The role of carbohydrate in the assembly and function of polymeric IgG

The carbohydrate present on glycoprotein can influence their biologic and functional properties. In the present paper we have assessed the role of oligosaccharides in the polymerization and effector functions of IgG with the 18 amino acid extension of IgM added to its carboxy terminus (IgGμtp). We found that IgG1μtp and IgG3μtp lacking the carbohydrate addition site in C_H2, in the tail-piece or both assembled into polymers as well as the glycosylated versions. Aglycosylated polymers retained the ability to activate complement as assayed by C1q binding and hemolysis, although they were not as effective as their wild type polymer counterparts. Although IgGμtp lacking the carbohydrate in the tail-piece was able to bind to FcγRII, completely aglycosylated polymers lost the ability to bind to both FcγRI and FcγRII, suggesting a critical role for the C_H2 sugar in FcR binding. Absence of the μtp carbohydrate increased the half life of polymeric IgG1, whereas absence of the carbohydrate in C_H2 accelerated the clearance rate.