Wang J Z, Gong C X, Zaidi T, Grundke-Iqbal I, Iqbal K
New York State Institute for Basic Research in Developmental Disabilities, Staten Island 10314.
J Biol Chem. 1995 Mar 3;270(9):4854-60. doi: 10.1074/jbc.270.9.4854.
Microtubule-associated protein tau is abnormally hyperphosphorylated in the brain of patients with Alzheimer disease and in this form is the major protein subunit of the paired helical filaments (PHF), the most prominent lesion of the disease. In this study the dephosphorylation of sparingly soluble PHF, PHF II-tau by brain protein phosphatase (PP)-2A1 and PP-2B, and the resulting biochemical, biological, and structural alterations were investigated. Both of the phosphatases dephosphorylated PHF II-tau at the sites of Ser-199/Ser-202 and partially dephosphorylated it at Ser-396/Ser-404; in addition, PHF II-tau was dephosphorylated at Ser-46 by PP-2A1 and Ser-235 by PP-2B. The relative electrophoretic mobility of PHF II-tau increased after dephosphorylation by either enzyme. Divalent cations, manganese, and magnesium increased the activities of PP-2A1 and PP-2B toward PHF II-tau. Dephosphorylation both by PP-2B and PP-2A1 decreased the resistance of PHF II-tau to proteolysis by the brain calcium-activated neutral proteases (CANP). The ability of PHF II-tau to promote the in vitro microtubule assembly was restored after dephosphorylation by PP-2A1 and PP-2B. Microtubules assembled by the dephosphorylated PHF II-tau were structurally identical to those assembled by bovine tau used as a control. The dephosphorylation both by PP-2A1 and PP-2B caused dissociation of the tangles and the PHF; some of the PHF dissociated into straight protofilaments/subfilaments. Approximately 25% of the total tau was released from PHF on dephosphorylation by PP-2A1. These observations demonstrate that PHF II-tau is accessible to dephosphorylation by PP-2A1 and PP-2B, and dephosphorylation makes PHF dissociate, accessible to proteolysis by CANP, and biologically active in promoting the assembly of tubulin into microtubules.
微管相关蛋白tau在阿尔茨海默病患者大脑中异常过度磷酸化，这种形式是成对螺旋丝（PHF）的主要蛋白质亚基，而PHF是该疾病最显著的病变。在本研究中，研究了脑蛋白磷酸酶（PP）-2A1和PP-2B对难溶性PHF即PHF II-tau的去磷酸化作用，以及由此产生的生化、生物学和结构改变。两种磷酸酶都使PHF II-tau在Ser-199/Ser-202位点去磷酸化，并使其在Ser-396/Ser-404位点部分去磷酸化；此外，PP-2A1使PHF II-tau在Ser-46位点去磷酸化，PP-2B使PHF II-tau在Ser-235位点去磷酸化。经任何一种酶去磷酸化后，PHF II-tau的相对电泳迁移率增加。二价阳离子、锰和镁增加了PP-2A1和PP-2B对PHF II-tau的活性。PP-2B和PP-2A1的去磷酸化作用均降低了PHF II-tau对脑钙激活中性蛋白酶（CANP）蛋白水解的抗性。经PP-2A1和PP-2B去磷酸化后，PHF II-tau促进体外微管组装的能力得以恢复。由去磷酸化的PHF II-tau组装的微管在结构上与用作对照的牛tau组装的微管相同。PP-2A1和PP-2B的去磷酸化作用均导致缠结和PHF解离；一些PHF解离成直的原纤维/亚纤维。经PP-2A1去磷酸化后，约25%的总tau从PHF中释放出来。这些观察结果表明，PP-2A1和PP-2B可使PHF II-tau去磷酸化，去磷酸化使PHF解离，易于被CANP进行蛋白水解，并在促进微管蛋白组装成微管方面具有生物活性。
