The role of Ca2+-activated K+ channel spliced variants in the tonotopic organization of the turtle cochlea.
1. Turtle auditory hair cells contain multiple isoforms of the pore-forming alpha-subunit of the large-conductance Ca2+-activated K+ (KCa) channel due to alternative splicing at two sites. Six splice variants were studied by expression in Xenopus oocytes. 2. The isoforms possessed differences in apparent Ca2+ sensitivity and kinetics. The lowest Ca2+ sensitivity was observed in a novel variant resulting from a 26 amino acid deletion around one of the splice sites. 3. Co-expression of a bovine beta-subunit slowed the current relaxation 10-fold compared with channels formed from alpha-subunits alone but preserved the original order of kinetic differences. The beta-subunit also increased the Ca2+ sensitivity of isoforms to bring them nearer the range of sensitivity of the native KCa channels of the hair cell. 4. With channels formed from alpha-subunits or alpha + beta-subunits, the half-activation voltage in a fixed Ca2+ concentration, and the time constant of the current relaxation, varied linearly with the combined size of the insertions/deletions at the splice sites. 5. Experiments in which the beta/alpha concentration ratio was varied indicated that the beta-subunit exerts an all-or-none effect on the Ca2+ sensitivity and kinetics of the channel. 6. Co-expression of an avian beta2-subunit had effects on kinetics and Ca2+ sensitivity of several alpha-isoforms which were qualitatively similar to those produced by the bovine beta-subunit. 7. We conclude that differential expression of alternatively spliced alpha-subunit variants and a non-uniform distribution of a beta-subunit can produce a range of KCa channel properties needed to explain the tonotopic organization of the turtle cochlea.
Department of Physiology, University of Wisconsin Medical School, Madison, WI 53706, USA.
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Jones
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Gray-Keller
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Fettiplace
D000161:Acoustic Stimulation; D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D002642:Chick Embryo; D003051:Cochlea / Q000502:physiology*; D004594:Electrophysiology; D006199:Hair Cells, Auditory, Inner / Q000378:metabolism / Q000502:physiology; D007536:Isomerism; D007700:Kinetics; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D051038:Large-Conductance Calcium-Activated Potassium Channel beta Subunits; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008969:Molecular Sequence Data; D009865:Oocytes / Q000378:metabolism; D015221:Potassium Channels / Q000235:genetics / Q000502:physiology*; D024681:Potassium Channels, Calcium-Activated; D011784:Quail; D012326:RNA Splicing; D012333:RNA, Messenger / Q000096:biosynthesis; D017384:Sequence Deletion / Q000235:genetics / Q000502:physiology; D014426:Turtles / Q000502:physiology*; D014981:Xenopus
D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D051038:Large-Conductance Calcium-Activated Potassium Channel beta Subunits; D051036:Large-Conductance Calcium-Activated Potassium Channels; D015221:Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D012333:RNA, Messenger