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J Biol Chem, Vol. 274, Issue 14, 9296-9304, April 2, 1999
1,4-GALACTOSYLTRANSFERASE I
From the Glycobiology Program, Cancer Research Center, The Burnham
Institute, La Jolla, California 92037 and the § School of
Dentistry, University of Copenhagen, DK-2200 Copenhagen, Denmark
I-branched poly-N-acetyllactosamine
is a unique carbohydrate composed of N-acetyllactosamine
branches attached to linear poly-N-acetyllactosamine, which
is synthesized by I-branching
1,6-N-acetylglucosaminyltransferase. I-branched
poly-N-acetyllactosamine can carry bivalent functional oligosaccharides such as sialyl Lewisx, which provide much
better carbohydrate ligands than monovalent functional
oligosaccharides. In the present study, we first demonstrate that
I-branching 1,6-N-acetylglucosaminyltransferase cloned
from human PA-1 embryonic carcinoma cells transfers 1,6-linked
GlcNAc preferentially to galactosyl residues of
N-acetyllactosamine close to nonreducing terminals. We then
demonstrate that among various 1,4-galactosyltransferases
(4Gal-Ts), 4Gal-TI is most efficient in adding a galactose to
linear and branched poly-N-acetyllactosamines. When a
1,6-GlcNAc branched poly-N-acetyllactosamine was
incubated with a mixture of 4Gal-TI and i-extension
1,3-N-acetylglucosaminyltransferase, the major product
was the oligosaccharide with one N-acetyllactosamine extension on the linear Gal1
4GlcNAc13 side chain. Only a
minor product contained galactosylated I-branch without
N-acetyllactosamine extension. This finding was explained
by the fact that 4Gal-TI adds a galactose poorly to 1,6-GlcNAc
attached to linear poly-N-acetyllactosamines, while
1,3-N-acetylglucosaminyltransferase and 4Gal-TI
efficiently add N-acetyllactosamine to linear
poly-N-acetyllactosamines. Together, these results strongly
suggest that galactosylation of I-branch is a rate-limiting step in
I-branched poly-N-acetyllactosamine synthesis, allowing
poly-N-acetyllactosamine extension mostly along the linear
poly-N-acetyllactosamine side chain. These findings are
entirely consistent with previous findings that
poly-N-acetyllactosamines in human erythrocytes, PA-1
embryonic carcinoma cells, and rabbit erythrocytes contain multiple,
short I-branches.
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