title: Inhibition of signaling through the B cell antigen receptor by the protooncogene product, c-Cbl, requires Syk tyrosine 317 and the c-Cbl phosphotyrosine-binding domain.
abstract: The Syk protein-tyrosine kinase couples the B cell Ag receptor (BCR) to intracellular biochemical pathways. Syk becomes phosphorylated on multiple tyrosine residues upon receptor cross-linking. Tyrosine 317 is a site of phosphorylation located within the linker region of Syk that separates the amino-terminal, tandem pair of SH2 domains from the carboxyl-terminal catalytic domain. The amino acid sequence surrounding phosphotyrosine 317 matches the consensus sequence for recognition by the phosphotyrosine-binding (PTB) domain of the protooncogene product, c-Cbl. The overexpression of c-Cbl in DT40 B cells inhibits Ag receptor-mediated activation of the NF-AT transcription factor. The ability of overexpressed c-Cbl to inhibit signaling requires both Syk tyrosine 317 and a functional c-Cbl PTB domain. Mutant forms of Syk lacking tyrosine 317 exhibit an enhanced ability to couple the BCR to pathways leading to the activation of both NF-AT and Elk-1. Coimmunoprecipitation experiments indicate that Syk phosphotyrosine 317 and the c-Cbl PTB domain enhance, but are not required for, all interactions between these two proteins. In unstimulated cells, c-Cbl and Syk can be isolated in a complex that also contains tubulin. A mutant form of Syk lacking tyrosine at position 317 exhibits an enhanced ability to interact with a diphosphopeptide modeled on the immunoreceptor tyrosine-based activation motif of the CD79a component of the Ag receptor. These studies indicate that c-Cbl may contribute to the regulation of BCR signaling by modulating the ability of Syk to associate with the BCR and couple the receptor to intracellular signaling pathways.
authors.affiliation: Department of Medicinal Chemistry and Molecular Pharmacology, Purdue University, West Lafayette, IN 47907, USA.
authors.forename: T M
authors.initials: TM
authors.lastname: Yankee
authors.forename: L M
authors.initials: LM
authors.lastname: Keshvara
authors.forename: S
authors.initials: S
authors.lastname: Sawasdikosol
authors.forename: M L
authors.initials: ML
authors.lastname: Harrison
authors.forename: R L
authors.initials: RL
authors.lastname: Geahlen
mesh_terms: D000818:Animals; D001665:Binding Sites; D002645:Chickens; D004268:DNA-Binding Proteins / Q000378:metabolism; D004792:Enzyme Precursors / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins; D050778:NFATC Transcription Factors; D009687:Nuclear Proteins; D019000:Phosphotyrosine; D011485:Protein Binding; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D050721:Proto-Oncogene Proteins c-cbl; D011947:Receptors, Antigen, B-Cell / Q000378:metabolism*; D015398:Signal Transduction; D000072377:Syk Kinase; D014157:Transcription Factors / Q000378:metabolism; D014443:Tyrosine; D044767:Ubiquitin-Protein Ligases; D050807:ets-Domain Protein Elk-1
chemical_list: D004268:DNA-Binding Proteins; D004792:Enzyme Precursors; D047908:Intracellular Signaling Peptides and Proteins; D050778:NFATC Transcription Factors; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D011947:Receptors, Antigen, B-Cell; D014157:Transcription Factors; D050807:ets-Domain Protein Elk-1; D019000:Phosphotyrosine; D014443:Tyrosine; D050721:Proto-Oncogene Proteins c-cbl; D044767:Ubiquitin-Protein Ligases; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase; C067419:CBL protein, human