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P/Q-type (Ca v 2.1) calcium channels support a host of Ca 2ϩ -driven neuronal functions in the mammalian brain. Alternative splicing of the main ␣ 1A (␣ 1 2.1) subunit of these channels may thereby represent a rich strategy for tuning the functional profile of diverse neurobiological processes. Here, we applied a recently developed "transcript-scanning" method for systematic determination of splice variant transcripts of the human ␣ 1 2.1 gene. This screen identified seven loci of variation, which together have never been fully defined in humans. Genomic sequence analysis clarified the splicing mechanisms underlying the observed variation. Electrophysiological characterization and a novel analytical paradigm, termed strength-current analysis, revealed that one focus of variation, involving combinatorial inclusion and exclusion of exons 43 and 44, exerted a primary effect on current amplitude and a corollary effect on Ca 2ϩ -dependent channel inactivation. These findings significantly expand the anticipated scope of functional diversity produced by splice variation of P/Q-type channels.

The tetrodotoxin-binding component associated with the voltage-sensitive sodium channel from electroplax membranes of Electrophorus electricus has been purified. The toxin-binding site could be efficiently solubilized with Lubrol-PX, resulting in an extract of high initial specific activity.Purification was facilitated by the development of a rapid, quantitative binding assay. The binding component was stabilized during purification by the use of mixed lipid/detergent micelles of defined composition, and by the saturation of the site with tetrodotoxin.The purification was achieved by means of a highly selective adsorption of the toxin-binding component to DEAE-Sephadex A-25, followed by desorption at hig ionic strength and chromatography over Sepharose 6B. Final peak specific activities were at least 50% of the specific activity expected for a pure, undenatured toxin-binding component of 230,000 molecular weight.

Preparations of the tetrodotoxin (TTX) and saxitoxin binding protein isolated from the electroplax of Electrophorus electricus are of high specific activity (greater than or equal to 2000 pmol of TTX binding sites/mg of protein) and appear to be homogeneous in that they contain only the large polypeptide previously identified to make up part of the voltage-sensitive sodium channel [Agnew, W. S., Moore, A. C., Levinson, S. R., & Raftery, M. S. (1980) Biochem. Biophys. Res. Commun. 92, 860-866]. This permits the inference that the TTX binding site, thought to be associated with the mouth of the ion channel, is located on this peptide. This peptide presumably corresponds to the large peptide, designated the alpha-peptide subunit, of the synaptosomal sodium channel [Hartshorne, R. P., & Catterall, W. A. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 4620-4624]. No convincing evidence for lower molecular weight peptides has yet been found for the electroplax protein. A rapid and convenient method is described for preparation of milligram quantities of the pure, sodium dodecyl sulfate (NaDodSO4) denatured form of the peptide, and its amino acid and carbohydrate compositions are reported. The peptide behaved anomalously on NaDodSO4-polyacrylamide gels. It was demonstrated that the molecular weight cannot be accurately quantified by this method but that the true value likely exceeds the value of 260 000 reported previously. The denatured peptide displayed an electrophoretic microheterogeneity which may be ascribed to variations in bulky carbohydrate substituents and an extremely high free mobility which is inferred to result from binding of unusually large amounts of NaDodSO4.

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