Deglycosylation of FcalphaR at N58 increases its binding to IgA.

FcalphaR (CD89) is the Fc receptor for immunoglobulin A (IgA) and plays important roles in IgA-mediated immune responses. It is a heavily glycosylated protein with six potential N-linked glycosylation sites. Previous reports showed that abnormal glycosylation of FcalphaR was involved in some diseases, including human immunodeficiency virus infection, alcoholic liver cirrhosis and IgA nephropathy. In this study, we examined the effects of N-glycosylation on interaction between FcalphaR and IgA. We found that depletion of N-glycosylation of FcalphaR transfected in Chinese hamster ovary (CHO) cells by tunicamycin resulted in increased IgA binding. To identify which glycosylation site is responsible for increased IgA binding, we performed site-directed mutagenesis at each N-linked glycosylation site by changing asparagine to glutamine. Flow cytometry analysis of IgA binding to CHO cells transfected with mutated FcalphaR showed that deglycosylation of FcalphaR at individual N44, N120, N156, N165 or N177 site did not affect IgA binding but deglycosylation at N58 resulted in marked increase of IgA binding. Similar result was shown for N58Q-FcalphaR transfected RBL2H3, a rat basophilic leukemia cell line. Furthermore, increased IgA binding was also observed on desialylated FcalphaR after neuraminidase treatment and desialylation of N58 contributed most to the increased IgA binding. These data demonstrated that glycosylation at N58 site influenced FcalphaR binding to IgA.