Shaker (Sh) mutants of Drosophila suffer from a characteristic leg-shaking behavioral defect. Previous genetic and physiological experiments suggested that Sh encodes at least one component of a fast, transient, or A-type K⁺ channel. To address questions pertaining to the structure, function and heterogeneity of K⁺ channels, we have undertaken a molecular analysis of Sh. We have isolated molecular clones for the genomic region encompassing Sh as part of a 350 kb chromosomal walk. Using a combination of classical and molecular genetics, we have mapped several Sh mutations within this region, and localized the Sh gene. Sh mutations scatter over at least 65 kb of genomic DNA, and the Sh gene itself is large, spanning at least 95 kb. Comparative studies on a collection of Sh cDNA clones show that Sh encodes a diverse array of gene products. The basis for this diversity is a mechanism that generates a limited number of different 5' and 3' end segments, and splices these segments onto a central constant region. This differential splicing mechanism produces at least 10, and possibly 28 or more, predicted Sh proteins that differ at the carboxyl and/or amino terminus. The primary structures of Sh proteins deduced from the cDNAs reveal two general types of polypeptide: a protein that contains seven potential membrane-spanning domains, including a positively charged segment that is similar to a sequence called S4 in Na⁺ channels, and a smaller protein that lacks S4 and contains only three potential membrane-spanning regions. Variants of the flrst protein type range in size from 493 a.a. to 656 a.a., whereas variants of the second type range from 303 a.a. to 337 a.a. These polypeptides may assemble as homomultimers and/or as heteromultimers to produce K⁺ channels with different features.
