Chemistry Reference
In-Depth Information
branching of
O
-mannosyl-linked glycans, has high activity in brain and testis, and
may mediate the adhesion and migration of human neuronal cells through integ-
rin - and laminin -dependent interactions. Tumor-related functions are reviewed in
Table 25.2 .
O
- Fuc and
O
-Glc glycosylation of EGF domains was originally identifi ed through
genes regulating Notch signaling and which were found to be glycosyltransferases
[8] . The specifi c protein - linked
-
O
-fucosyltransferase has been cloned from
Dro-
sophila
(OFUT1) and human (POFUT1) sources with transfer to the specifi c
consensus sequences identifi ed in EGF-like domains of protein acceptors. The
importance of the consensus sequence is emphasized by the fi nding that not
all EGF domains in Notch proteins are
O
- fucosylated [8] .
Drosophila
OFUT1 is
located in the ER, possesses chaperone properties required to achieve the correct
folding of the Notch EGF-like domain peptide and is also required for the traffi ck-
ing of wild-type Notch out of the ER. An extracellular interaction between Notch
and OFUT1 is essential for the normal endocytic transportation of Notch. The
importance of protein
O
-fucosylation is underlined by the embryonic lethality of
POFT1 deletion in mice. Chapter 24 deals with relevant aspects in embryogenesis.
A second protein- linked
O
-fucosyltransferase gene, POFUT2, showing substrate
specifi city for thrombospondin type 1 repeats (TSRs), notably in the ADAMTS
proteases and not for EGF domains, has also been reported [7] .
The extension of the protein
O
-Fuc unit occurs initially through the action of a
specifi c
α
1,3-N-acetylglucosaminyl transferase. The gene was detected in
Dro-
sophila
and three mammalian homologs have been identifi ed - Manic Fringe,
Lunatic Fringe and Radical Fringe (for phenotypes of KO mice, please see Table
23.1). Recognition of
O
-fucosylated EGF repeat amino acids has been shown for
all three mammalian Fringes. The Fringes show substrate activity with the Notch
proteins and their ligands Delta and Jagged [7] .
In mammalian species the GlcNAc
β
β
1,3Fuc -
α
-
O
-Ser/Thr disaccharide is a sub-
strate for a
1,4-galactosyltransferase. The investigation of Notch signaling in
Chinese hamster ovary (CHO ) cells defi cient in galactose transfer shows that
interaction of Notch with its ligands Delta and Jagged does not occur, and that
this can be rescued by the transfection of murine
β
β
4GalT1 - a
β
1,4 - galactosyltrans-
ferase. Further extension of the trisaccharide Gal
β
1,4GlcNAc
β
1,3Fuc -
α
-
O
- Ser/Thr
by a
2,3 sialyltransferase is proposed, but little information is currently available.
In place of the extension of Fuc-
α
α
-
O
-Ser/Thr by the action of the fringe genes a
β
1,3-glucosyltranferase has been reported to yield Glc
β
1,3Fuc -
α
-
O
- Ser/Thr. A
second
1,3 - glucosyltransferase was found with specifi city to TSRs. The biological
signifi cance of the Glc
β
-
O
-Ser/Thr has not been investigated.
The direct addition of glucose to proteins to generate Glc-
β
1,3Fuc -
α
-
O
- Ser/Thr has been
reported using recombinant factor VII EGF repeat as the acceptor. This study
showed that the enzymatic activity recognized the consensus sequence and also
the three-dimensional structure of the EGF repeat. Glc-
β
-
O
- Ser/Thr should serve
as a substrate for further elongation by the action of xylosyltransferases; however,
little data is available concerning these activities [7, 8]. Very recent evidence has
shown that collagen hydroxylysine glycosylation is initiated by specifi c
β
(1 - O)
galactosyltransferases [9]. Two proteins GLT25D1 and GLT25D2, were identifi ed
β
