Distinct properties and differential β subunit regulation of two C-terminal isoforms of the P/Q-type Ca 2+ -channel α 1A subunit

Description

Two C-terminal splice variants (BI-1 and BI-2, now termed Cav2.1a and Cav2.1b) of the neuronal voltage-gated P/Q-type Ca2+channel a1A pore-forming subunit have been cloned (Mori et al., 1991, Nature, 350, 398±402). BI-1 and BI-2 code for proteins of2273 and 2424 amino acids, respectively, and differ only by their extreme carboxyl-termini sequences. Here, we show that, inXenopus oocytes, the two isoforms direct the expression of channels with different properties. Electrophysiological analysisshowed that BI-1 and BI-2 have peak Ba2+ currents (IBa) at a potential of +30 and +20 mV, respectively. The different C-terminalsequence (amino acids 2229±2273) of BI-1 caused a shift in steady-state inactivation by +10 mV and decreased the proportion offast component of current inactivation twofold. Likewise, the biophysical changes in IBa caused by coexpression of the b4 auxiliarysubunit were substantially different in BI-1- and BI-2-containing channels in comparison to those induced by b3. Several of thesedifferences in b regulation were abolished by deleting the carboxyl-terminal splicing region. By creating a series of GST fusionproteins, we identi®ed two locations in the C-terminal (Leu2090±Gly2229 for BI-1 and BI-2, and Arg2230±Pro2424 for BI-2 only)that determine the differential interaction of b4 with the distinct a1A isoforms. These interactions appear to favour the binding of b4to the AID site, and also the plasma membrane expression of BI-2. These results demonstrate that the ®nal segment of theC-terminal affects a1A channel gating, interaction and regulation with/by the b subunits. The data will have several implications forthe understanding of the biophysical effects of many channelopathies in which the carboxyl-termini of a1A and b4 are affected.

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