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A feedback loop in the polo-like kinase activation pathway
Eleanor Erikson1, Timothy A J Haystead, Yue-Wei Qian
1Howard Hughes Medical Institute and Department of Pharmacology, University of Colorado School of Medicine, Denver, 80262, USA.
The Journal of Biological Chemistry
|May 29, 2004
Summary
Xenopus polo-like kinase 1 (Plx1) and its kinase, xPlkk1, were initially thought to form a positive feedback loop. However, new findings suggest xPlkk1 acts downstream of Plx1 during the G(2)/M transition.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Plx1 is crucial for mitotic entry and exit in Xenopus.
- Plx1 activation involves phosphorylation by upstream kinases.
- xPlkk1 was identified as a kinase that phosphorylates and activates Plx1 in vitro.
Purpose of the Study:
- To investigate the regulatory relationship between Plx1 and xPlkk1.
- To determine the role of xPlkk1 in the G(2)/M transition.
Main Methods:
- In vitro kinase assays using purified proteins.
- Phosphorylation site analysis using radiolabeling and sequencing.
- Site-directed mutagenesis to create non-phosphorylatable xPlkk1.
- Protein depletion experiments in oocyte extracts.
Main Results:
- A positive feedback loop was initially proposed, where Plx1 and xPlkk1 activate each other.
- xPlkk1 has specific phosphorylation sites targeted by Plx1, which are phosphorylated during M phase.
- A mutant xPlkk1 (xPlkk1(SA3)) lacking these sites was not activated by Plx1.
- Depletion of Plx1 blocked xPlkk1 activation, but xPlkk1 depletion did not block Plx1 activation.
Conclusions:
- The results indicate that xPlkk1 functions as a downstream target of Plx1.
- xPlkk1 activation by Plx1 is essential for the G(2)/M transition.
- The proposed positive feedback loop is re-evaluated, suggesting a different regulatory mechanism.