L. Toma et al., TRANSPORT OF UDP-GALACTOSE INTO THE GOLGI LUMEN REGULATES THE BIOSYNTHESIS OF PROTEOGLYCANS, The Journal of biological chemistry, 271(7), 1996, pp. 3897-3901
The lumen of the Golgi apparatus is the subcellular site where galacto
se is transferred, from UDP-galactose, to the oligosaccharide chains o
f glycoproteins, glycolipids, and proteoglycans. The nucleotide sugar,
which is synthesized in the cytosol, must first be transported into t
he Golgi lumen by a specific UDP-galactose transporter, Previously, a
mutant polarized epithelial cell (MDCKII-RCA(r)) with a 2% residual ra
te of transport of UDP-galactose into the lumen of Golgi vesicles was
described (Brandli, A. W., Hansson, G. C., Rodriquez-Boulan, E., and S
imons, K. (1988) J. Biol. Chem. 263, 16283-16290). The mutant has an e
nrichment in glucosyl ceramide and cell surface glycoconjugates bearin
g terminal N-acetylglucosamine, as well as a 75% reduction in sialylat
ion of cell surface glycoproteins and glycosphingolipids. We have now
studied the biosynthesis of galactose containing proteoglycans in this
mutant and the corresponding parental cell line. Wild-type Madin-Darb
y canine kidney cells synthesize significant amounts of chondroitin su
lfate, heparan sulfate, and keratan sulfate, while the above mutant sy
nthesizes chondroitin sulfate and heparan sulfate but not keratan sulf
ate, the only proteoglycan containing galactose in its glycosaminoglyc
an polymer, The mutant also synthesizes chondroitin 6-sulfate rather t
han only chondroitin 4-sulfate as wild-type cells. Together, the above
results demonstrate that the Golgi membrane UDP-galactose transporter
is rate-limiting in the supply of UDP-galactose into the Golgi lumen;
this in turn results in selective galactosylation of macromolecules.
Apparently, the K-m for galactosyltransferases involved in the synthes
is of Linkage regions of heparan sulfate and chondroitin sulfate are s
ignificantly lower than those participating in the synthesis of kerata
n sulfate polymer, glycoproteins, and glycolipids. The results also su
ggest that the 6-O-sulfotransferases, in the absence of their natural
substrates (keratan sulfate) may catalyze the sulfation of chondroitin
4-sulfate as alternative substrate.