Bratt T, Olsson H, Sjöberg E M, Jergil B, Akerström B
Department of Medical and Physiological Chemistry, University of Lund, Sweden.
Biochim Biophys Acta. 1993 Jun 11;1157(2):147-54. doi: 10.1016/0304-4165(93)90058-g.
alpha 1-Microglobulin, a plasma protein with immunoregulatory properties, and bikunin, the light chain of the proteinase inhibitors inter-alpha-inhibitor and pre-alpha-inhibitor, are translated as a precursor protein from the same mRNA. The cosynthesis of alpha 1-microglobulin and bikunin is unique compared to other proproteins such as procomplement components and prohormones, since alpha 1-microglobulin and bikunin have no known functional connection. Different forms of intracellular rat liver alpha 1-microglobulin were isolated and characterized by amino acid sequence analysis, lectin binding and glycosidase treatment. Their subcellular distribution was studied by Nycodenz and sucrose gradient centrifugation, pulse-chase experiments, and electrophoresis with subsequent immunoblotting, using pro-C3 and prohaptoglobin as reference proteins. Two alpha 1-microglobulin-bikunin precursors (40 and 42 kDa), containing one and two N-linked oligosaccharides, respectively, were detected in the endoplasmic reticulum. After transport to the Golgi apparatus, the precursors were cleaved, probably C-terminal to the sequence Arg-Ala-Arg-Arg immediately preceding the bikunin part, yielding free sialylated 28 kDa alpha 1-microglobulin, representing the mature protein. The cleavage was almost complete in phosphatidylinositol 4-kinase-enriched membranes, previously identified as a post-Golgi compartment. A fourth intracellular form of alpha 1-microglobulin, 26 kDa, lacked sialic acid. None of the intracellular forms carried the yellow-brown chromophore associated with alpha 1-microglobulin when purified from serum and urine, suggesting that this chromophore becomes linked to the protein after its secretion from the liver cells.
α1-微球蛋白是一种具有免疫调节特性的血浆蛋白，而比昆宁是蛋白酶抑制剂间α-抑制剂和前α-抑制剂的轻链，它们由同一mRNA翻译为前体蛋白。与其他前体蛋白如补体前体成分和前激素相比，α1-微球蛋白和比昆宁的共合成是独特的，因为α1-微球蛋白和比昆宁没有已知的功能联系。通过氨基酸序列分析、凝集素结合和糖苷酶处理，分离并鉴定了大鼠肝细胞内不同形式的α1-微球蛋白。以C3前体和触珠蛋白前体作为参考蛋白，通过 Nycodenz 和蔗糖梯度离心、脉冲追踪实验以及随后的免疫印迹电泳研究了它们的亚细胞分布。在内质网中检测到两种α1-微球蛋白-比昆宁前体（40 kDa和42 kDa），分别含有一个和两个N-连接寡糖。转运至高尔基体后，前体被切割，可能是在比昆宁部分之前紧邻的Arg-Ala-Arg-Arg序列的C末端进行切割，产生游离的唾液酸化28 kDaα1-微球蛋白，即成熟蛋白。在富含磷脂酰肌醇4-激酶的膜（先前鉴定为高尔基体后区室）中，切割几乎完全完成。α1-微球蛋白的第四种细胞内形式为26 kDa，缺乏唾液酸。从血清和尿液中纯化时，没有一种细胞内形式带有与α1-微球蛋白相关的黄褐色发色团，这表明该发色团在肝细胞分泌蛋白后与蛋白相连。
