C1 binding by mouse IgM. The effect of abnormal glycosylation at position 402 resulting from a serine to asparagine exchange at residue 406 of the mu-chain.

We have previously shown that IgM-Asn406, a mutant IgM which has asparagine in place of the serine which is normally found at position 406, also has an abnormally glycosylated mu-chain and is defective in complement-dependent cytolysis. Here we show by analyzing cyanogen bromide fragments from normal and mutant mu-chains that the site of abnormal glycosylation is at the neighboring position, Asn402. The cytolytic defect was shown to be due to impaired C1 binding. At physiological ionic strength, the C1 binding defect was estimated to be 12-fold, which correlates well with the measured defect in cytolytic activity; also, the severity of the defect in C1 binding by the mutant protein decreases with decreasing ionic strength. Kinetic studies showed that the difference in affinities is due to a proportional difference in the association rate for C1q. By comparing IgM made in the presence and absence of deoxymannojirimycin, we show further that the defect in cytolytic activity derives mostly from the abnormal oligosaccharide.