McLean R H, Niblack G, Julian B, Wang T, Wyatt R, Phillips J A, Collins T S, Winkelstein J, Valle D
Department of Pediatrics, University of Maryland Medical School, Baltimore 21201.
J Biol Chem. 1994 Nov 4;269(44):27727-31.
The fourth component of complement (C4) is encoded by two highly homologous genes, C4A and C4B. Only one hemolytically inactive C4A allotype (C4A6) has been reported. No hemolytically inactive C4B allotype has been described. We report the first hemolytically inactive (hi) allotype of C4B, C4B1 (hi). This unique variant was first recognized by hemolytic overlay assays and confirmed to segregate in the affected pedigree with the major histocompatibility complex haplotype A28,B35,CW4,DR6, C4A3,C4B1(hi), BFF,C2C. By single strand conformational polymorphism, we detected only a migration variant in exon 12 caused by a C to T transition in the second base of codon 459. This mutation results in a leucine substitution for proline (P459L) 1 residue downstream of a residue known to contribute to the C5-binding site. Allele-specific oligonucleotide analysis of samples demonstrated cosegregation of the mutation with the hemolytically inactive allotype in the affected pedigree. Site-directed mutagenesis and expression studies showed that the P459L mutation causes loss of hemolytic function. C4B1(hi) is the first example of a circulating C4B protein lacking detectable hemolytic activity and the P459L mutation expands our knowledge of the C5-binding site of C4.
补体第四成分（C4）由两个高度同源的基因C4A和C4B编码。据报道，只有一种无溶血活性的C4A同种异型（C4A6）。尚未描述无溶血活性的C4B同种异型。我们报道了首个无溶血活性的C4B同种异型C4B1（hi）。这种独特的变异体首先通过溶血覆盖试验被识别，并在受影响的家系中与主要组织相容性复合体单倍型A28、B35、CW4、DR6、C4A3、C4B1（hi）、BFF、C2C共分离。通过单链构象多态性分析，我们仅在第12外显子中检测到一个迁移变异体，该变异是由密码子459第二位的C到T转换引起的。此突变导致在已知对C5结合位点有贡献的一个残基下游1个残基处脯氨酸被亮氨酸取代（P459L）。对样本进行的等位基因特异性寡核苷酸分析表明，该突变与受影响家系中无溶血活性的同种异型共分离。定点诱变和表达研究表明，P459L突变导致溶血功能丧失。C4B1（hi）是循环C4B蛋白缺乏可检测溶血活性的首个实例，P459L突变扩展了我们对C4的C5结合位点的认识。
