Ebert D L, Maeda N, Lowe S W, Hasler-Rapacz J, Rapacz J, Attie A D
Department of Biochemistry, University of Wisconsin-Madison 53706.
J Lipid Res. 1988 Nov;29(11):1501-9.
Apolipoprotein B (apoB) is the predominant protein in low density lipoprotein (LDL) and is responsible for LDL binding to the LDL receptor. Although the primary amino acid sequence of human apoB has been determined, little is known about the structural domains involved in mediating apoB binding to the LDL receptor. Amino acid sequence comparisons across species lines provide a means of defining structures that are essential for function. We have sequenced a l.l kb fragment of pig apoB genomic DNA, corresponding to a 363 amino acid segment proposed to mediate human apoB binding to the LDL receptor. In human apoB this domain contains two regions enriched in positively charged amino acids flanking two disulfide-linked cysteine residues. The pig amino acid sequence shared 72% identity with the human sequence. However, there were differences that have significant structural and functional implications. Human apoB arginine-3,359 corresponds to a critical arginine (position 142) residue in the apoE LDL receptor binding domain. In the pig, this arginine residue was not conserved. Also, the two disulfide-linked cysteine residues found near the proposed apoB binding domain were not conserved in the pig. Despite these differences, pig LDL had a higher affinity than human LDL for both the pig and human LDL receptor. Thus, these features are not required for high affinity binding of pig LDL to the LDL receptor, and may not be necessary for the binding of human LDL to the LDL receptor.
载脂蛋白B(apoB)是低密度脂蛋白(LDL)中的主要蛋白质,负责LDL与LDL受体的结合。虽然人类apoB的一级氨基酸序列已被确定,但对于介导apoB与LDL受体结合的结构域却知之甚少。跨物种的氨基酸序列比较提供了一种定义功能所必需结构的方法。我们对猪apoB基因组DNA的一个1.1 kb片段进行了测序,该片段对应于一个363个氨基酸的区段,推测其介导人类apoB与LDL受体的结合。在人类apoB中,该结构域包含两个富含带正电荷氨基酸的区域,位于两个二硫键连接的半胱氨酸残基两侧。猪的氨基酸序列与人类序列有72%的同源性。然而,存在一些具有重要结构和功能意义的差异。人类apoB的精氨酸-3359对应于apoE LDL受体结合结构域中的一个关键精氨酸(第142位)残基。在猪中,这个精氨酸残基并不保守。此外,在所推测的apoB结合结构域附近发现的两个二硫键连接的半胱氨酸残基在猪中也不保守。尽管存在这些差异,但猪LDL对猪和人类LDL受体的亲和力都高于人类LDL。因此,这些特征对于猪LDL与LDL受体的高亲和力结合并非必需,对于人类LDL与LDL受体的结合可能也不是必需的。