| FCAR, also called FcalphaRI 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. 1-3 Human FCAR contains a 21 amino acid (aa) signal sequence and extracellular (ECD), TM and cytoplasmic domains of 206, 19 and 41 aa, respectively. 4 Arg230 within the TM domain supports interaction with the ITAM-containing signaling subunit, FcRgamma, which contains a TM Asp. 5-7 Two ECD C2-type Ig-like domains (EC1 and 2) are oriented at right angles. 8 Up to two molecules of FCAR can bind one molecule of serum IgA via EC1. 8 Many splice variants have been reported, but only two have been identified as proteins. 9, 10 The a.2 form, which lacks 22 aa 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. 1-3 Shedding and circulation of polymeric IgA/FCAR immune complexes has been reported. 11 Circulating neutrophils, eosinophils, and monocytes express FCAR. 12 Tissue expression of FCAR is mainly from neutrophils, FCAR is downregulated as monocytes differentiate to tissue macrophages. 12 On neutrophils, a significant amount of FCAR lacks FcRgamma, but can still be endocytosed to early endosomes and recycled to the cell surface. 5 Binding of serum IgA to FCAR is transient and anti- inflammatory, inhibiting IgG or IgE-induced degranulation. 6 Sustained aggregation of FCAR results in inflammatory responses. 6 FcRgamma signaling is required for these and for transport to late endosomes. 5-7 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, Fcalpha/µR, asialoglycoprotein receptor (ASGR1) and transferrin receptor (TfR). 1-3 Source: Human FCAR (Met 1-Asn 227), HHHHHH, DNA sequence encoding the extracellular domain of human FCAR (Met 1-Asn 227, Accession P24071) (Maliszewski, C.R., et al., 1990, J. Exp. Med. 172(6): 1665-1672) was fused to a C-terminal polyhistidine tag. The protein was expressed in a mouse myeloma cell line, NS0. Molecular Mass: The recombinant polyhistidine tagged human FCAR is a monomeric protein. Based on N-terminal sequencing, the N-terminus was blocked, suggesting that the recombinant protein may start at Gln 22 as the predicted N-terminus. The recombinant protein has a calculated molecular mass of 24.3kD. As a result of glycosylation, the recombinant protein migrates as an approximately 40-50 kD protein in SDS-PAGE under reducing conditions. Endotoxin Level: < 1.0 EU per 1 µg of the protein as determined by the LAL method. Activity: Measured by its ability to bind human lgA in a functional ELISA with an estimated Kd < 10 nM. Reconstitution: It is recommended that sterile PBS be added to the vial to prepare a working stock solution of no less than 100ug/ml. The carrier-free protein should be used immediately upon reconstitution to avoid losses in activity due to non-specific binding to the inside surface of the vial. For long term storage as a dilute solution, a carrier protein (e.g. 0.1% HSA or BSA) should be added to the vial. Storage and Stability: Lyophilized samples are stable for up to twelve months at -20C. Upon reconstitution, this protein, in the presence of a carrier protein, can be stored under sterile conditions at 2-8C for one month or at -20C in a manual defrost freezer for three months without detectable loss of activity. Avoid repeated freeze-thaw cycles. |
