No abstract

P2X receptors are trimeric membrane proteins that function as ion channels gated by extracellular ATP. We have engineered a P2X2 receptor that opens within milliseconds by irradiation at 440 nm, and rapidly closes at 360 nm. This requires bridging receptor subunits via covalent attachment of 4,4'-bis(maleimido)azobenzene to a cysteine residue (P329C) introduced into each second transmembrane domain. The cis-trans isomerization of the azobenzene pushes apart the outer ends of the transmembrane helices and opens the channel in a light-dependent manner. Lightactivated channels exhibited similar unitary currents, rectification, calcium permeability, and dye uptake as P2X2 receptors activated by ATP. P2X3 receptors with an equivalent mutation (P320C) were also light sensitive after chemical modification. They showed typical rapid desensitization, and they could coassemble with native P2X2 subunits in pheochromocytoma cells to form light-activated heteromeric P2X2/3 receptors. A similar approach was used to open and close human acid-sensing ion channels (ASICs), which are also trimers but are unrelated in sequence to P2X receptors. The experiments indicate that the opening of the permeation pathway requires similar and substantial movements of the transmembrane helices in both P2X receptors and ASICs, and the method will allow precise optical control of P2X receptors or ASICs in intact tissues. P 2X receptors and acid-sensing ion channels (ASICs) are trimeric membrane ion channels gated by binding extracellular ligands. P2X receptors are gated by extracellular ATP, and their physiological roles include neuroeffector transmission, primary afferent transmission (e.g., taste, hearing, chemoreception), central control of respiration, and neuroinflammation (1-3). ASICs are gated by protons and are involved in pain sensation (4, 5). The experimental study of ligand-gated channels in intact tissues is often hampered by difficulties in application of the appropriate ligand while recording ion channel activity in the millisecond time domain, and there are advantages to controlling channel activation by surrogate optical methods. The increase in our knowledge of molecular and atomic structure of ligand-gated channels over the past 10 years has allowed one such approach (photoswitchable tethered ligands) to become much more sophisticated, because cysteines can be introduced into the channel protein exactly where required to form an attachment point. The method has been applied to pentameric nicotinic receptors (6) and tetrameric glutamate receptors (7,8). Although attaching ligands through photoswitchable tethers is proving extremely valuable, an intimate structural knowledge of a closed and open state of a channel also allows for optical control of conformation at parts of the protein that are remote from the agonist binding site (9-11).High-resolution structures are available for P2X receptors (closed: ref. 12; open: ref. 13) and ASICs (closed: refs. 14 and 15; open: ref. 16). In both these trimeric channels the second of the...

Rab GTPases play key roles in the delivery, docking and fusion of intracellular vesicles. However, the mechanism by which spatial and temporal regulation of Rab GTPase activity is controlled is poorly understood. Here we describe a mechanism by which localized calcium release through a vesicular ion channel controls Rab GTPase activity. We show that activation of P2XA, an intracellular ion channel localized to the Dictyostelium discoideum contractile vacuole system, results in calcium efflux required for downregulation of Rab11a activity and efficient vacuole fusion. Vacuole fusion and Rab11a downregulation require the activity of CnrF, an EF hand containing Rab GAP found in a complex with Rab11a and P2XA. CnrF Rab GAP activity to Rab11a is enhanced by the presence of calcium and the EF-hand domain. These findings suggest that P2XA activation results in vacuolar calcium release, which triggers activation of CnrF Rab GAP activity and subsequent downregulation of Rab11a to allow vacuole fusion.

P2X receptors are trimeric membrane proteins. When they bind extracellular ATP, a conformational change occurs that opens a transmembrane ion channel. The ATP-binding pocket is formed in a cleft between two subunits, and a critical amino acid residue for ATP contact is Lys 69 (P2X2 numbering). In the present work, we sought to determine whether the binding of fewer than three ATP molecules could open the ion channel. We expressed eight concatenated cDNAs in human embryonic kidney cells, which encoded three serially joined, epitopetagged, subunits with either Lys or Ala at position 69 (denoted as KKK, KKA, KAK, AKK, KAA, AKA, AAK, and AAA). Western blotting of surface-biotinylated proteins indicated that breakdown of concatemers to individual subunits was minimal. Recording of membrane currents in response to ATP (whole cell and excised outside-out patch) showed that all formed functional channels except AAK, AKA, and AAA. There was no difference in the kinetics of activation and deactivation among KKK, KKA, KAK, and AKK channels, and amplitude of the unitary conductances was in all cases not different from that found after expression of a single wild-type subunit. Currents through KKA and KAK receptors were larger than those observed for AKK receptors. The results indicate that trimeric P2X receptors containing only two intact binding sites can be readily activated by ATP.

In the closed structure of the P2X cation channel, three α-helical transmembrane domains cross the membrane obliquely: in rat P2X2 receptors, these intersect at Thr339. Replacing Thr339 by lysine in one, two or three subunits progressively increased chloride permeability and reduced unitary conductance. This implies that the closed-open transition involves a symmetrical separation of the three subunits, and that Thr339 from each contributes symmetrically to the open channel permeation pathway.