The Caenorhabditis elegans Genes sqv-2and sqv-6, Which Are Required for Vulval Morphogenesis, Encode Glycosaminoglycan Galactosyltransferase II and Xylosyltransferase

Ho-Yon Hwang,Sara K Olson,Jillian R Brown,Jeffrey D Esko,H Robert Horvitz

doi:10.1074/jbc.c200518200

Abstract

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 GlcAbeta1,3Galbeta1, 3Galbeta1,4Xylbeta-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.

Highlights

From the ‡Howard Hughes Medical Institute, Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 and the §Department of Cellular and Molecular Medicine, Glycobiology Research and Training Center, University of California, San Diego, La Jolla, California 92093-0687
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 GlcA␤1,3Gal␤1, 3Gal␤1,4Xyl␤-O-(Ser), which is common to the two ma
Our results strongly support a model in which C. elegans vulval morphogenesis and zygotic cytokinesis depend on the expression of glycosaminoglycans

Summary

Accelerated Publication

The Caenorhabditis elegans Genes sqv-2 and sqv-6, Which Are Required for Vulval Morphogenesis, Encode Glycosaminoglycan Galactosyltransferase II and Xylosyltransferase*□S. 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. The backbones of chondroitin and heparan sulfate consist of repeating disaccharide units: GlcA␤1,3GalNAc␤1,4 for chondroitin and GlcA␤1,4GlcNAc␣1,4 for heparan sulfate (reviewed in Ref. 8) Their polymerization occurs on a tetrasaccharide primer (GlcA␤1,3Gal␤1,3Gal␤1,4Xyl␤-) that is linked to the protein core of a proteoglycan. The nuclei of the arrested progeny divide normally, but the extrusion of the polar bodies and the initiation of cytokinesis are impaired [10] These mutant eggs fail to form the normal fluid-filled extracellular space between the membrane of the egg and the eggshell. We show that sqv-6 encodes the xylosyltransferase that adds Xyl to the protein core, initiating GAG biosynthesis. sqv-2 encodes a galactosyltransferase that adds the second Gal residue to the linkage tetrasaccharide

EXPERIMENTAL PROCEDURES

RESULTS AND DISCUSSION

Enzyme activity