Evidence for two concentration-dependent processes for beta-subunit effects on alpha1B calcium channels.
beta-Subunits of voltage-dependent Ca(2+) channels regulate both their expression and biophysical properties. We have injected a range of concentrations of beta3-cDNA into Xenopus oocytes, with a fixed concentration of alpha1B (Ca(V)2.2) cDNA, and have quantified the corresponding linear increase of beta3 protein. The concentration dependence of a number of beta3-dependent processes has been studied. First, the dependence of the a1B maximum conductance on beta3-protein occurs with a midpoint around the endogenous concentration of beta3 (approximately 17 nM). This may represent the interaction of the beta-subunit, responsible for trafficking, with the I-II linker of the nascent channel. Second, the effect of beta3-subunits on the voltage dependence of steady-state inactivation provides evidence for two channel populations, interpreted as representing alpha1B without or with a beta3-subunit, bound with a lower affinity of 120 nM. Third, the effect of beta3 on the facilitation rate of G-protein-modulated alpha1B currents during a depolarizing prepulse to +100 mV provides evidence for the same two populations, with the rapid facilitation rate being attributed to Gbetagamma dissociation from the beta-subunit-bound alpha1B channels. The data are discussed in terms of two hypotheses, either binding of two beta-subunits to the alpha1B channel or a state-dependent alteration in affinity of the channel for the beta-subunit.
Cantí
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Department of Pharmacology, University College London, London WC1E 6BT, United Kingdom.
Davies
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A
Berrow
N S
NS
Butcher
A J
AJ
Page
K M
KM
Dolphin
A C
AC
D000818:Animals; D015220:Calcium Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary / Q000235:genetics / Q000378:metabolism; D018491:Dopamine Agonists / Q000494:pharmacology; D004553:Electric Conductivity; D015870:Gene Expression; D020962:Heterotrimeric GTP-Binding Proteins / Q000378:metabolism; D015640:Ion Channel Gating / Q000187:drug effects; D008564:Membrane Potentials; D009154:Mutation; D016376:Oligonucleotides, Antisense / Q000235:genetics; D009865:Oocytes / Q000378:metabolism; D011485:Protein Binding; D021122:Protein Subunits; D019257:Quinpirole / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D020349:Surface Plasmon Resonance; D014982:Xenopus laevis
D015220:Calcium Channels; D018076:DNA, Complementary; D018491:Dopamine Agonists; D016376:Oligonucleotides, Antisense; D021122:Protein Subunits; D011994:Recombinant Proteins; D019257:Quinpirole; D020962:Heterotrimeric GTP-Binding Proteins