3.3 CD23 - structure and function on B cells
The low-affinity IgE-receptor FcεRII, also known as CD23, is a single chain type II integral membrane protein of 45kD and belongs to the C-type (Calcium-dependent) lectin superfamily 3,48-50. The membrane-bound CD23 consists of three lectin ”head” domains spaced from the membrane by a three α-helical coiled-coil ”stalk”. The lectin head domains of CD23 in the human form contain the C-terminal tail sequence. The stalk region is susceptible to proteolytic cleavage. Adam 10, a desintegrin and a metalloproteinase has been shown to release soluble CD23 (sCD23). This proteolytic cleavage results in trimeric fragments of CD23 (37kD) containing the stalk or monomeric fragments (25kD, 16kD) lacking the stalk51. CD23 recognises the protein rather than the carbohydrate moiety of IgE. A single lectin head fragment can bind to the IgE-Fc portion with an affinity of ka =106-107 M-1 while the avidity of the trimeric CD23 to bind IgE-Fc results in the overall high-affinity binding (Ka = 108 -109 M-1)51,52. Two isoforms of CD23 which differ by seven (CD23a) or six amino acids (CD23b) have been defined. CD23a is constitutively expressed on antigen-activated B cells, and IL-4 induces CD23b expression in several cell types, including B cells and epithelial cells3,53,54.