title: Extracellular acid block and acid-enhanced inactivation of the Ca2+-activated cation channel TRPM5 involve residues in the S3-S4 and S5-S6 extracellular domains.
abstract: TRPM5, a member of the superfamily of transient receptor potential ion channels, is essential for the detection of bitter, sweet, and amino acid tastes. In heterologous cell types it forms a nonselective cation channel that is activated by intracellular Ca(2+). TRPM5 is likely to be part of the taste transduction cascade, and regulators of TRPM5 are likely to affect taste sensation. In this report we show that TRPM5, but not the related channel TRPM4b, is potently blocked by extracellular acidification. External acidification has two effects, a fast reversible block of the current (IC(50) pH = 6.2) and a slower irreversible enhancement of current inactivation. Mutation of a single Glu residue in the S3-S4 linker and a His residue in the pore region each reduced sensitivity of TRPM5 currents to fast acid block (IC(50) pH = 5.8 for both), and the double mutant was nearly insensitive to acidic pH (IC(50) pH = 5.0). Prolonged exposure to acidic pH enhanced inactivation of TRPM5 currents, and mutant channels that were less sensitive to acid block were also less sensitive to acid-enhanced inactivation, suggesting an intimate association between the two processes. These processes are, however, distinct because the pore mutant H896N, which has normal sensitivity to acid block, shows significant recovery from acid-enhanced inactivation. These data show that extracellular acidification acts through specific residues on TRPM5 to block conduction through two distinct but related mechanisms and suggest a possible interaction between extracellular pH and activation and adaptation of bitter, sweet, and amino acid taste transduction.
authors.affiliation: Division of Neurobiology, Department of Biological Sciences and Program in Neuroscience, University of Southern California, Los Angeles, California 90089, USA.
authors.forename: Dan
authors.initials: D
authors.lastname: Liu
authors.forename: Zheng
authors.initials: Z
authors.lastname: Zhang
authors.forename: Emily R
authors.initials: ER
authors.lastname: Liman
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002460:Cell Line; D002462:Cell Membrane / Q000737:chemistry; D004553:Electric Conductivity; D005110:Extracellular Space / Q000737:chemistry*; D015870:Gene Expression; D018698:Glutamic Acid / Q000737:chemistry; D049452:Green Fluorescent Proteins / Q000235:genetics; D006639:Histidine / Q000737:chemistry; D006801:Humans; D006863:Hydrogen-Ion Concentration; D042541:Intracellular Space / Q000737:chemistry; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D051379:Mice; D008969:Molecular Sequence Data; D016296:Mutagenesis; D011993:Recombinant Fusion Proteins; D016415:Sequence Alignment; D013329:Structure-Activity Relationship; D050053:TRPM Cation Channels; D013649:Taste / Q000502:physiology; D014162:Transfection
chemical_list: D008565:Membrane Proteins; D011993:Recombinant Fusion Proteins; D050053:TRPM Cation Channels; C404409:TRPM5 protein, human; C498617:Trpm5 protein, mouse; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; D018698:Glutamic Acid; D006639:Histidine; D002118:Calcium