To analyze the process of vulval morphogenesis, Herman and Horvitz (Cold Spring Harbor Quant. Biol. 62, 353, 1997) identified 25 mutants defective in vulval invagination but normal in vulval cell lineages. These mutants define eight genes,
sqv-1 to 8 (sqv, squashed vulva), the primary phenotype of which is reduced separation between the anterior and posterior halves of the vulva in the L4. Some of the mutants have additional defects: strong alleles of all sqv genes cause maternal-effect lethality, and
sqv-3 mutations cause male tail morphogenetic defects. Herman and Horvitz reported that three of the sqv genes encode proteins involved in glycosylation. SQV-3 is similar to a family of glycosyltransferases that add sugars onto glycoproteins and glycolipids. SQV-8 is similar to a UDP-glucuronosyltransferase that adds glucuronic acid to glycoproteins. SQV-7 is similar to LPG2, a Leishmania donovani putative Golgi GDP-mannose transporter. We have cloned and characterized two additional sqv genes.
sqv-1 encodes a 467 amino acid protein similar to a group of nucleotide-sugar biosynthetic enzymes, including UDP-glucose epimerases and dTDP-glucose dehydratases.
sqv-4 encodes a 478 amino acid protein similar to UDP-glucose dehydrogenases, which convert UDP-glucose to UDP-glucuronic acid. The expression patterns of
sqv-1::GFP and
sqv-4::GFP suggest that both genes are expressed in many tissues, including gut, neurons and the vulva.
sqv-4 cDNA expressed under heat-shock promoters can rescue the Sqv-4 mutant phenotype: heat-shock treatment of L1 and L2 larvae rescues the vulval defect but not the maternal-effect lethality, while heat-shock treatment of L3 and L4 larvae and adults can rescue maternal-effect lethality but not necessarily the vulval defect. We propose that SQV-1 and SQV-4 synthesize nucleotide sugars that are transported into the Golgi by nucleotide-sugar transporter(s), including SQV-7, then used as substrates for glycosyltransferases including SQV-3 and SQV-8. We speculate that glycosyltransferases encoded by
sqv-3 and
sqv-8 and possibly other yet-to-be-cloned sqv genes act on a small set of glycoconjugates, some of which modify cell-cell or cell-matrix interactions to modulate the vulval invagination process. The maternal-effect lethality of the sqv mutants may be caused by a defect in a biological process that shares components of a glycosylation pathway involved in vulval invagination. We are currently studying the biochemical properties of the cloned SQV proteins and searching for the target(s) of SQV-mediated glycosylation.