Loyet K M, Kowalchyk J A, Chaudhary A, Chen J, Prestwich G D, Martin T F
Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
J Biol Chem. 1998 Apr 3;273(14):8337-43. doi: 10.1074/jbc.273.14.8337.
The calcium-dependent activator protein for secretion (CAPS) is a novel neural/endocrine-specific cytosolic and peripheral membrane protein required for the Ca2+-regulated exocytosis of secretory vesicles. CAPS acts at a stage in exocytosis that follows ATP-dependent priming, which involves the essential synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). In the present studies, CAPS is shown to bind liposomes that contain acidic phospholipids and binding was markedly enhanced by inclusion of PtdIns(4,5)P2 but not other phosphoinositides in the absence of Ca2+. PtdIns(4,5)P2, but not other phosphoinositides including PtdIns(3, 4)P2 and PtdIns(3,4,5)P3, altered the susceptibility of CAPS to proteolysis by trypsin and proteinase K, suggesting that phosphoinositide binding promoted a conformational change. Photoaffinity labeling studies with a photoactivatable benzoylcinnimidyl acyl chain derivative of PtdIns(4,5)P2 confirmed the phosphoinositide-binding properties of CAPS and suggested a hydrophobic aspect of the interaction. CAPS, as one of very few characterized proteins with a binding specificity for 4-, 5-phosphorylated inositides over 3-phosphorylated inositides, may function in regulated exocytosis as an effector of PtdIns(4,5)P2.
分泌型钙依赖性激活蛋白（CAPS）是一种新型的神经/内分泌特异性胞质和外周膜蛋白，是分泌囊泡Ca2+调节性胞吐作用所必需的。CAPS在胞吐作用中作用于ATP依赖性引发之后的阶段，这一阶段涉及磷脂酰肌醇4,5-二磷酸（PtdIns(4,5)P2）的必需合成。在本研究中，CAPS被证明可结合含有酸性磷脂的脂质体，并且在不存在Ca2+的情况下，通过包含PtdIns(4,5)P2而非其他磷酸肌醇，结合作用显著增强。PtdIns(4,5)P2而非其他磷酸肌醇（包括PtdIns(3,4)P2和PtdIns(3,4,5)P3）改变了CAPS对胰蛋白酶和蛋白酶K蛋白水解的敏感性，这表明磷酸肌醇结合促进了构象变化。用PtdIns(4,5)P2的光活化苯甲酰亚胺基酰基链衍生物进行的光亲和标记研究证实了CAPS的磷酸肌醇结合特性，并提示了相互作用的疏水方面。作为极少数对4,5-磷酸化肌醇具有比对3-磷酸化肌醇更高结合特异性的已鉴定蛋白质之一，CAPS可能在调节性胞吐作用中作为PtdIns(4,5)P2的效应器发挥作用。
