Sec1/Munc-18 (SM) protein family is involved in various vesicular trafficking from yeast to mammals, but its function is less well understood. To investigate systematically molecular structure and function of SM family in multicellular organism, we isolated deletion mutants of SM genes and found that four of six SM gene mutations lead to a lethal phenotype. Hence, SM genes appear to be also indispensable factors for intracellular trafficking in C. elegans. The putative null mutation of
vps-45 (
tm246), whose orthologous protein in yeast plays an important role in membrane trafficking from the Golgi to vacuole, showed a ts lethality and defects of endocytosis. Maternal rescued homozygotes (m+ z-) exhibit a Cup (coelomocyte uptake defective) and Rme (receptor mediated endocytosis defective) phenotypes in restrictive temperature, suggesting that
vps-45 may be required for a common pathway between Rme and non-Rme endocytosis. Most SM proteins interact with syntaxin family, a subclass of Q-SNAREs, and may regulate SNARE complex formation. To identify the cognate syntaxin for C. elegans
vps-45, we performed co-immunoprecipitation and yeast two hybrid studies between
vps-45 and eight syntaxin genes identified on the C. elegans genome (Jantsch-Plunger and Glotzer, 1999). We found that
vps-45 exhibits specific interactions with
tlg-2 and
pep-12. To elucidate whether
vps-45 functions with the cognate syntaxins in vivo, we isolated deletion mutations of these syntaxins.
tlg-2 (
tm1560),
pep-12 (
tm1198,
tm1764),
tlg-2;
pep-12 were viable, and endocytic trafficking in coelomocytes of these mutant animals were not severely blocked. These results suggest that specific binding of SM proteins to syntaxins has probably important aspects, but SM proteins might also have syntaxin-independent functions in endocytic trafficking. Biosynthetic pathways from Golgi to lysosome in coelomocytes of the isolated mutants are now analyzed using a visible marker of cathepsin D/ fluorescent fusion protein.