Interaction of aglycosyl immunoglobulins with the IgG Fc transport receptor from neonatal rat gut: comparison of deglycosylation by tunicamycin treatment and genetic engineering.

The role of carbohydrate in the structure and function of immuno-globulin Fc regions has been studied using the interaction of a monoclonal mouse IgG2b anti-NIP antibody with the IgG Fc transport receptor from neonatal rat gut. An aglycosyl variant of this immunoglobulin, in which site-directed mutagenesis had been used to eliminate the carbohydrate attachment site in the CH2 domain by changing Asn297 to Ala, was compared in this system to aglycosyl immunoglobulin prepared from immunoglobulin-secreting cells treated with tunicamycin to inhibit N-linked glycosylation. Loss of carbohydrate from the heavy chain was confirmed for both methods by Western blotting of the separated chains with Concanavalin A, and no significant differences in circular dichroism spectra were found between glycosylated and non-glycosylated mutants. Removal of carbohydrate by site-directed mutagenesis had no effect on binding of the immunoglobulin to the Fc transport receptor (FcTR) in vitro or transport from the gut to blood in vivo. Short-term clearance from circulation and degradation by gut contents in vitro were similarly unaffected. Mutation of Glu235 to Leu, an alteration that allows binding to human monocyte Fc gamma RI, did not alter the interaction with FcTR. However, treatment of wild-type or aglycosyl mutant cells with tunicamycin resulted in immunoglobulin which was less stable, cleared more rapidly and was transported slightly less efficiently. These findings indicate that the binding site for the FcTR may be unique among Fc-binding ligands, and that tunicamycin treatment may cause alterations in the immunoglobulin molecule in addition to loss of N-linked carbohydrate.