BIOGENESIS AND FUNCTION OF IGM - THE ROLE OF THE CONSERVED MU-CHAIN TAILPIECE GLYCANS

The tailpiece of secretory Ig-mu-chains (mu(s)tp) is highly conserved throughout evolution: in particular, a carboxy-terminal cysteine resid ue (Cys575) and a glycan linked to Asn563 are found in all species seq uenced so far. Here we show that the mu(s)tp oligosaccharide moieties are important for the binding of J-chains and for the process of IgM p olymerization. In the absence of the mu(s)tp glycans pentamers cannot be assembled and polymers containing six or more subunits are secreted . Despite their increased valency, these molecules have a lower associ ation rate with antigen than wild-type polymers. Unexpectedly, the C-t erminal oligosaccharides also affect kinetic parameters on unpolymeriz ed subunits. Thus, monomers lacking the C-terminal sugars because of e ither site-directed mutagenesis or selective enzymatic deglycosylation with endoglycosidase H, have a lower k(on) for the antigen. Taken tog ether, our results indicate that the C-terminal mu-chain glycans can s hape the structure of mu(s2)L(2) subunits and their further assembly i nto polymers. (C) 1998 Elsevier Science Ltd. All rights reserved.