The Sec1/Munc18 (SM) family members are essential factors that regulate docking/fusion of membrane-bound organelles in various eukaryotic cells. To elucidate the functions of SM proteins in membrane trafficking and organelle biogenesis in a multicellular organism, we have systematically isolated deletion mutations for C. elegans</I> SM genes (
vps-33.1</I>,
vps-33.2</I>,
vps-45</I>, F43D9.3 and T07A9.10). In this study, we focused on the SM genes,
vps-33.1</I> and
vps-45</I>. The Vps33p is a yeast class C vacuolar protein sorting (Vps) protein that is involved in the vacuolor biogenesis and the protein trafficking to vacuoles. There are two homologues of Vps33p on the C. elegans</I> genome:
vps-33.1</I> (homologous to human VPS33A) and
vps-33.2</I> (homologous to human VPS33B). The buff</I> mouse caused by a mutation in VPS33A gene has been suggested as an animal model for Hermansky-Pudlak syndrome (HPS) with a hypopigmentation phenotype. We found that the putative null mutation of
vps-33.1</I> results in a maternal effect embryonic lethality. The
vps-33.1</I> (m-z-</I>) embryos were arrested before morphogenesis with complete penetrance. The maternally rescued hermaphrodites exhibited the coelomocyte uptake defective (Cup) phenotypes. To investigate which organelles are affected in the mutant coelomocytes, we used the organelle-specific GFP markers. We found that both the endosomes and the lysosomes are markedly abnormal in the
vps-33.1</I> mutants. These endocytic phenotypes were rescued by expression of a functional fusion gene,
vps-33.1::mRFP</I>. Another SM gene,
vps-45</I>, whose mutant showed a larval lethal phenotype. Transgenic analyses using fluorescent reporters revealed that both fluid-phase and receptor-mediated endocytosis pathways are impaired in the
vps-45</I> mutant. In the
vps-45</I> coelomocytes, aberrantly small endosomes were accumulated, however, size of lysosomes was almost normal. These results suggest that VPS-33.1 and VPS-45 may have important roles in the regulation of the endosomal/lysosomal system via specific functions. It has previously been suggested that SM proteins directly interact with syntaxins (t-SNAREs) and Rab effectors in the docking/fusion step. To elucidate the functional relationships between these SM proteins and their partners, biochemical and knockout studies are in progress.