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Published on: October 30, 2012
Molecular cloning of a docking protein, BRDG1, that acts downstream of the Tec tyrosine kinase
K Ohya1, S Kajigaya, A Kitanaka
1Department of Molecular Biology, Jichi Medical School, Kawachi-gun, Tochigi 329-0498, Japan.
Abstract:
Tec, Btk, Itk, Bmx, and Txk constitute the Tec family of protein tyrosine kinases (PTKs), a family with the distinct feature of containing a pleckstrin homology (PH) domain. Tec acts in signaling pathways triggered by the B cell antigen receptor (BCR), cytokine receptors, integrins, and receptor-type PTKs. Although upstream regulators of Tec family kinases are relatively well characterized, little is known of the downstream effectors of these enzymes. The yeast two-hybrid system has identified several proteins that interact with the kinase domain of Tec, one of which is now revealed to be a previously unknown docking protein termed BRDG1 (BCR downstream signaling 1). BRDG1 contains a proline-rich motif, a PH domain, and multiple tyrosine residues that are potential target sites for Src homology 2 domains. In 293 cells expressing recombinant BRDG1 and various PTKs, Tec and Pyk2, but not Btk, Bmx, Lyn, Syk, or c-Abl, induced marked phosphorylation of BRDG1 on tyrosine residues. BRDG1 was also phosphorylated by Tec directly in vitro. Efficient phosphorylation of BRDG1 by Tec required the PH and SH2 domains as well as the kinase domain of the latter. Furthermore, BRDG1 was shown to participate in a positive feedback loop by increasing the activity of Tec. BRDG1 transcripts are abundant in the human B cell line Ramos, and the endogenous protein underwent tyrosine phosphorylation in response to BCR stimulation. BRDG1 thus appears to function as a docking protein acting downstream of Tec in BCR signaling.
Insights
Researchers identified BRDG1, a novel docking protein that acts downstream of Tec kinase in B cell receptor signaling. BRDG1 phosphorylation by Tec enhances Tec activity, suggesting a positive feedback loop crucial for B cell signaling pathways.
Area of Science:
- Biochemistry
- Cell Biology
- Immunology
Background:
- Tec family kinases, including Tec, are crucial for B cell receptor (BCR) signaling.
- While upstream regulators are known, downstream effectors of Tec family kinases remain largely uncharacterized.
Purpose of the Study:
- To identify and characterize novel downstream effectors of Tec family kinases, specifically focusing on BCR signaling.
- To elucidate the function and regulatory mechanisms of a newly identified protein, BRDG1.
Main Methods:
- Yeast two-hybrid system to identify Tec-interacting proteins.
- Recombinant protein expression and in vitro kinase assays in 293 cells.
- Analysis of BRDG1 phosphorylation by various protein tyrosine kinases (PTKs).
- Assessment of BRDG1's effect on Tec activity and its role in BCR signaling in B cells.
Main Results:
- A novel docking protein, BRDG1, was identified through yeast two-hybrid screening as a Tec-interacting protein.
- Tec and Pyk2, but not other kinases, phosphorylated BRDG1 on tyrosine residues.
- Tec directly phosphorylated BRDG1 in vitro, requiring Tec's PH, SH2, and kinase domains.
- BRDG1 enhanced Tec kinase activity, indicating a positive feedback loop.
- BRDG1 is expressed in B cells and phosphorylated upon BCR stimulation, confirming its role in BCR signaling.
Conclusions:
- BRDG1 functions as a critical downstream docking protein for Tec kinase in BCR signaling pathways.
- The interaction and phosphorylation of BRDG1 by Tec contribute to a positive feedback mechanism that regulates Tec activity.
- BRDG1 represents a novel component in the BCR signaling cascade, offering potential targets for therapeutic intervention.
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