infiltrating ability. When coexpressed with full-length P2X7 (P2X7A), P2X7B coassembled with P2X7A into a heterotrimer and potentiated all known responses mediated by this latter receptor. P2X7B mRNA was found to be widely distributed in human tissues, especially in the immune and nervous systems, and to a much higher level than P2X7A. Finally, P2X7B expression was increased on mitogenic stimulation of peripheral blood lymphocyte. Altogether, these data show that P2X7B is widely expressed in several human tissues, modulates P2X7A functions, participates in the control of cell growth, and may help understand the role of the P2X7 receptor in the control of normal and cancer cell proliferation.

The known seven mammalian receptor (R) subunits (P2X1-7) form cationic channels gated by ATP. Three subunits compose a receptor-channel. Each subunit is a polypeptide consisting of two transmembrane regions (TM1, TM2), intracellular Nand C-termini, and a bulky extracellular loop. Crystallization allowed the identification of the 3D-structure and gating cycle of P2XRs. The agonist binding pocket is located at the intersection of two neighboring subunits. In addition to the mammalian P2XRs their primitive ligand-gated counterparts with little structural similarity have also been cloned. Selective agonists for P2XR subtypes are not available, but medicinal chemistry supplied a range of subtype selective antagonists, as well as positive and negative allosteric modulators. Knockout mice and selective antagonists helped to identify pathological functions due to defective P2XRs, such as male infertility (P2X1), hearing loss (P2X2), pain/cough (P2X3), neuropathic pain (P2X4), inflammatory bone loss (P2X5), and faulty immune reactions (P2X7).

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