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Activation of the Rap1 GTPase by the B cell antigen receptor
S J McLeod1, R J Ingham, J L Bos
1Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
Abstract:
The B cell antigen receptor (BCR) activates Ras, a GTPase that promotes cell proliferation by activating the Raf-1/MEK/ERK signaling module and other signaling enzymes. In its active GTP-bound form, the Rap1 GTPase may act as a negative regulator of Ras-mediated signaling by sequestering Ras effectors (e.g., Raf-1) and preventing their activation. In this report, we show that BCR engagement activates Rap1 and that this is dependent on production of diacylglycerol (DAG) by phospholipase C-gamma. Activation of Rap1 by the BCR was greatly reduced in phospholipase C-gamma-deficient B cells, whereas both a synthetic DAG and phorbol dibutyrate could activate Rap1 in B cells. We had previously shown that C3G, an activator of Rap1, associates with the Crk adaptor proteins in B cells and that BCR engagement causes Crk to bind to the Cas and Cbl docking proteins. However, the DAG-dependent pathway by which the BCR activates Rap1 apparently does not involve Crk signaling complexes since phorbol dibutyrate could activate Rap1 without inducing the formation of these complexes. Thus, the BCR activates Rap1 via a novel DAG-dependent pathway.
Insights
The B cell antigen receptor (BCR) activates Rap1 GTPase via a novel pathway dependent on diacylglycerol (DAG) production. This signaling mechanism, distinct from prior pathways, involves BCR engagement and regulates cell proliferation.
Area of Science:
- Immunology
- Cell Signaling
- Molecular Biology
Background:
- The B cell antigen receptor (BCR) triggers Ras signaling, promoting cell proliferation via the Raf-1/MEK/ERK pathway.
- The Rap1 GTPase, in its active form, may negatively regulate Ras-mediated signaling by sequestering effectors like Raf-1.
Purpose of the Study:
- To investigate the mechanism by which BCR engagement activates Rap1 GTPase.
- To determine the role of diacylglycerol (DAG) and phospholipase C-gamma in BCR-mediated Rap1 activation.
Main Methods:
- Investigated Rap1 activation in response to BCR engagement in B cells.
- Utilized phospholipase C-gamma-deficient B cells to assess DAG-dependent signaling.
- Administered synthetic DAG and phorbol dibutyrate to evaluate Rap1 activation pathways.
Main Results:
- BCR engagement activates Rap1, a process dependent on diacylglycerol (DAG) produced by phospholipase C-gamma.
- Rap1 activation by BCR was significantly reduced in cells lacking phospholipase C-gamma.
- Synthetic DAG and phorbol dibutyrate activated Rap1 independently of Crk signaling complexes, indicating a novel pathway.
Conclusions:
- The BCR activates Rap1 through a novel DAG-dependent pathway.
- This pathway does not rely on the previously identified Crk signaling complexes.
- Understanding this Rap1 activation mechanism provides insights into BCR signaling regulation.