FCAR, also called FcaRI or CD89, is a variably glycosylated 50-100 kD myeloid-specific type I transmembrane (TM) Fc receptor for IgA that is a member of the multichain immune recognition receptor (MIRR) family. Human FCAR contains a 21 amino acid (aa) signal sequence and extracellular (ECD), TM and cytoplasmic domains of 206, 19 and 41 aa, respectively. Arg230 within the TM domain supports interaction with the ITAM-containing signaling subunit, FcRg, which contains a TM Asp. 5-7 Two ECD C2-type Ig-like domains (EC1 and 2) are oriented at right angles. Up to two molecules of FCAR can bind one molecule of serum IgA via EC1. Many splice variants have been reported, but only two have been identified as proteins. The a.2 form, which lacks 22aa just prior to the TM domain, is exclusively expressed in alveolar macrophages. The a.3 form lacks EC2. FCAR binds monomeric, polymeric and secretory IgA, but does not mediate the barrier function of secretory IgA in mucosal epithelium. Shedding and circulation of polymeric IgA/FCAR immune complexes has been reported. Circulating neutrophils, eosinophils, and monocytes express FCAR. Tissue expression of FCAR is mainly from neutrophils; FCAR is downregulated as monocytes differentiate to tissue macrophages. On neutrophils, a significant amount of FCAR lacks FcRg, but can still be endocytosed to early endosomes and recycled to the cell surface. Binding of serum IgA to FCAR is transient and anti- inflammatory, inhibiting IgG or IgE-induced degranulation. Sustained aggregation of FCAR results in inflammatory responses. FcRg signaling is required for these and for transport to late endosomes. Human FCAR shows 55-58% aa identity with rat, horse and cow FCAR. No ortholog occurs in mouse. FCAR structure resembles the KIR/ILT/LIR/MIR family more than other IgA receptors, including pIgR, Fca/uR, asialoglycoprotein receptor (ASGR1) and transferrin receptor (TfR).
Applications:
Suitable for use in ELISA, Flow Cytometry, Immunoprecipitation, and Western Blot. Other applications not tested.
Recommended Dilution:
Flow Cytometry: 1:50-1:100; 10ul labels 10e6 cells in 100ul
Optimal dilutions to be determined by the researcher.
Storage and Stability:
May be stored at 4 degrees C for short-term only. For long-term storage and to avoid repeated freezing and thawing, add sterile 40-50% glycerol, aliquot and store at -20 degrees C. Aliquots are stable for at least 12 months at -20 degrees C. For maximum recovery of product, centrifuge the original vial after thawing and prior to removing the cap. Further dilutions can be made in assay buffer.
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