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GM3 synthase
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Ganglioside GM3 is a common precursor of the major ganglio-series
ganglio-sides, and is distributed in almost all mammalian tissues.
GM3 is synthesized by the transfer of sialic acid from CMP-sialic
acid to non-reduced terminal galactose residue of lactosylceramide
through the alpha 2,3 glycosyl bond, and the reaction is catalyzed
by GM3 synthase (CMP-NeuAc:lactosylceramide alpha 2,3-sialyltransferase:
EC 2.4.99.9). The enzyme is active at the branch point of the
extension of the sugar chain on glycosphingolipids, and the regulatory
expression of SAT-I activity is considered to affect the biosynthesis
not only for ganglio-series but also for lacto/neolacto-,globo-
and/or isogloboseries glycosphingolipids. |
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| Fig.1 GM3 synthase competes with four other glycosyltransferases
and one sulfotransferase for a common substrate, lactosylceramide.
Expression of each transferase is regulated developmentally and
spatially in the process of embryogenesis, differentiation and
carcinogenesis, resulting in proper composition of glycosphingolipids
in each tissue. |
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In 1991, Melkerson-Watson and Sweeley reported that SAT-I
was enriched to 43,000-fold from rat liver, and in 1993, the
enzyme was purified to 7,200-fold from rat brain by Preuss et
al. It is unclear whether the two purified enzymes are the same
protein, since they showed differences in molecular weight, substrate
specificity, etc. Gu et al. proposed the possibility that SAT-I
activity might be regulated through phosphorylation/dephosphorylation
mechanisms.
One of the biological functions of GM3 is the differentiation
induction of certain cells. During the monocytic differentiation
of HL-60 cells with phorbol ester as the differentiation inducer,
the level of GM3 is dramatically enhanced with a concomitant
increase in GM3 synthase activity. Furthermore, the treatment
of HL-60 cells with GM3 induced differentiation into monocytoid,
as shown in the treatment of HL-60 cells with phorbol ester.
In 1997, the SAT-I gene was cloned by a modified expression cloning
using the monocytic differentiation system of HL-60 cells. SAT-I
is a novel protein belonging to the sialyltransferase family
(STs), although an invariant aspartic acid in sialylmotif L of
all other sialyltransferases is replaced by histidine. This amino
acid substitution is found in mouse and rat SAT-Is.
Unlike other sialyltransferases using glycosphingolipids as sialic
acid acceptor, the substrate specificity of sat-I product was
found to be highly restricted to lactosylceramide. This result
seems to be incompatible with those of purified SAT-I described
above. The expression of SAT-I mRNA shows a tissue- and species-specific
pattern, although distribution of GM3 is ubiquitous in various
tissues of animals. In human, brain, skeletal muscle and testis
express high levels of the transcript of SAT-I gene, whereas
low levels of the mRNA were found in liver and kidney. In mouse,
the highest level of the mRNA was detected in liver, while the
lowest was found in the kidney. On the other hand, kidney, as
well as heart, brain, and spleen, express high levels of the
mRNA in rat.
It is expected that studies on biological functions of GM3 have
been stimulated by the cloning of the SAT-I gene. It would be
interesting to investigated the origin of GM3 in an attempt to
elucidated its biological significance in animals using the SAT-I
gene as a clue. |
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| Fig.2 Schematic structure of human GM3 synthase. GM3 synthase
belongs to sialyltransferase family with a transmembrane portion
at NH2-terminal, a large lumenal catalytic domain with two conserved
regions, so called sialylmotifs, while an invariant aspartic
acid in the sialylmotif Ls of all other sialyltransferases is
replaced by histidine in GM3 synthase (indicated by yellow letter).
Red letters indicate amino acids conserved in sialyltransferases
from mammalian sources. TM is transmembrane portion, and triangles
show a potential N-glycosylation sites. |
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Atsushi Ishii, Msaki Saito
(National Cancer Center Research Institute, Virology Division) |
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| References |
(1) |
Melkerson-Watson, LJ, Sweeley, CC, J. Biol. Chem. 266, 4448-4457,
1991 |
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(2) |
Preuss, U, Gu, X, Gu, T, Yu, RK, J. Biol. Chem. 268, 26273-26278,
1993 |
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(3) |
Gu, X, Preuss, U, Gu, T, Yu, RK, J. Neurochem. 64, 2295-
2302, 1995 |
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(4) |
Nojiri, H, Takaku, F, Terui, Y, Miura, Y, Saito, M, Proc.
Natl. Acad. Sci. USA, 83, 782-786, 1986 |
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(5) |
Ishii, A et al. J. Biol. Chem. 273, in press, 1998 |
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| Dec.15, 1998 |
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