An Open Rectifier Potassium Channel with Two Pore Domains in Tandem Cloned from Rat Cerebellum

Abstract

Tandem pore domain K ϩ channels represent a new family of ion channels involved in the control of background membrane conductances. We report the structural and functional properties of a TWIK-related acid-sensitive K ϩ channel (rTASK), a new member of this family cloned from rat cerebellum. The salient features of the primary amino acid sequence include four putative transmembrane domains and, unlike other cloned tandem pore domain channels, a PDZ (postsynaptic density protein, disk-large, zo-1) binding sequence at the C terminal. rTASK has distant overall homology to a putative Caenorhabditis elegans K ϩ channel and to the mammalian clones TREK-1 and TWIK-1. rTASK expression is most abundant in rat heart, lung, and brain. When exogenously expressed in Xenopus oocytes, rTASK currents activate instantaneously, are noninactivating, and are not gated by voltage. Because rTASK currents satisfy the Goldman-Hodgkin-Katz current equation for an open channel, rTASK can be classified an open rectifier. Activation of protein kinase A produces inhibition of rTASK, whereas activation of protein kinase C has no effect. rTASK currents were inhibited by extracellular acidity. rTASK currents also were inhibited by Zn 2ϩ (IC 50 ϭ 175 M), the local anesthetic bupivacaine (IC 50 ϭ 68 M), and the anti-convulsant phenytoin (ϳ50% inhibition at 200 M). By demonstrating open rectification and open probability independent of voltage, we have established that rTASK is a baseline potassium channel.

sup-9, sup-10, andunc-93May Encode Components of a Two-Pore K+Channel that Coordinates Muscle Contraction inCaenorhabditis elegans

“…Mammalian TASK-1 and TASK-3 form a subfamily of two-pore K ϩ channels that are activated by high pH, volatile anesthetics, and neurotransmitters (Duprat et al, 1997;Leonoudakis et al, 1998;Patel et al, 1999;Kim et al, 2000;Millar et al, 2000;Rajan et al, 2000;Talley et al, 2000). Our findings indicate that UNC-93 and SUP-10 associate with a SUP-9 two-pore K ϩ channel and suggest that UNC-93 and SUP-10 may be regulatory subunits of this channel.…”

“…SUP-9 is 49 -51% identical in amino acid sequence to human TASK-1(KCNK3), TASK-3(KCNK9), and TASK-5(KCNK15) channels as well as to two predicted Drosophila proteins. Mammalian TASK-1 and TASK-3 behave as pH-sensitive background K ϩ channels when expressed heterologously in mammalian cell lines (Duprat et al, 1997;Kim et al, 1998;Leonoudakis et al, 1998;Rajan et al, 2000), whereas the properties of TASK-5 channels remain unknown. All other identified human two-pore K ϩ channels, such as TREK-1(KCNK2), share Ͻ30% amino acid identity with SUP-9.…”

TASK-3, a New Member of the Tandem Pore K+ Channel Family

“…Searching the GenBank TM data base using the BLAST sequence alignment program (26) indicated that the DNA sequence of the new clone is most similar to that of TASK-1, a 4TM K ϩ channel that was cloned earlier (20,23,27). A 2P/4TM K ϩ channel clone named TASK-2 has been described recently but has low homology with that of TASK-1 or the new clone (21).…”

TWIK-2, an Inactivating 2P Domain K+ Channel

“…TASK subunits are background K ϩ channels that are inhibited by mild external acidosis (3,7,23). The opening of TASK-1 is stimulated by inhalational anesthetics including halothane and isoflurane (11).…”