C. elegans embryonic intestinal cells contain birefringent gut granules that we show are equivalent to the autofluorescent lysosomes seen in the adult intestine. Gut granules stain with lysosomal markers and their formation is disrupted in mutants affecting lysosome biogenesis. Gut granule formation requires the activity of genes implicated in the AP-3 lysosomal trafficking pathway, including
apt-6,
apt-7,
vps-16, and
vps-41. Gut granule formation, however, does not appear to require the function of vps genes whose homologues are implicated in the yeast CPY endosomal trafficking pathway. To identify genes involved in lysosome biogenesis in C. elegans, we have isolated a collection of mutants that mislocalize birefringent material into the embryonic intestinal lumen. The glo (gut granule loss) mutants identified in our screen define at least three genes:
glo-1,
glo-2, and
glo-3. Here we present our phenotypic and molecular analysis of
glo-1. Both embryonic and adult stage
glo-1(-) animals lack acidified intestinal lysosomes. The C. elegans V-ATPase is localized to intestinal lysosomes and functions in their acidification. We have found that two subunits of the V-ATPase, VHA-11 and FUS-1, are not localized to intestinal organelles in
glo-1(-) embryos. In contrast,
glo-1(-) embryos appear to properly localize early endosomal (RME-1, RAB-5) and late endosomal (RME-8) markers. We have cloned the
glo-1 gene and found that it encodes a Rab GTPase most similar to mammalian Rab38 and Drosophila Rab-RP1. Both of these Rabs associate with, and are necessary for, the formation of specialized pigment containing lysosomes. We find that
glo-1 is expressed in the intestine from early embryogenesis through adulthood and the GLO-1 protein is localized to intestinal lysosomes. In searches of the C. elegans genome we have identified at putative guanine nucleotide exchange factor (GEF) that is most similar to Drosophila Claret, a Rab-RP1 GEF. Mutations in the C. elegans gene result in a Glo phenotype, suggesting that it might function as a GEF for GLO-1. Our studies show that the Glo phenotype provides a means to identify evolutionarily conserved genes that function during lysosomal trafficking and establish C. elegans gut granules as a model system for studying the molecular mechanisms controlling lysosomal biogenesis.