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SH2-B family members differentially regulate JAK family tyrosine kinases
Karen B O'Brien1, John J O'Shea, Christin Carter-Su
1Department of Physiology, University of Michigan Medical School, Ann Arbor, Michigan 48109-0622, USA.
Abstract:
Activation of JAK tyrosine kinases is an essential step in cell signaling by multiple hormones, cytokines, and growth factors, including growth hormone (GH) and interferon-gamma. Previously, we identified SH2-B beta as a potent activator of JAK2 (Rui, L., and Carter-Su, C. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 7172-7177). Here, we investigated whether the activation of JAK2 by SH2-B beta is specific to JAK2 and SH2-B beta or extends to other JAKs or other members of the SH2-B beta family. When SH2-B beta was overexpressed with JAK1 or JAK3, SH2-B beta failed to increase their activity. However, SH2-B beta bound to both and was tyrosyl-phosphorylated by JAK1. In contrast to SH2-B beta, APS decreased tyrosyl phosphorylation of GH-stimulated JAK2 as well as Stat5B, a substrate of JAK2. APS also decreased tyrosyl phosphorylation of JAK1, but did not affect the activity or tyrosyl phosphorylation of JAK3. Overexpressed APS bound to and was tyrosyl-phosphorylated by all three JAKs. Consistent with these data, in 3T3-F442A adipocytes, endogenous APS was tyrosyl-phosphorylated in response to GH and interferon-gamma. These results suggest that 1) SH2-B beta specifically activates JAK2, 2) APS negatively regulates both JAK2 and JAK1, and 3) both SH2-B beta and APS may serve as adapter proteins for all three JAKs independent of any role they have in JAK activity.
Insights
SH2-B beta specifically activates Janus kinase 2 (JAK2), while APS negatively regulates JAK2 and JAK1. Both proteins may act as adapter proteins for JAK family kinases.
Area of Science:
- Cellular signaling pathways
- Protein-protein interactions
- Kinase regulation
Background:
- Janus kinases (JAKs) are crucial for cell signaling, mediating responses to hormones like growth hormone (GH) and interferon-gamma.
- SH2-B beta was previously identified as an activator of JAK2.
Purpose of the Study:
- To investigate the specificity of SH2-B beta's activation of JAK2.
- To determine if SH2-B beta or APS proteins interact with and affect the activity of other JAK family members (JAK1, JAK3).
Main Methods:
- Overexpression of SH2-B beta and APS with different JAKs in cell lines.
- Assessment of JAK activity and tyrosyl phosphorylation.
- Co-immunoprecipitation to detect protein binding.
Main Results:
- SH2-B beta specifically activated JAK2, but not JAK1 or JAK3, although it bound to JAK1 and JAK3.
- APS decreased the tyrosyl phosphorylation of GH-stimulated JAK2 and JAK1, but not JAK3.
- APS bound to and was phosphorylated by all three JAKs (JAK1, JAK2, JAK3).
Conclusions:
- SH2-B beta is a specific activator of JAK2.
- APS acts as a negative regulator for JAK2 and JAK1.
- Both SH2-B beta and APS can function as adapter proteins for JAK1, JAK2, and JAK3.