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Molecular & Cellular Proteomics 8:681-695, 2009.
© 2009 by The American Society for Biochemistry and Molecular Biology, Inc.

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Research

Identification and Characterization of Neuronal Mitogen-activated Protein Kinase Substrates Using a Specific Phosphomotif Antibody^*,S

Dieter Edbauer^,, Dongmei Cheng^¶, Matthew N. Batterton, Chi-Fong Wang, Duc M. Duong^¶, Michael B. Yaffe^||, Junmin Peng^¶,** and Morgan Sheng^,

From The Picower Institute for Learning and Memory, Howard Hughes Medical Institute, RIKEN-MIT Neuroscience Research Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, ^¶ Department of Human Genetics, Center for Neurodegenerative Disease, Emory University, Atlanta, Georgia 30322, and ^|| Koch Institute for Integrative Cancer Research, Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142

Mitogen-activated protein kinases (MAPKs) control neuronal synaptic function; however, little is known about the synaptic substrates regulated by MAPKs. A phosphopeptide library incorporating the MAPK consensus motif (PX(pS/pT)P where pS is phosphoserine and pT is phosphothreonine) was used to raise a phosphospecific antibody that detected MAPK-mediated phosphorylation. The antibody (termed "5557") recognized a variety of phosphoproteins in the brain, many of which were enriched in postsynaptic density fractions. The immunoblot pattern changed rapidly in response to altered synaptic activity and with the inhibition of specific MAPKs and protein phosphatases. By immunoaffinity purification with 5557 antibody followed by mass spectrometry, we identified 449 putative MAPK substrates of which many appeared dynamically regulated in neuron cultures. Several of the novel candidate MAPK substrates were validated by in vitro phosphorylation assays. Additionally 82 specific phosphorylation sites were identified in 34 proteins, including Ser-447 in -catenin, a component of the cadherin adhesion complex. We further raised another phosphospecific antibody to confirm that -catenin Ser-447 is modified in neurons by the MAPK JNK in a synaptic activity-dependent manner. Ser-447 phosphorylation by JNK appears to be correlated with -catenin degradation, and a -catenin mutant defective in Ser-447 phosphorylation showed enhanced ability to promote dendrite branching in cultured neurons. Thus, phosphomotif-based affinity purification is a powerful approach to identify novel substrates of MAPKs in vivo and to reveal functionally significant phosphorylation events.

An investigator of the Howard Hughes Medical Institute. To whom correspondence may be addressed: The Picower Inst. for Learning and Memory/Howard Hughes Medical Inst., Massachusetts Inst. of Technology, 77 Massachusetts Ave. (46-4303), Cambridge, MA 02139. Tel.: 617-452-3716; Fax: 617-452-3692; E-mail: msheng{at}mit.edu

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