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Figure 1.
Figure 1. Schematic diagram of the Ig fold domains in human
IgA1 and IgG1. Each heavy chain contains the V[H], C[H]1, C[H]2
and C[H]3 domains, and each light chain contains the V[L]and
C[L]domains, each of which is represented by a rectangle. The
four-domain Fab fragments are linked to the four-domain Fc
fragment by a 23 residue hinge in IgA1. The C terminus of each
IgA1 heavy chain contains an 18 residue tailpiece (broken line).
The hinge of IgG1 is one residue shorter and there is no
tailpiece. Each Ig fold contains a conserved internal Cys-Cys
disulphide bridge (S-S). The C[H]1 domain of IgA1 has an
additional disulphide bridge between Cys196-Cys220. The heavy
and light chains of IgA1 are linked between Cys133 in the C[H]1
domain, and the C-terminal Cys residue of the light chain. The
heavy and light chains of IgG1 are linked by a Cys residue in
the hinge and the C-terminal Cys residue of the light chain. In
IgA1, Cys241 and Cys242 in the hinge and Cys299 and Cys301 in
the C[H]2 domain form intra and inter-heavy chain disulphide
bridges. In IgG1, two Cys residues in the hinge form two
inter-chain bridges. In IgA1, Cys311 on the C[H]2 domain and
Cys471 in the tailpiece are shown free. IgA1 has two N-linked
oligosaccharide sites on β-strand B of the C[H]2 domain and on
the tailpiece (•). That on the C[H]2 domain of IgA1 is at a
different position to that on the C[H]2 domain of IgG1 which
occupies a central cavity in the IgG1 Fc structure and which is
conserved in the other human Ig classes. IgA1 also has five
O-linked oligosaccharide sites in the hinge (○) which are not
present in IgG1. PTerm455 is a recombinant IgA1 molecule which
lacks the tailpiece.
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