The CD23 Multifunctional Molecule and its Soluble Fragments (IgE-Binding Factors or Soluble CD23)

The low affinity receptor for IgE (FceRII), recently identified as CD23 antigen, is a structurally and functionally unique immunoglobulin receptor primarily expressed on B lymphocytes. The CD23 is a 45 KD type II integral membrane glycoprotein (carboxy-terminus outside), with a short intracytoplasmic tail (23 residues), a single transmembrane domain (20 residues) and a large extracellular domain (277 residues) that shares a significant homology with calcium dependent (C-type) animal lectins (Ludin 1987; Ikuta 1987; Kikutani 1986a). Nearby the carboxy-end of the molecule, is an “RGD” sequence in a reverse configuration. RGD is a common recognition site of the integrin receptor. Three consensus sequences of 21 residues each, are located between the N-glycosylation site (position 63) and the lectin homology domain. Within this repetitive region, there are five heptadic repeats of leucine or (isoleucine) forming a “leucine zipper” motif, that may be involved in the formation of CD23 dimers (Beavil 1992). Unlike other Fc receptors, CD23 does not belong to the Ig superfamily but it is a member of a novel supergene family of type II membrane proteins displaying a lectin motif.