Kojima S, Terabe M, Taguchi S, Momose H, Miura K
Institute for Biomolecular Science, Gakushuin University, Tokyo, Japan.
Biochim Biophys Acta. 1994 Jul 20;1207(1):120-5. doi: 10.1016/0167-4838(94)90060-4.
Protein proteinase inhibitors showing sequence homology with Streptomyces subtilisin inhibitor (SSI) have been found to be distributed widely in Streptomyces species, and accordingly have been named SSI-like (SIL) proteins. SIL1 from S. cacaoi was the first of these proteins to be isolated and to be given a serial number. To study the structure-function relationship of SIL proteins, we determined the primary structure of SIL1 and measured its inhibitory activities. It was found to be composed of 110 amino acids and to exist in dimer form. The amino-acid sequence of SIL1 was unique among other characterized SIL proteins in having a one-residue deletion in two regions and a three-residue insertion in the flexible loop region. Sequence comparison indicated that SIL1 was distantly related to other members of the SSI family, and that amino-acid replacements had occurred not only on the surface of the SIL1 molecule but also in the beta-sheet region. The reactive site of SIL1 was considered to be Arg70-Glu71 from sequence alignment with other SSI-family inhibitors. SIL1 inhibited subtilisin BPN' strongly with an inhibitor constant (Ki) of 2.8 x 10(-11) M, like other members of the SSI family possessing an Arg residue at the P1 site. In contrast, SIL1 exhibited weak inhibition toward trypsin with a Ki value of 5.5 x 10(-8) M, possibly as a consequence of insertion of the three residues in the flexible loop region near the reactive site. This contrast seems to be due to the difference in the subsite structure of the two proteinases.
已发现与链霉菌枯草杆菌蛋白酶抑制剂（SSI）具有序列同源性的蛋白质蛋白酶抑制剂广泛分布于链霉菌属物种中，因此被命名为SSI样（SIL）蛋白。来自可可链霉菌的SIL1是这些蛋白质中第一个被分离并被赋予序列号的。为了研究SIL蛋白的结构-功能关系，我们测定了SIL1的一级结构并测量了其抑制活性。发现它由110个氨基酸组成，以二聚体形式存在。SIL1的氨基酸序列在其他已表征的SIL蛋白中是独特的，在两个区域有一个残基缺失，在柔性环区域有一个三个残基的插入。序列比较表明，SIL1与SSI家族的其他成员关系较远，并且氨基酸替换不仅发生在SIL1分子的表面，也发生在β-折叠区域。通过与其他SSI家族抑制剂的序列比对，认为SIL1的活性位点是Arg70-Glu71。与在P1位点具有Arg残基的SSI家族的其他成员一样，SIL1以2.8×10^(-11) M的抑制常数（Ki）强烈抑制枯草杆菌蛋白酶BPN'。相比之下，SIL1对胰蛋白酶的抑制作用较弱，Ki值为5.5×10^(-8) M，这可能是由于在活性位点附近的柔性环区域插入了三个残基所致。这种差异似乎是由于两种蛋白酶的亚位点结构不同。
