Key points are not available for this paper at this time.
Microtubule-associated protein τ 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-τ 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-τ at the sites of Ser-199/Ser-202 and partially dephosphorylated it at Ser-396/Ser-404; in addition, PHF II-τ was dephosphorylated at Ser-46 by PP-2A1 and Ser-235 by PP-2B. The relative electrophoretic mobility of PHF II-τ increased after dephosphorylation by either enzyme. Divalent cations, manganese, and magnesium increased the activities of PP-2A1 and PP-2B toward PHF II-τ. Dephosphorylation both by PP-2B and PP-2A1 decreased the resistance of PHF II-τ to proteolysis by the brain calcium-activated neutral proteases (CANP). The ability of PHF II-τ to promote the in vitro microtubule assembly was restored after dephosphorylation by PP-2A1 and PP-2B. Microtubules assembled by the dephosphorylated PHF II-τ were structurally identical to those assembled by bovine τ 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 τ was released from PHF on dephosphorylation by PP-2A1. These observations demonstrate that PHF II-τ 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. Microtubule-associated protein τ 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-τ 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-τ at the sites of Ser-199/Ser-202 and partially dephosphorylated it at Ser-396/Ser-404; in addition, PHF II-τ was dephosphorylated at Ser-46 by PP-2A1 and Ser-235 by PP-2B. The relative electrophoretic mobility of PHF II-τ increased after dephosphorylation by either enzyme. Divalent cations, manganese, and magnesium increased the activities of PP-2A1 and PP-2B toward PHF II-τ. Dephosphorylation both by PP-2B and PP-2A1 decreased the resistance of PHF II-τ to proteolysis by the brain calcium-activated neutral proteases (CANP). The ability of PHF II-τ to promote the in vitro microtubule assembly was restored after dephosphorylation by PP-2A1 and PP-2B. Microtubules assembled by the dephosphorylated PHF II-τ were structurally identical to those assembled by bovine τ 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 τ was released from PHF on dephosphorylation by PP-2A1. These observations demonstrate that PHF II-τ 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.
Wang et al. (Wed,) studied this question.