A MURINE NEURAL-SPECIFIC HOMOLOG CORRECTS CHOLINERGIC DEFECTS IN CAENORHABDITIS-ELEGANS UNC-18 MUTANTS

Caenorhabditis elegans UNC-18 protein, homologous to yeast Sec1p, is i mportant in neurotransmitter release, because the unc-18 mutation lead s to severe paralysis and presynaptic acetylcholine (ACh) accumulation . To examine the functional conservation in mammals, we tried to isola te unc-18 isoforms from mouse and human brain cDNA libraries and obtai ned two classes of isoforms-neural genes and ubiquitous genes. Neural genes were identical to Munc-18 (also known as n-Sec1 or rbSec1), iden tified in rat and bovine brains as a syntaxin-binding protein. Accordi ng to ''Munc-18'' terminology, we call the neural genes Munc-18-1 and the ubiquitous genes Munc-18-3. These mammalian isoforms exhibit 58% ( Munc-18-1) and 42-43% (Munc-18-3) amino acid sequence identity with UN C-18. Next, we constructed transgenic unc-18 mutants to test biologica l activity of mouse Munc-18-1 and Munc-18-3 under the control of C. el egans unc-18 promoter. Munc-18-1 compensates for severe locomotion dis ability and cholinergic defects, e.g., abnormal sensitivities to choli nesterase inhibitors and cholinergic receptor agonists in unc-18 mutan ts, but Munc-18-3 fails. These data suggest that Munc-18-1 and C. eleg ans unc-18 may play positive roles in ACh release and that the molecul ar mechanism of neuronal regulated secretion has been partially conser ved from nematodes to mammals.