Targeted mutations in beta1,4-galactosyltransferase I reveal its multiple cellular functions.
Beta1,4-galactosyltransferase I (GalT I) is one of the most extensively studied glycosyltransferases. It is localized in the trans-Golgi compartment of most eukaryotic cells, where it participates in the elongation of oligosaccharide chains on glycoproteins and glycolipids. GalT I has also been reported in non-Golgi locations, most notably the cell surface, where it has been suggested to function non-biosynthetically as a receptor for extracellular glycoside substrates. Cloning of the GalT I cDNAs revealed that the gene encodes two similar proteins that differ only in the length of their cytoplasmic domains. Whether these different GalT I proteins, or isoforms, have similar or different biological roles is a matter of active investigation. The functions of the GalT I proteins have been addressed by targeted mutations that eliminate either both GalT I isoforms or just the long GalT I isoform. Eliminating both GalT I proteins abolishes most, but not all, GalT activity, an observation that led to the realization that other GalT family members must exist. The loss of both GalT I isoforms leads to neonatal lethality due to a wide range of phenotypic abnormalities that are most likely the result of decreased galactosylation. When the long isoform of GalT I is eliminated, galactosylation proceeds grossly normal via the short GalT I isoform, but specific defects in cell interactions occur that are thought to depend upon a non-biosynthetic function of the long GalT I isoform.
Rodeheffer
Carey
C
Department of Cell Biology, Emory University School of Medicine, Atlanta, GA 30322, USA.
Shur
Barry D
BD
D000818:Animals; D018390:Gene Targeting; D006801:Humans; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D016296:Mutagenesis; D009237:N-Acetyllactosamine Synthase / Q000235:genetics / Q000378:metabolism*
D007527:Isoenzymes; D009237:N-Acetyllactosamine Synthase