Rall S C, Weisgraber K H, Innerarity T L, Mahley R W
Proc Natl Acad Sci U S A. 1982 Aug;79(15):4696-700. doi: 10.1073/pnas.79.15.4696.
The three major isoforms of human apolipoprotein E (apo-E2, -E3, and -E4) are coded for by three alleles (epsilon 2, epsilon 3, and epsilon 4) which have a common genetic locus. Previously, we demonstrated that E2, E3, and E4 differ in primary structure from one another at two substitution sites, site A (residue 112) and site B (residue 158). At sites A/B, apo-E2, -E3, and -E4 contain cysteine/cysteine, cysteine/arginine, and arginine/arginine, respectively. We demonstrated that the substitution of cysteine for arginine at site B is at least partly responsible for the defective binding of apo-E2 to human fibroblast low density lipoprotein receptors, compared to the normal binding activity of apo-E3 or -E4. Subjects with the genetic disorder type III hyperlipoproteinemia are phenotypically homozygous for apo-E2, but the binding activity of apo-E to the fibroblast receptor differs considerably from one type III individual to another. We therefore undertook a partial comparative sequence analysis of apo-E2 from three type III subjects whose apo-E displayed this heterogeneity. The subject with the poorest binding apo-E2 was genotypically homozygous for an apo-E allele (epsilon 2); cysteine was found at sites A and B. The subject with the most active apo-E2 was genotypically homozygous for an apo-E allele (epsilon 2); cystine was found at site A and at a new site (site C, residue 145). The epsilon 2 allele specifies a protein that has arginine at site B (residue 158); the epsilon 2 allele specifies a protein that has arginine at site C (residue 145). Therefore, the two alleles differ from one another by cysteine/arginine interchanges at two positions, sites B and C. The third subject, whose apo-E2 displayed binding activity intermediate between the activities of the other two, was genotypically heterozygous, having one epsilon 2 allele and one epsilon 2 allele. The intermediate binding activity of apo-E2 from this subject resulted from having a mixture of severely defective apo-E (specified by epsilon 2) and slightly defective apo-E (specified by epsilon 2).
人类载脂蛋白E的三种主要异构体（apo-E2、-E3和-E4）由具有共同基因座的三个等位基因（ε2、ε3和ε4）编码。此前，我们证明E2、E3和E4在两个替换位点，即位点A（第112位残基）和位点B（第158位残基）的一级结构彼此不同。在A/B位点，apo-E2、-E3和-E4分别含有半胱氨酸/半胱氨酸、半胱氨酸/精氨酸和精氨酸/精氨酸。我们证明，与apo-E3或-E4的正常结合活性相比，位点B处半胱氨酸替代精氨酸至少部分导致apo-E2与人成纤维细胞低密度脂蛋白受体的结合缺陷。患有III型高脂蛋白血症这种遗传疾病的受试者在表型上是apo-E2的纯合子，但apo-E与成纤维细胞受体的结合活性在不同的III型个体之间有很大差异。因此，我们对三名apo-E表现出这种异质性的III型受试者的apo-E2进行了部分比较序列分析。apo-E2结合能力最差的受试者在基因型上是一个apo-E等位基因（ε2）的纯合子；在位点A和B发现了半胱氨酸。apo-E2活性最高的受试者在基因型上是一个apo-E等位基因（ε2）的纯合子；在位点A和一个新位点（位点C，第145位残基）发现了胱氨酸。ε2等位基因指定的蛋白质在位点B（第158位残基）有精氨酸；ε2等位基因指定的蛋白质在位点C（第145位残基）有精氨酸。因此，这两个等位基因在两个位置，即位点B和位点C，通过半胱氨酸/精氨酸互换而彼此不同。第三名受试者的apo-E2表现出的结合活性介于其他两名受试者之间，其基因型是杂合子，有一个ε2等位基因和一个ε2等位基因。该受试者的apo-E2的中间结合活性是由严重缺陷的apo-E（由ε2指定）和轻度缺陷的apo-E（由ε2指定）的混合物导致的。
