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Insulin-stimulated MAP-2 kinase phosphorylates and activates ribosomal protein S6 kinase II
T W Sturgill1, L B Ray, E Erikson
1Department of Internal Medicine, University of Virginia School of Medicine, Charlottesville 22903.
Abstract:
Ribosomal protein S6 is a component of the eukaryotic 40S ribosomal subunit that becomes phosphorylated on multiple serine residues in response to a variety of mitogens, including insulin, growth factors, and transforming proteins of many oncogenic viruses. Recently, an activated S6 kinase (S6 K II) has been purified to homogeneity from Xenopus eggs, and characterized immunologically and at the molecular level. Purified S6 K II can be deactivated in vitro by incubation with either protein phosphatase 1 or protein phosphatase 2A. Reactivation and phosphorylation of S6 K II occurs in vitro with an insulin-stimulated microtubule-associated protein-2 (MAP-2) protein kinase which is itself a phosphoprotein that can be deactivated by protein phosphatase 2A. These studies suggest that a step in insulin signalling involves sequential activation by phosphorylation of at least two serine/threonine protein kinases.
Insights
Insulin signaling involves sequential activation of protein kinases. Phosphorylation of ribosomal protein S6 and S6 kinase II by other kinases suggests a cascade in cellular growth regulation.
Area of Science:
- Molecular Biology
- Cell Signaling
- Biochemistry
Background:
- Ribosomal protein S6 is a key component of the eukaryotic 40S ribosomal subunit.
- S6 phosphorylation occurs in response to mitogens like insulin, growth factors, and viral oncoproteins.
- Understanding the regulation of S6 phosphorylation is crucial for deciphering cellular growth and proliferation pathways.
Purpose of the Study:
- To investigate the molecular mechanisms underlying insulin signaling.
- To characterize the activated S6 kinase II (S6 K II) purified from Xenopus eggs.
- To explore the sequential activation of protein kinases involved in insulin signal transduction.
Main Methods:
- Purification of activated S6 kinase II (S6 K II) from Xenopus eggs.
- In vitro deactivation assays using protein phosphatase 1 and protein phosphatase 2A.
- In vitro reactivation and phosphorylation studies using an insulin-stimulated MAP-2 protein kinase.
Main Results:
- Purified S6 K II can be deactivated by protein phosphatase 1 or 2A.
- S6 K II reactivation and phosphorylation in vitro involves an insulin-stimulated MAP-2 protein kinase.
- The MAP-2 protein kinase is itself a phosphoprotein regulated by protein phosphatase 2A.
Conclusions:
- Insulin signaling appears to involve a sequential activation cascade of at least two serine/threonine protein kinases.
- This cascade culminates in the phosphorylation of S6 kinase II and subsequently ribosomal protein S6.
- The findings provide insights into the intricate regulation of protein kinases in response to growth factor signaling.