Takahashi N, Takahashi Y, Putnam F W
Proc Natl Acad Sci U S A. 1985 Apr;82(7):1906-10. doi: 10.1073/pnas.82.7.1906.
The complete primary structure of the 3.1S leucine-rich alpha 2-glycoprotein (LRG) present in human plasma has been determined. This protein (Mr approximately 45,000) consists of a single polypeptide chain with one galactosamine and four glucosamine oligosaccharides attached. The polypeptide has two intrachain disulfide bonds and contains 312 amino acid residues of which 66 are leucine. The amino acid sequence can be exactly divided into 13 segments of 24 residues each, eight of which exhibit a periodic pattern in the occurrence of leucine, proline, and asparagine. The consensus sequence for the repeating tetracosapeptide unit is Pro-Xaa-Xaa-Leu-Leu-Xaa-Xaa-Xaa-X aa-Xaa-Leu-Xaa-Xaa-Leu-Xaa-Leu-Xaa-Xaa-Asn-Xaa-Leu-Xaa-Xaa-Leu. This periodicity suggests that the unique structure of LRG arose from a series of unequal crossovers of a precursor oligonucleotide sequence that encoded a building block rich in leucine. Overall, the amino acid sequence of LRG is not significantly homologous to the continuous sequence of any protein in the current data base. However, the consensus tetracosapeptide sequence shows strong homology to segments of many mitochondrial proteins, viral envelope proteins, and oncogene proteins that have a high leucine content and transmembrane domains. Tandem repetition of similar segments also occurs in apolipoproteins that have amphipathic helical potential. Prediction of the secondary structure by the Chou-Fasman rules and calculation of the hydrophilic/hydrophobic profile by several methods confirm the tandem repetition of largely hydrophobic structural units; these begin with a beta-turn that leads into an organized structure with alpha-helical or beta-sheet potential. These structural characteristics and the homology to mitochondrial proteins and apolipoproteins suggest that LRG is a membrane-derived or membrane-associated protein containing a series of domains capable of bipolar surface orientation.
已确定人血浆中存在的3.1S富含亮氨酸的α2-糖蛋白（LRG）的完整一级结构。这种蛋白质（Mr约为45,000）由一条带有一个半乳糖胺和四个葡糖胺寡糖的单多肽链组成。该多肽有两个链内二硫键，包含312个氨基酸残基，其中66个是亮氨酸。氨基酸序列可精确地分为13个各含24个残基的片段，其中8个在亮氨酸、脯氨酸和天冬酰胺的出现上呈现出周期性模式。重复的二十四肽单元的共有序列为Pro-Xaa-Xaa-Leu-Leu-Xaa-Xaa-Xaa-Xaa-Xaa-Leu-Xaa-Xaa-Leu-Xaa-Leu-Xaa-Xaa-Asn-Xaa-Leu-Xaa-Xaa-Leu。这种周期性表明LRG的独特结构源于编码富含亮氨酸的构件的前体寡核苷酸序列的一系列不等交换。总体而言，LRG的氨基酸序列与当前数据库中任何蛋白质的连续序列均无明显同源性。然而，共有二十四肽序列与许多线粒体蛋白、病毒包膜蛋白和具有高亮氨酸含量及跨膜结构域的癌基因蛋白的片段显示出很强的同源性。具有两亲性螺旋潜力的载脂蛋白中也存在类似片段的串联重复。通过Chou-Fasman规则预测二级结构并通过几种方法计算亲水/疏水分布图，证实了主要为疏水结构单元的串联重复；这些单元始于一个β-转角，进而形成具有α-螺旋或β-折叠潜力的有序结构。这些结构特征以及与线粒体蛋白和载脂蛋白的同源性表明，LRG是一种膜衍生或膜相关蛋白，含有一系列能够进行双极表面取向的结构域。
