No abstract

Protein phosphatase 2A (PP2A) is a family of multifunctional serine/threonine phosphatases all composed of a catalytic C, a structural A, and a regulatory B subunit. Assembly of the complex with the appropriate B subunit forms the key to the functional specificity and regulation of PP2A. Emerging evidence suggests a crucial role for methylation and phosphorylation of the PP2A C subunit in this process. In this study, we show that PP2A C subunit methylation was not absolutely required for binding the PR61/B and PR72/B؆ subunit families, whereas binding of the PR55/B subunit family was determined by methylation and the nature of the C-terminal amino acid side chain. Moreover mutation of the phosphorylatable Tyr 307 or Thr 304 residues differentially affected binding of distinct B subunit family members. Down-regulation of the PP2A methyltransferase LCMT1 by RNA interference gradually reduced the cellular amount of methylated C subunit and induced a dynamic redistribution of the remaining methylated PP2A C between different PP2A trimers consistent with their methylation requirements. Persistent knockdown of LCMT1 eventually resulted in specific degradation of the PR55/B subunit and apoptotic cell death. Together these results establish a crucial foundation for understanding PP2A regulatory subunit selection.Protein phosphatase 2A (PP2A) 2 represents a family of heterotrimeric serine/threonine phosphatases implicated in the regulation of a plethora of cellular processes such as apoptosis, transcription, translation, DNA replication, signal transduction, protection against tumorigenesis, and cell division (for reviews, see Refs. 1 and 2). It is estimated that, depending on the cell type, PP2A holoenzymes are responsible for 30 -50% of the total cellular serine/threonine dephosphorylating activity, demonstrating the importance of this enzyme system for almost any aspect of life.The basis of this broad functional importance is formed by the diversity of specific PP2A subunit compositions. Typically the PP2A core enzyme exists as a dimer (PP2A D ) consisting of a catalytic subunit (C subunit/PP2A C ) and a scaffolding A subunit (PR65/A subunit). Proper functioning and regulation of PP2A is achieved by the association of regulatory B subunits 3 with the PP2A core enzyme, resulting in the formation of heterotrimeric PP2A holoenzymes with specific catalytic properties, subcellular locations, and substrate specificities. At present, three multigene families of B-type subunits have been described, PR55/B, PR61/BЈ, and PR72/BЉ, all with specific cellular functions. Therefore, the assembly of the complex with the appropriate B-type subunit is the key to specificity and regulation of PP2A (2). In this process, the highly conserved C-terminal PP2A C tail seems to play a crucial role (3, 4). Recently a major breakthrough has been achieved by elucidating the crystal structure of a heterotrimeric PP2A T61␥ holoenzyme (5, 6). It was shown that the C-terminal PP2A C tail recognizes a surface groove at the interface between the PR65 a...

Functional diversity of protein phosphatase 2A (PP2A) enzymes mainly results from their association with distinct regulatory subunits. To analyze the functions of one such holoenzyme in vivo, we generated mice lacking PR61/B'δ (B56δ), a subunit highly expressed in neural tissues. In PR61/B'δ-null mice the microtubule-associated protein tau becomes progressively phosphorylated at pathological epitopes in restricted brain areas, with marked immunoreactivity for the misfolded MC1-conformation but without neurofibrillary tangle formation. Behavioral tests indicated impaired sensorimotor but normal cognitive functions. These phenotypical characteristics were further underscored in PR61/B'δ-null mice mildly overexpressing human tau. PR61/B'δ-containing PP2A (PP2A T61δ ) poorly dephosphorylates tau in vitro, arguing against a direct dephosphorylation defect. Rather, the activity of glycogen synthase kinase-3β, a major tau kinase, was found increased, with decreased phosphorylation of Ser-9, a putative cyclin-dependent kinase 5 (CDK5) target. Accordingly, CDK5 activity is decreased, and its cellular activator p35, strikingly absent in the affected brain areas. As opposed to tau, p35 is an excellent PP2A T61δ substrate. Our data imply a nonredundant function for PR61/B'δ in phospho-tau homeostasis via an unexpected spatially restricted mechanism preventing p35 hyperphosphorylation and its subsequent degradation.brain stem | knockout mouse | AT8 | AT180 | transgenic

We have described recently the purification and cloning of PP2A (protein phosphatase 2A) leucine carboxylmethyltransferase. We studied the purification of a PP2A-specific methylesterase that co-purifies with PP2A and found that it is tightly associated with an inactive dimeric or trimeric form of PP2A. These inactive enzyme forms could be reactivated as Ser/Thr phosphatase by PTPA (phosphotyrosyl phosphatase activator of PP2A). PTPA was described previously by our group as a protein that stimulates the in vitro phosphotyrosyl phosphatase activity of PP2A; however, PP2A-specific methyltransferase could not bring about the activation. The PTPA activation could be distinguished from the Mn2+ stimulation observed with some inactive forms of PP2A, also found associated with PME-1 (phosphatase methylesterase 1). We discuss a potential new function for PME-1 as an enzyme that stabilizes an inactivated pool of PP2A.

The protein phosphatase 2A (PP2A) phosphatase activator (PTPA) is an essential protein involved in the regulation of PP2A and the PP2A-like enzymes. In this study we demonstrate that PTPA and its yeast homologues Ypa1 and Ypa2 can induce a conformational change in some model substrates. Using these model substrates in different assays with and without helper proteases, this isomerase activity is similar to the isomerase activity of FKBP12, the human cyclophilin A, and one of its yeast homologs Cpr7 but dissimilar to the isomerase activity of Pin1. However, neither FKBP12 nor Cpr7 can reactivate the inactive form of PP2A. Therefore, PTPA belongs to a novel peptidyl-prolyl cis/trans-isomerase (