De Jonge H, Schmeeda H, Shaltiel S
Department of Chemical Immunology, Weizmann Institute of Science, Rehovot, Israel.
Eur J Biochem. 1987 Dec 15;169(3):503-9. doi: 10.1111/j.1432-1033.1987.tb13638.x.
The active site of the rat intestinal brush-border membranal proteinase [Alhanaty E. and Shaltiel S. (1979) Biochem. Biophys. Res. Commun. 89, 323-332], which splits the catalytic subunit (C) of cAMP-dependent protein kinase with a remarkable specificity [Alhanaty E., Tauber-Finkelstein, M., Schmeeda, H. and Shaltiel, S. (1985) Curr. Topics Cell. Regul. 27, 267-277], is shown to face predominantly the cell exterior; vesicles prepared from these brush-borders (mostly sealed and right-side-out) fully express the proteinase activity as judged by the fact that there is no increase in activity upon rupture or solubilization of the vesicles. Although the brush-border vesicles contain a cAMP-dependent protein kinase, this membrane-bound kinase is not likely to be the physiological target of the proteinase, since it appears to have an intracellular orientation and, at least in the vesicles, to be inaccessible to the proteinase. It is, therefore, suggested that the physiological substrate of the proteinase might be either an extracellular cAMP-dependent protein kinase, which is lost (e.g. removed, inactivated or degraded) in the course of vesicle isolation, or a kinase domain in one of the family of proteins recently shown to have a considerable structural and conformational homology with C. Alternatively the physiological site of action of this kinase-splitting proteinase might be an intracellular organelle to which it is translocated by endocytosis.
大鼠肠道刷状缘膜蛋白酶[阿尔哈纳蒂E.和沙尔蒂尔S.（1979年）《生物化学与生物物理学研究通讯》89卷，323 - 332页]的活性位点主要面向细胞外；从这些刷状缘制备的囊泡（大多为封闭的、外表面外翻的）充分表达蛋白酶活性，这可通过囊泡破裂或溶解后活性不增加这一事实来判断。尽管刷状缘囊泡含有一种依赖cAMP的蛋白激酶，但这种膜结合激酶不太可能是蛋白酶的生理靶点，因为它似乎具有细胞内取向，并且至少在囊泡中，蛋白酶无法接近它。因此，有人提出蛋白酶的生理底物可能要么是一种细胞外依赖cAMP的蛋白激酶，它在囊泡分离过程中丢失了（例如被去除、失活或降解），要么是最近显示与催化亚基（C）具有相当结构和构象同源性的蛋白质家族中的一个激酶结构域。或者，这种裂解激酶的蛋白酶的生理作用位点可能是通过内吞作用转运到的一个细胞内细胞器。
