Structure of IgE

Like antibodies of any other class, IgF. is composed of four polypeptide chains, two heavy chains and two X. or k light chains; its main features are described in Table 1. IgE is characterized by its e heavy chains, which have four invariant or constant (C) region domains, C£l-C64 (Figure 1), corresponding to those in the p. chain of IgM (CM1-CM4), but differing from the three that occur in the y- and a- heavy chains of IgG, IgD and IgA, respectively. The extra domain is Ce2 (or C^2 in IgM), which occupies the equivalent position to the hinge found in IgG, IgD and IgA. IgE can be proteolytically cleaved into Fab and Fc fragments in the same way as IgG. The cleavage site lies between CJ and Q2, and thus the Fc fragment has three (Q2-Q4) rather than two (C72 and Cy3) immunoglobulin domains per chain. The homology between the Q3-Q4 region of IgE and the IgG Fc, and the existence of a high-resolution crystal structure of the IgG 1 Fc, has provided the basis for a model of this region of the IgE structure (Figure 1).

IgE is heavily glycosylated, the carbohydrate contributing 13% of the total mass. There are seven putative glycosylation sites, three in the C, 1 domain of the Fab and four in the Fc region. The latter include the conserved site in Q3 (Asn397; homolo-

Table 1 Characteristics of IgE

Cell distribution Molecular weight Glycosylation sites (human) Receptors

Domain structure - heavy chain

Isoforms

Committed and selected B cells ~ 190000

Asn145, 173, 219, 265, 371, 383, 394 FceRI, FceRII, galectin 3 (e-BP or Mac-1) VH - C.1 - C,2 - C.3 - C,4 VL-CL

Secreted, and long and short membrane forms

.tP kip

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