The FcaR receptor (CD89) binds to the constant region of Immunoglobulin (Ig) A to mediate mucosal
immunity [1–2]. FcaR consist of five exons: two that code for the signal peptide regions S1 & S2, two for
the extracellular regions EC1 and EC2, and the final exon for the transmembrane/cytoplasmic tail region
. Previously, we reported that the EC1 region plays an essential role for extracellular membrane
localization of the receptor , where the absence of EC1 would prevent the variants from localizing to the
cell surface, even with a full signal peptide. In the case of FcaR Variant 4 (lacking the S2 region only), there
was some “leakiness” to membrane surface localization.