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A serine/threonine kinase p90rsk1 phosphorylates the anti-proliferative protein Tob
T Suzuki1, S Matsuda, J K Tsuzuku
1Department of Oncology, Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan.
Background:
tob is a member of a gene family with anti-proliferative function. Over-expression of Tob in NIH3T3 cells results in the suppression of cell proliferation. The growth suppression is hampered by the presence of activated ErbB2 kinase. The molecular mechanisms by which Tob suppresses cell growth and by which ErbB2 abrogates Tob function remain to be elucidated.
Results:
We show that Tob is phosphorylated on serines and threonines, but not tyrosines, by a kinase(s) that associates with Tob in the lysates of various cells, including ErbB2-over-expressed cells. We also show that a 95 kDa kinase associates with Tob in vitro. The autophosphorylation activity of this kinase co-chromatographes with Tob-phosphorylating activity, suggesting that the 95 kDa kinase phosphorylates Tob. Among the known kinases with molecular mass around 95 kDa, p90rsk1 associates with Tob in vitro and in vivo, and phosphorylates Tob at least in vitro. Therefore, it is likely that p90rsk1 represents the 95 kDa kinase and is involved in the regulation of Tob function through phosphorylation.
Conclusion:
p90rsk1 associates with and phosphorylates Tob. Because p90rsk1 is activated downstream of receptor tyrosine kinases, we propose that Tob function is at least in part under the control of growth factor-stimulated tyrosine kinases through its phosphorylation by p90rsk1.
Insights
The Tob protein, which inhibits cell proliferation, is phosphorylated by p90rsk1. This phosphorylation suggests growth factor signaling pathways regulate Tob
Area of Science:
- Cellular Biology
- Molecular Biology
- Signal Transduction
Background:
- Tob is a gene family member with anti-proliferative functions.
- Tob overexpression suppresses NIH3T3 cell proliferation.
- Activated ErbB2 kinase can hinder Tob's growth suppression activity.
Purpose of the Study:
- To elucidate the molecular mechanisms of Tob-mediated growth suppression.
- To understand how ErbB2 abrogates Tob function.
- To identify the kinase responsible for Tob phosphorylation.
Main Methods:
- Cell lysis and protein association assays.
- In vitro kinase assays.
- Co-chromatography and Western blotting.
Main Results:
- Tob is phosphorylated on serine and threonine residues by an associated kinase.
- A 95 kDa kinase associates with Tob and phosphorylates it.
- p90rsk1 (a 95 kDa kinase) associates with and phosphorylates Tob in vitro and in vivo.
Conclusions:
- p90rsk1 associates with and phosphorylates Tob.
- Tob function is regulated by growth factor-stimulated tyrosine kinases via p90rsk1 phosphorylation.
- This identifies a novel regulatory pathway for Tob's anti-proliferative role.