Wright J F, Shulman M J, Isenman D E, Painter R H
Department of Biochemistry, University of Toronto, Ontario, Canada.
J Biol Chem. 1990 Jun 25;265(18):10506-13.
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.
我们之前已经表明，IgM-Asn406是一种突变型IgM，其406位的丝氨酸被天冬酰胺取代，其μ链糖基化也异常，并且在补体依赖性细胞溶解方面存在缺陷。在此，我们通过分析正常和突变μ链的溴化氰片段表明，异常糖基化位点位于相邻位置Asn402。细胞溶解缺陷被证明是由于C1结合受损所致。在生理离子强度下，C1结合缺陷估计为12倍，这与所测得的细胞溶解活性缺陷密切相关；此外，突变蛋白C1结合缺陷的严重程度随离子强度降低而降低。动力学研究表明，亲和力差异是由于C1q结合速率的比例差异所致。通过比较在有和没有脱氧甘露基野霉素的情况下产生的IgM，我们进一步表明细胞溶解活性缺陷主要源于异常寡糖。
