Figure 1. Structure of hair cell K_Ca channel splice variants.

A, schematic diagram of the K_Ca channel α-subunit, showing seven transmembrane domains, a pore (P) region, and an extended intracellular C-terminus (Meera et al. 1997). Positions of a putative Ca²⁺-binding site (Schreiber & Salkoff, 1997) and the two splice sites, SS1 and SS2, are indicated. The amphipathic helical domains S7 to S10 are drawn arbitrarily in antiparallel configuration. B, polyacrylamide gel of PCR products from hair cells isolated from four quarters of the basilar papilla, lane 1 corresponding to apical low-frequency cells and lane 4 to basal high-frequency cells. Turtle mRNA was amplified with a forward primer specific for SS1 = 4 (SRKR), and a reverse primer downstream of SS2 (see Methods). This revealed four of the splice variants including the 4:-26 that contains a 26 amino acid deletion around SS2. The right-hand lane contains 100 bp size markers. The 31 amino acid insert in the 31:3 variant is composed of 4 amino acids (SRKR) + 27 amino acids. Note that each variant shows a different distribution. C, amino acid sequences in the region around SS1 and SS2 of five of the isoforms studied, with the inserts denoted in bold. The asterisks mark the donor and acceptor positions for a new splice site (see text). A sixth variant, 0:3, contained IYF in SS2. thc1, 3, 4, 5, 7 refer to the naming of the turtle hair cell Slo variants in the GenBank database.