Switching proteins between two functional states via reversible phosphorylation is a fundamental process in signal transduction. Protein phosphatases are integral components of all phosphorylation-based signaling pathways. Our laboratory wants to understand the molecular basis of phosphatase action in normal and pathological cell signaling.
One project is focused on the role of the PR70 subunit of protein phosphatase 2A (PP2A) in targeting the enzyme to the Cdc6 protein. Cdc6 triggers DNA replication at the beginning of S phase. PR70 is a Cdc6-interacting protein identified using the yeast two-hybrid assay. Mutagenesis and biochemical analyses identified two EF-hand calcium binding motifs and regions of PR70 involved in binding to PP2A and Cdc6. Mass spectrometric analysis identified two in vivo phosphorylation sites in PR70. We are testing a model in which the PR70/PP2A complex maintains Cdc6 in the dephosphorylated state until the G1/S boundary. Changes in calcium or phosphorylation of PR70 could trigger dissociation of PP2A allowing rapid phosphorylation of Cdc6 and initiation of DNA replication.
A second area involves defining the roles of protein phosphorylation and phosphatases in signaling through mTOR pathway. The mTOR kinase is the target of the clinically used immunosuppressant drug rapamycin. The mTOR kinase and its associated signaling pathways play critical roles in the ability of cells to integrate growth and metabolism with environmental inputs including energy levels, nutrient availability, and growth factor signals. Two proteins involved in regulating mTOR (TSC2 and LKB1) are tumor suppressors. Genetic data from yeast has suggested a model in which members of the PP2A family of phosphatases (PP2A, PP4, and PP6) are normally sequestered away from ribosomal S6 kinase (a downstream target of mTOR) when mTOR is active. This allows phosphorylation of S6 kinase and stimulates protein synthesis. Inhibition of mTOR signaling, by starvation or other stresses, induces the association of these phosphatases with S6 kinase resulting in rapid dephosphorylation, kinase inactivation, and inhibition of protein synthesis. We are testing this model in both Drosophila and mammalian cells using RNAi, pharmacological inhibitors, phosphospecific antibodies, and mass spectrometry.
RESEARCH INTERESTS
Cellular Signal Transduction
Nutrient signaling
Protein Phosphatases
Proteomics
Protein Phosphorylation
RECENT PUBLICATIONS
Bielinski, V.A. and Mumby, M.C., "Functional analysis of the PP2A subfamily of protein phosphatases in regulating Drosophila S6 kinase" Exp Cell Research, 313:3117-3129, 2007
Mumby, M., "The 3D structure of protein phosphatase 2A: new insights into a ubiquitous regulator of cell signaling" ACS Chemical Biology, 2:99-103, 2007
Mumby, M., "PP2A: Unveiling a reluctant tumor suppressor" Cell, 130:21-24, 2007
Tang Z, Shu H, Qi W, Mahmood N, Mumby MC, Yu H., "PP2A is required for centromeric localization of Sgo1 and proper chromosomes segregation" Developmental Cell, 10:575-585, 2006
Liu W, Silverstein AM, Shu H, Martinez B, Mumby MC., "A functional genomics analysis of the B56-isoforms of Drosophila protein phosphatase 2A" Mol Cell Proteomics, 6:319-332, 2007
SIGNIFICANT PUBLICATIONS
Sontag E, Federov S, Kamibayashi C, Robbins D, Cobb M, Mumby M, "The interaction of SV40 small tumor antigen with protein phosphatase 2A stimulates the MAP kinase pathway and induces cell proliferation" Cell, 75:887-897, 1993
Sontag E, Nunbhakdi-Craig V, Brandt R, Lee G, Bloom GS, Mumby MC, "Regulation of the phosphorylation state and microtubule-binding activity of tau by protein phosphatase 2A" Neuron, 17:1201-1207, 1996
Yan Y, Shay JW, Wright W, Mumby MC, "Inhibition of protein phosphatase activity induces p53-dependent apoptosis in the absence of p53 transactivation" J Biol Chem, 272:15220-15226, 1997
Silverstein AM, Barrow CA, Davis AJ, Mumby MC, "The actions of PP2A on the MAP kinase pathway and apoptosis are mediated by distinct regulatory subunits" Proc. Nat. Acad. Sci. USA, 99:4221-4226, 2002
Yan Z, Fedorov SA, Mumby MC, Williams RS, "PR48, a novel regulatory subunit of PP2A, interacts with Cdc6 and modulates DNA replication in human cells" Mol Cell Biol, 20:1021-1029, 2000
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