Co

to u

Figure 1 Structure of IgE. Each of the immunoglobulin domains is represented by rectangles. Intrachain and interchain disulfide bridges are represented by S-S, and external glycosylation sites are shown as black circles. Also shown is the a-carbon backbone trace of the Ce3 and Ct4 domains, modeled on the homologous lgG1 structure, with the conserved internal (buried) carbohydrate between the two Ce3 domains.

Table 2 Characteristics of IgE receptors FceRI and FceRII

Feature

FceRI

FceRII

Structure

High-affinity IgE receptor Mast cells, basophils, eosinophils, Langerhans cells, activated monocytes

Alternative names Cellular distribution

Subunit composition Transmembrane regions Orientation Structural motifs

High-affinity IgE receptor Mast cells, basophils, eosinophils, Langerhans cells, activated monocytes

One a, one p and two y chains o = 1, p = 4, 7 = 1 (x2), total = 7 Intracellular C-termini a = two immunoglobulin domains

Post-translational modification Seven /V-glycosylation sites in a, Ser, Thr,

Tyr phosphorylation in a

a Chain, membrane proximal Ig domain Degranulation of mast cells

Affinity for monomeric IgE Binding site on IgE Binding site on receptor Function

Low-affinity IgE receptor, CD23 B Ceils, T cells, platelets, macrophages, natural killer (NK) ceils, follicular dendritic cells, eosinophils, epithelial cells Homotrimer, cleaved to soluble fragments 1 per monomer, total = 3 Intracellular N-terminus

C-type animal lectin, «-helical coiled coil, reverse

RGD (Arg Gly Asp) adhesion sequence Single /V-glycosylation site

Lectin domain; all soluble fragments bind Growth and differentiation factor for hematopoietic cells; rescues germinal center B cells from apoptosis; IgE antibody dependent antigen presentation; regulation of IgE expression; IgE antibody dependent cellular cytotoxicity (ADCC)

hans cells and a wide variety of effector functions in mast cells, basophils, macrophages, eosinophils and platelets.

FceRII/CD23 is a type II integral membrane protein (cytoplasmic N-terminus), which occurs in two forms, the a-form and the b-form, differing by 6-7 amino acids at their N-termini. The former is consti-tutively expressed in activated B cells and the latter is upregulated by interleukin 4 (IL-4) on a variety of cell types. FceRII/CD23a mediates IgE antibody-dependent presentation of antigen to T cells and FceRII/CD23b the phagocytic activity of macrophages. The IgE-binding portion of FceRII/CD23 is a C-type (Ca2 +-dependent) lectin domain, differing from other known Fc receptors expressed on cell membranes, all of which utilize immunoglobulin domains. The lectin domain is near the C-terminus of the protein, which lies at the distal end of the extracellular sequence. Separating this domain from the transmembrane sequence is a region that forms an a-helical coiled-coil stalk, responsible for the association of FceRII/CD23 into trimers in the cell membrane. FceRII/CD23 is cleaved by endogenous proteases into fragments with important cytokine activities, not least of which is in uprcgulation of IgE synthesis via complement receptor 2, also known as CD21. Binding of IgE protects FceRII/CD23 from such fragmentation. The membrane-bound form of FceRII/CD23 is involved in feedback regulation of IgE synthesis.

A third protein, galectin 3 (also known as e-bind ing protein or e-BP), an S-type lectin, binds to IgE (and indeed to FceRI), recognizing carbohydrate chains, and may be another modifier of the IgE response.

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