title: Identification of major ERK-related phosphorylation sites in Gab1.
abstract: Gab1 (Grb2-associated binder1) belongs to a family of multifunctional docking proteins that play a central role in the integration of receptor tyrosine kinase (RTK) signaling, i.e., mediating cellular growth response, transformation, and apoptosis. In addition to RTK-specific tyrosine phosphorylation, these docking proteins also can be phosphorylated on serine/threonine residues affecting signal transduction. Since serine and threonine phosphorylation are capable of modulating the initial signal one major task to elucidate signal transduction via Gab1 is to determine the exact localization of distinct phosphorylation sites. To address this question in this report we examined extracellular signal-regulated kinases 1/2 (ERK) specific serine/threonine phosphorylation of the entire Gab1 engaged in insulin signaling in more detail in vitro. To elucidate the ERK1/2-specific phosphorylation pattern of Gab1, we used phosphopeptide mapping by two-dimensional HPLC analysis. Subsequently, phosphorylated serine/threonine residues were identified by sequencing the separated phosphopeptides using matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) and Edman degradation. Our results demonstrate that ERK1/2 phosphorylate Gab1 at six serine/threonine residues (T312, S381, S454, T476, S581, S597) in consensus motifs for MAP kinase phosphorylation. Serine residues S454, S581, S597, and threonine residue T476 represent nearly 80% of overall incorporated phosphate. These sites are located adjacent to src homology region-2 (SH2) binding motifs (YVPM-motif: Y447, Y472, Y619) specific for the phosphatidylinositol 3kinase (PI3K). The biological role of identified phosphorylation sites was proven by PI3K and Akt activity in intact cells. These data demonstrate that ERK1/2 modulate insulin action via Gab1 by targeting serine and threonine residues beside YXXM motifs. Accordingly, insulin signaling is blocked at the level of PI3K.
authors.affiliation: Institute for Clinical Biochemistry and Pathobiochemistry, German Diabetes Center at the Heinrich-Heine-University, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Lehr
authors.forename: Jorg
authors.initials: J
authors.lastname: Kotzka
authors.forename: Haluk
authors.initials: H
authors.lastname: Avci
authors.forename: Albert
authors.initials: A
authors.lastname: Sickmann
authors.forename: Helmut E
authors.initials: HE
authors.lastname: Meyer
authors.forename: Armin
authors.initials: A
authors.lastname: Herkner
authors.forename: Dirk
authors.initials: D
authors.lastname: Muller-Wieland
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D002851:Chromatography, High Pressure Liquid; D006224:Cricetinae; D004789:Enzyme Activation / Q000187:drug effects; D005786:Gene Expression Regulation; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism*; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D010449:Peptide Mapping; D010750:Phosphoproteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D012694:Serine / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013912:Threonine / Q000235:genetics / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C098587:GAB1 protein, human; D007328:Insulin; D010446:Peptide Fragments; D010750:Phosphoproteins; D011518:Proto-Oncogene Proteins; D013912:Threonine; D012694:Serine; C494918:AKT1 protein, human; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases