Bhargava M M, Listowsky I, Arias I M
J Biol Chem. 1978 Jun 25;253(12):4116-9.
Several lines of evidence indicate that ligandin consists of two different subunits. The protein dissociates into two components that are detected by electrophoresis in a discontinuous sodium dodecyl sulfate system, or in acid-urea gels, and by isoelectric focusing in the presence of urea. The apparent molecular weights of the two polypeptides are 25,000 and 22,000. Alkylated or succinylated ligandins also exhibit subunit heterogeneity and resolved into two bands in these electrophoretic systems. Cross-linked ligandin showed only one band in sodium dodecyl sulfate-gel electrophoresis indicating that the two subunits are part of a heterodimeric protein rather than monomers of two different proteins. No dansylated terminal amino acids were detected suggesting that the NH2-terminal residues of both chains are blocked. One mole of arginine or phenylalanine was released per mole of ligandin after digestion with carboxypeptidase B or A, respectively. Tryptic maps of succinylated ligandin were consistent with identical disposition of arginine residues in both chains, but several additional tryptic peptides were obtained with native ligandin as compared to the predicted number if both subunits were identical. These observations are consistent with the possibility that both subunits contain common sequences and that a small peptide of about 25 to 30 amino acid residues is cleaved from the COOH-terminal of the larger subunit to produce the smaller subunit.
几条证据表明，配体蛋白由两个不同的亚基组成。在不连续十二烷基硫酸钠系统或酸性尿素凝胶中进行电泳，以及在有尿素存在的情况下进行等电聚焦时，该蛋白质会解离成两个组分。这两种多肽的表观分子量分别为25,000和22,000。烷基化或琥珀酰化的配体蛋白在这些电泳系统中也表现出亚基异质性，并可分离成两条带。交联的配体蛋白在十二烷基硫酸钠-凝胶电泳中仅显示一条带，这表明这两个亚基是异二聚体蛋白的一部分，而不是两种不同蛋白质的单体。未检测到丹磺酰化的末端氨基酸，这表明两条链的NH2末端残基均被封闭。分别用羧肽酶B或A消化后，每摩尔配体蛋白释放出一摩尔精氨酸或苯丙氨酸。琥珀酰化配体蛋白的胰蛋白酶图谱与两条链中精氨酸残基的相同排列一致，但与两个亚基相同情况下预测的数量相比，天然配体蛋白获得了一些额外的胰蛋白酶肽。这些观察结果与以下可能性一致，即两个亚基都包含共同的序列，并且从较大亚基的COOH末端切割出一个约25至30个氨基酸残基的小肽以产生较小的亚基。
