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Phosphoinositide 3-kinase in rat liver nuclei
1Division of Medicinal Chemistry and Pharmaceutics, College of Pharmacy, University of Kentucky, Lexington 40536, USA.
Biochemistry
|May 16, 1998
Summary
This study identifies phosphoinositide 3-kinase (PI 3-kinase) in rat liver nuclei, demonstrating its autonomous function. Nuclear PI 3-kinase activity is regulated by proteins like CapG, linking it to signal transduction pathways.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Phosphoinositide 3-kinases (PI 3-kinases) are crucial signaling enzymes.
- Their presence and function within the cell nucleus are not fully understood.
- Investigating nuclear PI 3-kinase activity is essential for understanding intracellular signal transduction.
Purpose of the Study:
- To investigate the presence and activity of PI 3-kinase within rat liver nuclei.
- To characterize the enzymes involved in phosphoinositide metabolism in the nucleus.
- To explore the regulatory mechanisms of nuclear PI 3-kinase, including its interaction with nuclear proteins.
Main Methods:
- Biochemical and immunochemical assays to detect PI 3-kinase.
- [gamma-32P]ATP labeling of nuclear phosphoinositides.
- Subnuclear fractionation to localize PI 3-kinase activity.
- In vitro assays to examine the effect of nuclear proteins (CapG) on PI 3-kinase activity.
Main Results:
- A functional p85/p110 PI 3-kinase was identified in rat liver nuclei, producing phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] and phosphatidylinositol 3-phosphate [PtdIns(3)P].
- Nuclear soluble fractions contain PI 3-kinase along with phospholipase C (PLC), phosphoinositide phosphatase, and diacylglycerol (DAG) kinase, indicating an active inositide metabolism.
- Nuclear PI 3-kinase, analogous to PI 3-kinase alpha, constitutes about 5% of the total cellular enzyme and its activity can be inhibited by the nuclear actin-regulatory protein CapG, particularly under conditions of PKC phosphorylation and elevated calcium.
- This CapG-mediated regulation suggests a cross-communication link between nuclear PLC and PI 3-kinase pathways.
Conclusions:
- Rat liver nuclei possess an autonomous phosphoinositide 3-kinase cycle.
- Nuclear PI 3-kinase activity is regulated by interactions with nuclear proteins like CapG, influenced by calcium and PKC.
- The findings suggest a significant role for nuclear PI 3-kinase in signal transduction from the plasma membrane to the nucleus.