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The cytokine-activated tyrosine kinase JAK2 activates Raf-1 in a p21ras-dependent manner
K Xia1, N K Mukhopadhyay, R C Inhorn
1Dana-Farber Cancer Institute, Boston, MA, USA.
Abstract:
JAK2, a member of the Janus kinase superfamily was found to interact functionally with Raf-1, a central component of the ras/mitogen-activated protein kinase signal transduction pathway. Interferon-gamma and several other cytokines that are known to activate JAK2 kinase were also found to stimulate Raf-1 kinase activity toward MEK-1 in mammalian cells. In the baculovirus coexpression system, Raf-1 was activated by JAK2 in the presence of p21ras. Under these conditions, a ternary complex of p21ras, JAK2, and Raf-1 was observed. In contrast, in the absence of p21ras, coexpression of JAK2 and Raf-1 resulted in an overall decrease in the Raf-1 kinase activity. In addition, JAK2 phosphorylated Raf-1 at sites different from those phosphorylated by pp60v-src. In mammalian cells treated with either erythropoietin or interferon-gamma, a small fraction of Raf-1 coimmunoprecipitated with JAK2 in lysates of cells in which JAK2 was activated as judged by its state of tyrosine phosphorylation. Taken together, these data suggest that JAK2 and p21ras cooperate to activate Raf-1.
Insights
Janus kinase 2 (JAK2) and p21ras cooperate to activate Raf-1 kinase. This interaction is crucial for cytokine signaling pathways, particularly in response to interferon-gamma and erythropoietin.
Area of Science:
- Cellular signaling pathways
- Protein kinase interactions
- Molecular biology
Background:
- Janus kinase 2 (JAK2) is a key enzyme in cytokine signaling.
- Raf-1 is a central component of the ras/mitogen-activated protein kinase (MAPK) pathway.
- Cross-talk between JAK2 and MAPK pathways is not fully understood.
Purpose of the Study:
- To investigate the functional interaction between JAK2 and Raf-1.
- To elucidate the role of p21ras in JAK2-mediated Raf-1 activation.
- To understand how cytokines like interferon-gamma and erythropoietin influence this interaction.
Main Methods:
- Baculovirus coexpression system to study protein interactions in vitro.
- Kinase assays to measure Raf-1 activity.
- Coimmunoprecipitation to detect protein complexes in mammalian cells.
- Phosphorylation site analysis.
Main Results:
- JAK2 activates Raf-1 kinase activity, particularly in the presence of p21ras, forming a ternary complex.
- In the absence of p21ras, JAK2 decreases Raf-1 activity.
- JAK2 phosphorylates Raf-1 at distinct sites compared to pp60v-src.
- Activated JAK2 coimmunoprecipitates with Raf-1 in mammalian cells stimulated by cytokines.
Conclusions:
- JAK2 and p21ras cooperate to activate Raf-1.
- This cooperative activation is a significant mechanism in cytokine-induced signaling.
- The findings reveal a novel cross-talk between the JAK2 and MAPK pathways.