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Molecular and Cellular Biology, April 2007, p. 2572-2581, Vol. 27, No. 7
0270-7306/07/$08.00+0 doi:10.1128/MCB.01611-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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Department of Biochemistry and Cell Biology, Graduate School of Medical Sciences, Nagoya City University, 1 Kawasumi, Mizuho-ku, Mizuho-cho, Nagoya 467-8601, Japan,1 Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117597, Singapore2
Received 29 August 2006/ Returned for modification 2 October 2006/ Accepted 3 January 2007
Chk1 is a multifunctional protein kinase that plays essential roles in cell survival and cell cycle checkpoints. Chk1 is phosphorylated at multiple sites by several protein kinases, but the precise effects of these phosphorylations are largely unknown. Using a knockout-knockin system, we examined the abilities of Chk1 mutants to reverse the defects of Chk1-null cells. Wild-type Chk1 could rescue all the defects of Chk1-null cells. Like endogenous Chk1, wild-type Chk1 localized in both the cytoplasm and the nucleus, and its centrosomal association was enhanced by DNA damage. The mutation at S345 resulted in mitotic catastrophe, impaired checkpoints, and loss of the ability to localize in the cytoplasm, but the mutant retained the ability to be released from chromatin upon encountering genotoxic stressors. In contrast, the mutation at S317 resulted in impaired checkpoints and loss of chromatin release upon encountering genotoxic stressors, but its mutant retained the abilities to prevent mitotic catastrophes and to localize in the cytoplasm, suggesting the distinct effects of these phosphorylations. The forced immobilization of S317A/S345A in centrosomes resulted in the prevention of apoptosis in the presence or absence of DNA damage. Thus, two-step phosphorylation of Chk1 at S317 and S345 appeared to be required for proper localization of Chk1 to centrosomes.
Published ahead of print on 22 January 2007.
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