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Premature chromatin condensation upon accumulation of NIMA
M J O'Connell1, C Norbury, P Nurse
1Cell Cycle Laboratory, Imperial Cancer Research Fund, London, UK.
Abstract:
The NIMA protein kinase of Aspergillus nidulans is required for the G2/M transition of the cell cycle. Mutants lacking NIMA arrest without morphological characteristics of mitosis, but they do contain an activated p37nimX kinase (the Aspergillus homologue of p34cdc2). To gain a better understanding of NIMA function we have investigated the effects of expressing various NIMA constructs in Aspergillus, fission yeast and human cells. Our experiments have shown that the instability of the NIMA protein requires sequences in the non-catalytic C-terminus of the protein. Removal of this domain results in a stable protein that, once accumulated, promotes a lethal premature condensation of chromatin without any other aspects of mitosis. Similar effects were also observed in fission yeast and human cells accumulating Aspergillus NIMA. This phenotype is independent of cell cycle progression and does not require p34cdc2 kinase activity. As gain of NIMA function by accumulation results in premature chromatin condensation, and loss of NIMA function results in an inability to enter mitosis, we propose that NIMA functions in G2 to promote the condensation of chromatin normally associated with entry into mitosis.
Insights
The NIMA protein kinase is crucial for cell cycle progression. Accumulating NIMA causes premature chromatin condensation, while its absence prevents mitosis entry, highlighting its G2 phase role.
Area of Science:
- Cell Biology
- Molecular Biology
- Genetics
Background:
- The NIMA protein kinase in Aspergillus nidulans regulates the G2/M cell cycle transition.
- NIMA is essential for proper mitotic entry, as NIMA-deficient mutants arrest without mitotic features but retain active p34cdc2 kinase activity.
Purpose of the Study:
- To elucidate the function of NIMA by examining the effects of expressing various NIMA constructs in Aspergillus, fission yeast, and human cells.
- To understand the structural requirements for NIMA protein stability and its role in cell cycle regulation.
Main Methods:
- Expression of different NIMA protein constructs in Aspergillus nidulans, fission yeast, and human cell lines.
- Analysis of cell cycle progression, mitotic entry, and chromatin condensation phenotypes.
- Investigation of the role of the C-terminal domain in NIMA protein stability and function.
Main Results:
- NIMA protein instability is mediated by sequences in its non-catalytic C-terminus.
- Removal of the C-terminus yields a stable NIMA protein that induces lethal premature chromatin condensation independent of cell cycle progression or p34cdc2 kinase activity.
- Expression of Aspergillus NIMA in fission yeast and human cells recapitulated the premature chromatin condensation phenotype.
Conclusions:
- NIMA kinase activity is essential for promoting chromatin condensation during the G2 phase, a prerequisite for mitotic entry.
- Gain of NIMA function leads to premature chromatin condensation, while loss of function results in an inability to initiate mitosis.
- NIMA plays a critical role in orchestrating the events leading to mitosis, specifically chromatin condensation.