In mutants defective in any of eight Caenorhabditis elegans sqv (squashed vulva) genes, the vulval extracellular space fails to expand during vulval morphogenesis. Strong sqv mutations result in maternal-effect lethality, caused in part by the failure of the progeny of homozygous mutants to initiate cytokinesis and associated with the failure to form an extracellular space between the egg and the eggshell. Recent studies have implicated glycosaminoglycans in these processes. Here we report the cloning and characterization of sqv-2 and sqv-6. sqv-6 encodes a protein similar to human xylosyltransferases. Transfection of sqv-6 restored xylosyltransferase activity to and rescued the glycosaminoglycan biosynthesis defect of a xylosyltransferase mutant hamster cell line. sqv-2 encodes a protein similar to human galactosyltransferase II. A recombinant SQV-2 fusion protein had galactosyltransferase II activity with substrate specificity similar to that of human galactosyltransferase II. We conclude that C. elegans SQV-6 and SQV-2 likely act in concert with other SQV proteins to catalyze the stepwise formation of the proteoglycan core protein linkage tetrasaccharide GlcA1,3Gal1, 3Gal1,4Xyl-O-(Ser), which is common to the two major types of glycosaminoglycans in vertebrates, chondroitin and heparan sulfate. Our results strongly support a model in which C. elegans vulval morphogenesis and zygotic cytokinesis depend on the expression of glycosaminoglycans.
Glycosaminoglycans (GAGs)1 are important in animal development, and defects in GAGs are responsible for certain human disorders. For example, mutations in the Drosophila melanogastger genes tout-velu (1) and sulfateless (2), which encode homologs of heparan sulfate co-polymerase and heparan sulfate N-deacetylase/N-sulfotransferase, respectively, cause zygotic lethality and defects in segmentation. Mutations in the mouse tout-velu homolog EXT1 disrupt gastrulation and the generation of mesoderm (3), while mutations in human EXT1 and EXT2 have been associated with hereditary multiple exostoses (reviewed in Ref. 4). Mutations in the human galactosyltransferase I have been associated with a progeroid variant of the connective-tissue disorder Ehlers-Danlos syndrome (EDS) (5-7). EDS is a group of heritable disorders characterized by hyperelasticity of the skin and hypermobile joints. Tout-velu, EXT-1, EXT-2, and Sulfateless affect the biosynthesis of heparan sulfate specifically, while galactosyltransferase I deficiency affects the biosynthesis of both chondroitin and heparan sulfate.The backbones of chondroitin and heparan sulfate consist of repeating disaccharide units: GlcA1,3GalNAc1,4 for chondroitin and GlcA1,4GlcNAc␣1,4 for heparan sulfate (reviewed in Ref. 8). Their polymerization occurs on a tetrasaccharide primer (GlcA1,3Gal1,3Gal1,4Xyl-) that is linked to the protein core of a proteoglycan. The addition of these four sugars is catalyzed stepwise in the lumen of the Golgi apparatus and requires three nucleotide sugars, UDP-Xyl, UDP-Gal, and UDP-GlcA, and fou...