Leucine Carboxyl Methyltransferase 1 (LCMT1)-dependent Methylation Regulates the Association of Protein Phosphatase 2A and Tau Protein with Plasma Membrane Microdomains in Neuroblastoma Cells

Abstract

Down-regulation of protein phosphatase 2A (PP2A) methylation occurs in Alzheimer disease (AD). However, the regulation of PP2A methylation remains poorly understood. We have reported that altered leucine carboxyl methyltransferase (LCMT1)-dependent PP2A methylation is associated with down-regulation of PP2A holoenzymes containing the Bα subunit (PP2A/Bα) and subsequent accumulation of phosphorylated Tau in N2a cells, in vivo and in AD. Here, we show that pools of LCMT1, methylated PP2A, and PP2A/Bα are co-enriched in cholesterol-rich plasma membrane microdomains/rafts purified from N2a cells. In contrast, demethylated PP2A is preferentially distributed in non-rafts wherein small amounts of the PP2A methylesterase PME-1 are exclusively present. A methylation-incompetent PP2A mutant is excluded from rafts. Enhanced methylation of PP2A promotes the association of PP2A and Tau with the plasma membrane. Altered PP2A methylation following expression of a catalytically inactive LCMT1 mutant, knockdown of LCMT1, or alterations in one-carbon metabolism all result in a loss of plasma membrane-associated PP2A and Tau in N2a cells. This correlates with accumulation of soluble phosphorylated Tau, a hallmark of AD and other tauopathies. Thus, our findings reveal a distinct compartmentalization of PP2A and PP2A regulatory enzymes in plasma membrane microdomains and identify a novel methylation-dependent mechanism involved in modulating the targeting of PP2A, and its substrate Tau, to the plasma membrane. We propose that alterations in the membrane localization of PP2A and Tau following down-regulation of LCMT1 may lead to PP2A and Tau dysfunction in AD.