ISOLATION OF A HUMAN-BRAIN SODIUM-CHANNEL GENE ENCODING 2 ISOFORMS OFTHE SUBTYPE-III ALPHA-SUBUNIT

Voltage-gated sodium channels are members of a multigene family of tra nsmembrane proteins that are important determinants of electrical exci tability in cell membranes. These proteins are typically composed of a large alpha-subunit and one or two beta-subunits. The primary structu re of alpha-subunits is highly conserved among different subtypes and different species. Based on the conserved sequences and application of the rapid amplification of cDNA ends (RACE) reaction, we have isolate d three overlapping clones from human brain. These sequences share hig hest homology (89%) to the rat brain subtype III gene and cover a 4.2- kb expanse of the transcript. The 5'-most clone has a translation star t site located in the same region as other mammalian brain sodium chan nel genes. A 92-nucleotide insert was found in domain I at a location previously demarcated by published splice sites in rat brain sodium ch annels IIN/IIA and IIIN/IIIA. It is most likely that this transcript r epresents the two isoforms (neonatal and adult) of the human brain sod ium channel gene, SCN3A (GenBank accession numbers AF035685 and AF0356 86). As is the case for rat brain sodium channels IIN/IIA and IIIN/III A, these isoforms are generated through an alternative splicing mechan ism. The conservation of the exon structure suggests that alternative RNA splicing is a common feature for sodium channel mRNA processing an d may play an important role in modulating the channel function.