Potassium channels modulate cellular excitability by gating repolarizing potassium ion efflux. These channels function as tetrameric complexes, either by homomeric assembly of identical subunits or by heteromeric assembly of closely related subunits. The role of subunit assembly and mixing in the regulation of cellular excitability, in vivo, is poorly understood. The Shaw-1(
shw-1) gene was cloned by PCR using degenerate primers encoding conserved portions of voltage-gated potassium channel subunits.
shw-1 is a homologue of the Shaw (Kv3) subfamily of voltage-gated potassium channel subunits.
shw-1 maps to cosmid F14F11 (II), sequenced by the Genome Sequencing Consortium (GSC). Two additional Shaw-type genes,
shw-2 on R07A4 (X), and
shw-3 on R186 (V) are revealed by BLAST searches of the GSC database. An unusual genomic organization of
shw-1 was revealed by inspection of the F14F11 sequence. Five additional exons were discovered 0.5 Kb downstream of 11 exons encoding a complete subunit. These additional exons encode another near complete subunit, missing only a cytoplasmic N-terminal domain. This suggests alternative splicing to generate two transcripts,
shw-1a and
shw-1b, encoding two subunits that share identical N-terminal domains appended to different "cores" consisting of S1-S6, and C-terminal segments. This pattern of alternative splicing was confirmed by isolation of cDNAs correctly splicing all predicted alternative exons.
shw-1::GFP fusions express in neurons and body-wall muscle. Expression appears widespread at early developmental stages, with progressive restriction to fewer cells in adults.
shw-1 was targeted by Tc-1 mediated mutagenesis. One allele,
shw-1(
r1159), was recovered deleting
shw-1a specific exons, with no apparent mutant phenotype. This is consistent with a non-essential role for
shw-1a, in vivo. Screens for additional deletion alleles are underway. Xenopus oocytes injected with
shw-1a cRNA failed to produce functional channels. However, coinjection with cRNA encoding a human Shaw-type (Kv3.4) subunit suggests that
shw-1a can modify the functional properties of channels formed by Kv3.4 subunits. We hypothesize that
shw-1a cRNA encodes a regulatory "silent" subunit, which requires heteromeric assembly with a similar subunit to form functional channels. This hypothesis is presently being tested by coinjection with
shw-1b cRNA. The unique genomic organization of
shw-1 may insure expression of a fixed stiochiometry of
shw-1a and
shw-1b subunits necessary for normal function, in neurons and body-wall muscles.