Chemistry Reference
In-Depth Information
proteins (see Tables 7.2-7.4 and Figure 7.1). The subcellular organization of the
glycosyltransferases into functional units acting on individual proteins delivers the
specifi c glycosylation patterns found for each protein [2].
O
- Glycan processing
leading to biologically active glycoproteins occurs via several routes. First, the
sequential action of glycosyltransferases delivers completed structures. 'Incom-
plete' or truncated structures, which also have biological function, result from the
action of a reduced number of glycosyltransferases in the biosynthetic units.
Alternatively, the action of catabolic enzymes including glycosidases, sulfatases or
esterases on the completed
O
-glycan chains may occur. Thus,
O
- glycans can be
processed while attached to proteins analogous to the normal catabolic pathways
leading to degradation and recycling of glycoproteins. This involves removal of
terminal residues such as sialic acids, fucose or sulfate or total removal of
O
- glycan
chains through the combined action of exoglycosidases, endoglycosidases and
O
-glycanases responsible for the internal cleavage of the oligosaccharide chains or
removal from the peptide backbone. Table 7.5 indicates that glycan processing acts
at different levels and these are described below.
Protein synthesis and expression can be regulated during and after biosynthesis
through the presence of glycan chains. The synthesis of mature glycophorin A
requires
O
-glycans when expressed in glycosylation- defi cient CHO cells (please
see Info Box 3 in Chapter 6 on origin of mutant CHO cells). The
N
- glycans nor-
mally present are not necessary for expression. The expression of glycophorin A
without
O
-glycans was dependent on specifi c
N
-glycans, and indicates that inde-
pendent expression of
O
- and
N
-glycans can enable glycophorin A expression.
Proteolytic processing of certain proteins demonstrates a requirement for
O
- glycans at specifi c sites in order to prevent proteolytic cleavage which eliminates
biological activity or prevents continued residence/activity of the intact protein at
its designated subcellular location. An example is the cell- surface expression of
low-density lipoprotein receptors that require
O
- glycans to prevent proteolytic
cleavage of their extracellular domains. The transport of the transferrin receptor
(TfR) between the cell surface and endosomes also shows such glycosylation
dependency. Soluble TfR is formed by proteolytic cleavage in the endosomes.
O
-Glycosylation of Thr104 prevents the action of the relevant protease. The loss
of sialic acids from these
O
-glycans abolishes the protection of the
O
- glycans
against proteolytic activity.
Subcellular localization of proteins is mediated by
O
- glycosylation. The transport
of proteins to the Golgi apparatus from the ER is an example here. Studies using
brefeldin A, which induces a microtubule- dependent back - fl ow of Golgi compo-
nents to the ER and
- galactosyltransferase, an established trans - Golgi enzyme,
demonstrate disruption of the biosynthesis, maturation and intracellular trans-
port. Targeting of the
β
β
-galactosyltransferase to the Golgi apparatus depends on
its
O
- glycosylation.
Developmental expression during oogenesis, spermatogenesis, fertilization, pre-
implantation implantation and post-implantation depends on glycosylation,
including expression of
O
- glycans (see Chapter 24). From this large literature two
examples have been chosen to illustrate this phenomenon.
