We have shown previously that
egl-19 (aka.
eat-12,
pat-5) plays a key role in controlling muscle contraction and depolarization (Lee et al., WBG 13(1): 45). Putative null mutations of
egl-19 lead to a Pat phenotype, suggesting that embryonic muscles are unable to contract. Partial loss-of-function alleles show feeble contraction of pharyngeal, body, and egg-laying muscles while gain-of-function alleles lead to muscle hypercontraction. Based on the electrophysiological analysis of the pharyngeal phenotype, we argue that these contraction defects can be explained at least in part by defects in muscle excitation (loss-of-function) and repolarization (gain-of-function). We have cloned
egl-19 (Lobel et al., WBG 13(2): 71). It encodes a polypeptide of 1,787 amino acids and shares more than 60% sequence identity with the alpha1 subunit of L-type voltage-activated Ca2+ channels. To address the question of how
egl-19 affects muscle excitation, we localized its expression by constructing an
egl-19::GFP translational fusion. EGL-19::GFP was first detected in body muscles in 1-1/2-fold embryos, before the onset of embryonic rolling. This result is consistent with the Pat phenotype seen in null mutants, suggesting a cell-autonomous muscle defect. By hatching, GFP fluorescence is found in pharyngeal muscles
pm3,
pm4,
pm5, and
pm7; in body muscles; and in the anal depressor muscle. The muscle expression pattern is again consistent with a muscle cell-autonomous defect caused by mutations. Interestingly, however, we also found expression in the nervous system including the pharyngeal neuron M4 and many neurons in the nerve ring, the ventral nerve cord, and the pre-anal ganglion. We do not know the significance of the apparent nervous system expression since most of the phenotypes of
egl-19 mutants are consistent with defects of muscle function. (This is not surprising, as failure of muscle would hide most nervous system phenotypes.) One exception is that two gain-of-function alleles have a Daf-d phenotype. Although some dauers do form, their modification appears incomplete as they are less resistant to SDS treatment. We could not detect expression in egg-laying muscles. It may be that the level is too low, or that the promoter fragment we used lacks elements for vulval muscle expression. In order to learn at a molecular level how gain-of-function mutations of
egl-19 affect muscle repolarization, we sequenced the coding regions of
ad695sd and
n2368sd. We did not find any mutation in the ORF of
ad695sd. One possiblity is that
ad695sd may affect gene expression. In
egl-19(
n2368sd), however, we found a G->A mutation in the putative 6th transmembrane domain of repeat I (IS6). This mutation would change a conserved glycine residue to an arginine. The IS6 region has previously been shown to determine the rate of voltage-dependent inactivation of L-type channels expressed in Xenopus oocytes (Zhang et al., Nature 372: 97). The myotonic phenotype (delayed relaxation/repolarization) of
n2368sd can be reasonably explained by slower inactivation of muscle Ca2+ channels.