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Mol Cell Biol, July 1998, p. 4262-4271, Vol. 18, No. 7
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Activated Polo-Like Kinase Plx1 Is Required at Multiple Points during Mitosis in Xenopus laevis

Yue-Wei Qian, Eleanor Erikson, Chuan Li, and James L. Maller*

Howard Hughes Medical Institute and Department of Pharmacology, University of Colorado School of Medicine, Denver, Colorado 80262

Received 26 January 1998/Returned for modification 10 March 1998/Accepted 21 April 1998

Entry into mitosis depends upon activation of the dual-specificity phosphatase Cdc25C, which dephosphorylates and activates the cyclin B-Cdc2 complex. Previous work has shown that the Xenopus polo-like kinase Plx1 can phosphorylate and activate Cdc25C in vitro. In the work presented here, we demonstrate that Plx1 is activated in vivo during oocyte maturation with the same kinetics as Cdc25C. Microinjection of wild-type Plx1 into Xenopus oocytes accelerated the rate of activation of Cdc25C and cyclin B-Cdc2. Conversely, microinjection of either an antibody against Plx1 or kinase-dead Plx1 significantly inhibited the activation of Cdc25C and cyclin B-Cdc2. This effect could be reversed by injection of active Cdc25C, indicating that Plx1 is upstream of Cdc25C. However, injection of Cdc25C, which directly activates cyclin B-Cdc2, also caused activation of Plx1, suggesting that a positive feedback loop exists in the Plx1 activation pathway. Other experiments show that injection of Plx1 antibody into early embryos, which do not require Cdc25C for the activation of cyclin B-Cdc2, resulted in an arrest of cleavage that was associated with monopolar spindles. These results demonstrate that in Xenopus laevis, Plx1 plays important roles both in the activation of Cdc25C at the initiation of mitosis and in spindle assembly at late stages of mitosis.

* Corresponding author. Mailing address: Howard Hughes Medical Institute and Department of Pharmacology, University of Colorado School of Medicine, Biomedical Research Bldg., Room 433, 4200 Ninth Ave., Box C-236, Denver, CO 80262-0236. Phone: (303) 315-7075. Fax: (303) 315-7160. E-mail: mallerj{at}essex.uchsc.edu.

Mol Cell Biol, July 1998, p. 4262-4271, Vol. 18, No. 7
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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