Exploring Functions for Glycosylation in Host Defence
333
on the cell surface. By limiting non-specific protein-protein interactions and controlling the spacing, the glycans may influence the distribution of CD59 molecules at the cell surface where GPI anchored proteins may associate in microdomains in dynamic equilibrium with isolated individual molecules (van den
Berg et aI., 1995). The large N-glycans may also be important in preventing proteolysis of the extracellular domain since N-glycosylation has been shown to
increase the dynamic stability of a protein while different glycoforms variably
increase its resistance to protease digestion (Rudd et al., 1994).
(iii) Function for GPI Anchors on DAF and CD59
GPI membrane anchors are used by a wide variety of cell surface glycoproteins
(McConville et aI., 1993). The tetrasaccharide backbone (Man3 inositol) may be
extended with other sugars as the anchored protein moves through the secretory
pathway. In the anchored form of CD59 the bulky, hydrophilic glycans at the Cterminus would be expected to limit interactions with the lipid bilayer and in this
way facilitate the diffusion of the glycosylated protein in the top leaflet of the cell
membrane. DAF has been shown to reside in micro domains on the surface of
MDCK cells. These domains contain at least 15 molecules (Friedrichson and
Kurzchalia, 1998).
7
Glycosylation Provides a Mechanism for Fine Tuning the Affinity
of (02 and (048 for their Receptors
One aspect of the cellular immune response involves the cell adhesion molecules
CD2 and CD48 in rat (CD58 in human), which are present on cells expressing the
TCR and on antigen presenting cells, respectively. CD2 and its ligand interact
through homologous binding surfaces located in the amino-terminal domains
distal to the membrane surface (Davis et aI., 1999).
The N-glycans attached to the highly conserved, membrane proximal glycosylation site at the base of CD2 and CD48 limit the conformational space available
to the protein to a cone of 520 described by CD2 (Dustin et aI., 1996). This also
promotes trans-interactions with rat CD48 on other cells and discourages cisinteractions with other human CD2 molecules on the same cell. The sugars
attached to human soluble CD2 and rat soluble CD48 (Fig. 23.9) were analysed
(Fig. 23.10). In neither case are any of the sugars close to the binding site, and
they may therefore be expected to protect the protein surfaces against proteases
without hindering the function of the protein.
Since the length of the CD2/CD48 cell adhesion pair is the same as that of the
TCR/MHC pair it has been proposed that CD2 and CD48 mediate the alignment
of the cell surfaces of cytolytic T-Iymphocytes carrying the TCR complex with
those of target cells carrying loaded HLA class 1 molecules. CD48 on rat (CD58
on human) molecules cluster when presented with CD2 receptors (Dustin et aI.,
1996). During clustering non-specific protein interactions may be inhibited by
the heterogeneous arrays of sugars which variably shield the protein surface.
Human sCD2 and rat sCD48 have 3 and 5 N-glycosylation sites, respectively.
Both molecules contain mainly complex type glycans (Fig. 23.10). Human sCD2
333
on the cell surface. By limiting non-specific protein-protein interactions and controlling the spacing, the glycans may influence the distribution of CD59 molecules at the cell surface where GPI anchored proteins may associate in microdomains in dynamic equilibrium with isolated individual molecules (van den
Berg et aI., 1995). The large N-glycans may also be important in preventing proteolysis of the extracellular domain since N-glycosylation has been shown to
increase the dynamic stability of a protein while different glycoforms variably
increase its resistance to protease digestion (Rudd et al., 1994).
(iii) Function for GPI Anchors on DAF and CD59
GPI membrane anchors are used by a wide variety of cell surface glycoproteins
(McConville et aI., 1993). The tetrasaccharide backbone (Man3 inositol) may be
extended with other sugars as the anchored protein moves through the secretory
pathway. In the anchored form of CD59 the bulky, hydrophilic glycans at the Cterminus would be expected to limit interactions with the lipid bilayer and in this
way facilitate the diffusion of the glycosylated protein in the top leaflet of the cell
membrane. DAF has been shown to reside in micro domains on the surface of
MDCK cells. These domains contain at least 15 molecules (Friedrichson and
Kurzchalia, 1998).
7
Glycosylation Provides a Mechanism for Fine Tuning the Affinity
of (02 and (048 for their Receptors
One aspect of the cellular immune response involves the cell adhesion molecules
CD2 and CD48 in rat (CD58 in human), which are present on cells expressing the
TCR and on antigen presenting cells, respectively. CD2 and its ligand interact
through homologous binding surfaces located in the amino-terminal domains
distal to the membrane surface (Davis et aI., 1999).
The N-glycans attached to the highly conserved, membrane proximal glycosylation site at the base of CD2 and CD48 limit the conformational space available
to the protein to a cone of 520 described by CD2 (Dustin et aI., 1996). This also
promotes trans-interactions with rat CD48 on other cells and discourages cisinteractions with other human CD2 molecules on the same cell. The sugars
attached to human soluble CD2 and rat soluble CD48 (Fig. 23.9) were analysed
(Fig. 23.10). In neither case are any of the sugars close to the binding site, and
they may therefore be expected to protect the protein surfaces against proteases
without hindering the function of the protein.
Since the length of the CD2/CD48 cell adhesion pair is the same as that of the
TCR/MHC pair it has been proposed that CD2 and CD48 mediate the alignment
of the cell surfaces of cytolytic T-Iymphocytes carrying the TCR complex with
those of target cells carrying loaded HLA class 1 molecules. CD48 on rat (CD58
on human) molecules cluster when presented with CD2 receptors (Dustin et aI.,
1996). During clustering non-specific protein interactions may be inhibited by
the heterogeneous arrays of sugars which variably shield the protein surface.
Human sCD2 and rat sCD48 have 3 and 5 N-glycosylation sites, respectively.
Both molecules contain mainly complex type glycans (Fig. 23.10). Human sCD2