Chain subclasses

Four subclasses are known in human (yl, y2, y3 and y4) and also in mouse IgG (yl, y2a, y2b and y3) originally defined on the basis of specific antisera. There is little correspondence between the subclasses of the two species and it is thought that the subclasses evolved from a common ancestor indepen dently and subsequent to the divergence of the two species. While there arc major differences in the biological properties of the subclasses in each species the sequence differences are small and confined mainly to the hinge region. The human subclass constant domains are approximately 95% identical but there are major structural differences in the hinge regions in terms of the number of disulfide bonds and the length of the region. The y3 hinge contains four tandem replications of the 15 residue segment (216-230) of the y1 hinge sequence. Electron micrographs reveal that this results in a considerably extended structure for the y3 protein. There are differences too in the point of attachment of the light chain disulfide bond to the heavy chain. In the IgG2, 3 and 4 subclasses the cysteine lies between the Vn domain and the C71 domain (residue 131) while in IgG 1 the cysteine lies at the N-terminal end of the hinge (residue 220). The consequences of this arc minimal since through the folding of the H chain these two points of attachment are close to one another.

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