Poly-N-acetyllactosamine synthesis in branched N-glycans is controlled by complemental branch specificity of i-extension enzyme and beta 1,4-galactosyltransferase I
M. Ujita et al., Poly-N-acetyllactosamine synthesis in branched N-glycans is controlled by complemental branch specificity of i-extension enzyme and beta 1,4-galactosyltransferase I, J BIOL CHEM, 274(24), 1999, pp. 16717-16726
Poly-N-acetyllactosamine is a unique carbohydrate that can carry various fu
nctional oligosaccharides, such as sialyl Lewis X. It has been shown that t
he amount of poly-N-acetyllactosamine is increased in N-glycans, when they
contain Gal beta 1 --> 4GlcNAc beta 1 --> 6(Gal beta 1 --> 4GlcNAc beta 1 -
-> 2)Man alpha 1 --> branched structure. To determine how this increased sy
nthesis of poly-N-acetyllactosamines takes place, the branched acceptor was
incubated with a mixture of i-extension enzyme (iGnT) and beta 1,4-galacto
syltransferase I(beta 4Gal-TI), First, N-acetyllactosamine repeats were mor
e readily added to the branched acceptor than the summation of poly-N-acety
llactosamines formed individually on each unbranched acceptor. Surprisingly
, poly-N-acetyllactosamine was more efficiently formed on Gal beta 1 --> 4G
lcNAc beta 1 --> 2Man alpha --> R side chain than in Gal beta 1 --> 4GlcNAc
beta 1 --> 6Man alpha --> R, due to preferential action of iGnT on Gal bet
a 1 --> 4GlcNAc beta 1 --> 2Man alpha --> R side chain. On the other hand,
galactosylation was much more efficient on pl,G-linked GlcNAc than beta 1,a
-linked GlcNAc, preferentially forming Gal beta 1 --> 4GlcNAc beta 1 --> 6(
GlcNA beta 1 --> 2)Man alpha 1 --> 6Man beta --> R. Starting with this pref
ormed acceptor, N-acetyllactosamine repeats were added almost equally to Ga
l beta 1 --> 4GlcNAc beta 1 --> 6Man alpha --> R and Gal beta 1 --> 4GlcNA
beta 1 --> 2Man alpha --> R side chains. Taken together, these results indi
cate that the complemental branch specificity of iGnT and beta 4Gal-TI lead
s to efficient and equal addition of N-acetyllactosamine repeats on both si
de chains of GlcNAc beta 1 --> 6(GlcNAc beta 1 --> 2)Man alpha 1 --> 6Man b
eta --> R structure, which is consistent with the structures found in natur
e. The results also suggest that the addition of Gal beta 1 --> 4GlcNA beta
1 --> 6 side chain on Gal beta 1 --> 4GlcNAc beta 1 --> 2Man --> R side ch
ain converts the acceptor to one that is much more favorable for iGnT and b
eta 4Gal-TI.