With apologies for the title, we would like to report that
unc-110 encodes a potassium channel subunit of the TWIK (2-pore) family. The first member of this poorly understood family to be defined by mutant analysis was
sup-9. We became interested in
unc-110 for its phenotypic similarity to other Mac (Muscle Activation) mutants. Mac genes were independently identified by several groups by semi-dominant mutations, most of which have been demonstrated to be gain-of-function, which we hypothesized exert their effects by altering muscle membrane excitability. Analysis of Mac mutants identifies genes that may contribute to membrane excitability, and the gain-of-function mutations provide insight into the activation of these gene products.
unc-110 was originally defined by the dominant allele
e1913, which causes a strong Unc phenotype when heterozygous and genetic lethality when homozygous.
unc-110 was previously mapped on LGX to the right of
unc-27 and to the left of
aex-2. We further mapped
unc-110 with respect to the two cloned genes
unc-115 and
egl-15. The strain
unc-110(
e1913sd) +
egl-15(
n484)/ +
unc-115(
e2225) + segregates 1/4 dead animals, 1/2
unc-110 non-Egl, and 1/4
unc-115 non-Egl.
unc-110 Egl recombinants from this strain are picked without bias for
unc-115, and the approximate physical position of
unc-110 is predicted from the ratio of + +
egl-15 to +
unc-115 egl-15 recombinant chromosomes. The data from 112 recombinants indicated that
unc-110 maps near a predicted potassium channel subunit of the TWIK family. We reasoned that a gain-of-function mutation in this gene could cause excessive potassium efflux in body-wall muscle, which would inhibit muscle membrane excitation and thereby prevent muscle contraction. By sequencing PCR products from wild-type and
unc-110 mutants, we found that
e1913 changes a conserved glycine to aspartate in the predicted coding region of this TWIK channel subunit. Additional alleles of
unc-110 have been isolated as revertants of
e1913sd, first by Danielle Thierry-Mieg and recently by us. Seven revertant alleles,
e2283 and
sa559 to
sa564 carry the missense mutation found in
e1913, and second site mutations have been found for several of the alleles, one of which is a nonsense mutation. Translational fusions of partial-length
unc-110 and over 3 Kb of promoter to gfp are expressed only in body-wall muscle, consistent with the dominant phenotype of defective body-wall muscle contraction. In addition, we have isolated a full-length cDNA in order to examine
unc-110-dependent potassium currents in Xenopus oocytes. Two fundamental questions remain: how do TWIK subunits assemble into channels, and how does the rubberband phenotype (a quick snap-back of the flaccid body-wall muscle when prodded) arise? For the former question, due to the molecular similarity of
unc-110 and
sup-9, we are examining possible genetic interactions of
unc-110 with the
unc-93 group. For the latter, we suggest that strong prodding activates excitatory motorneurons, which initiates excitation of body-wall muscle membrane. This excitation, however, is dampened in duration and magnitude because the change in voltage also leads to increased potassium efflux through TWIK channels, due to either/both a simple increase in potassium driving force through already opened channels, or from increased channel opening if, contrary to present notions, TWIK channel activation is modulated by voltage.