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The Inner Nuclear Membrane Protein Src1 Is Required for Stable Post-Mitotic Progression into G1 in Aspergillus nidulans.

Liu HL, Osmani AH, Osmani SA - PLoS ONE (2015)

Bottom Line: How membranes and associated proteins of the nuclear envelope (NE) are assembled specifically and inclusively around segregated genomes during exit from mitosis is incompletely understood.We suggest the term "reboot regulation" to define this mode of cell cycle regulation.The findings are discussed in relationship to recent studies showing the Cdk1 master oscillator can entrain subservient oscillators that when uncoupled cause cell cycle transitions to be repeated.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Genetics, Ohio State University, Columbus, Ohio 43210, United States of America.

ABSTRACT
How membranes and associated proteins of the nuclear envelope (NE) are assembled specifically and inclusively around segregated genomes during exit from mitosis is incompletely understood. Inner nuclear membrane (INM) proteins play key roles by providing links between DNA and the NE. In this study we have investigated the highly conserved INM protein Src1 in Aspergillus nidulans and have uncovered a novel cell cycle response during post mitotic formation of G1 nuclei. Live cell imaging indicates Src1 could have roles during mitotic exit as it preferentially locates to the NE abscission points during nucleokinesis and to the NE surrounding forming daughter G1 nuclei. Deletion analysis further supported this idea revealing that although Src1 is not required for interphase progression or mitosis it is required for stable post-mitotic G1 nuclear formation. This conclusion is based upon the observation that in the absence of Src1 newly formed G1 nuclei are structurally unstable and immediately undergo architectural modifications typical of mitosis. These changes include NPC modifications that stop nuclear transport as well as disassembly of nucleoli. More intriguingly, the newly generated G1 nuclei then cycle between mitotic- and interphase-like states. The findings indicate that defects in post-mitotic G1 nuclear formation caused by lack of Src1 promote repeated failed attempts to generate stable G1 nuclei. To explain this unexpected phenotype we suggest a type of regulation that promotes repetition of defective cell cycle transitions rather than preventing progression past the defective cell cycle transition. We suggest the term "reboot regulation" to define this mode of cell cycle regulation. The findings are discussed in relationship to recent studies showing the Cdk1 master oscillator can entrain subservient oscillators that when uncoupled cause cell cycle transitions to be repeated.

No MeSH data available.


Related in: MedlinePlus

Src1 preferentially locates around chromatin during mitotic exit in the absence of mitotic spindle function.Cells expressing the indicated pairs of tagged protein were treated with the microtubule poison benomyl and imaged during spindle independent mitotic exit (SIME) [17]. (A) Time-lapse images of Src1-GFP with the DNA marker H1-chRFP during the transition from a SAC imposed mitotic arrest and SIME, the start of which is indicated as time 0. Src1 can be observed to retract from the nuclear region to the right and locate preferentially around the chromatin located in the left side of the nucleus. When this process is completed (time 7’30”) the chromatin begins to undergo decondensation. (B) A similar time course as A but following the nucleolar marker Bop1 and Src1 to show that it is the nucleolus from which Src1 retracts when it preferentially locates to mitotic chromatin during mitotic exit. The arrow at time 7’ 30” indicates completion of the removal of Src1 from around the nucleolus. Scale bar 5μm.
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pone.0132489.g002: Src1 preferentially locates around chromatin during mitotic exit in the absence of mitotic spindle function.Cells expressing the indicated pairs of tagged protein were treated with the microtubule poison benomyl and imaged during spindle independent mitotic exit (SIME) [17]. (A) Time-lapse images of Src1-GFP with the DNA marker H1-chRFP during the transition from a SAC imposed mitotic arrest and SIME, the start of which is indicated as time 0. Src1 can be observed to retract from the nuclear region to the right and locate preferentially around the chromatin located in the left side of the nucleus. When this process is completed (time 7’30”) the chromatin begins to undergo decondensation. (B) A similar time course as A but following the nucleolar marker Bop1 and Src1 to show that it is the nucleolus from which Src1 retracts when it preferentially locates to mitotic chromatin during mitotic exit. The arrow at time 7’ 30” indicates completion of the removal of Src1 from around the nucleolus. Scale bar 5μm.

Mentions: In A. nidulans several mitotic events can occur in the absence of the mitotic spindle including exclusion of the nucleolus from the nucleus, its subsequent disassembly in the cytoplasm, and reassembly within nuclei [17, 46]. During such spindle independent mitotic exit (SIME) core NPC proteins, including Nup170, relocate from around the entire nuclear periphery to preferentially locate around the mitotic chromatin leaving the nucleolus outside the nucleus. Dispersed peripheral NPC proteins, such as Nup49, are also preferentially recruited back to the NE surrounding chromatin during SIME. We therefore determined if Src1 might similarly display a preferential redistribution to chromatin during SIME. During mitotic arrest, due to spindle assembly checkpoint (SAC) activation (entry into mitosis after addition of benomyl to depolymerize microtubules), chromatin condensed and Src1 remained around the nuclear periphery, surrounding both the condensed chromatin and the nucleolus (Fig 2A, compare the Interphase and Mitosis +Benomyl panels). After a defined period of time (~45 minutes) the SAC is inactivated in a regulated manner allowing exit from mitosis in the absence of spindle function [46]. During exit from SAC arrest there was a clear transition in the location of Src1; it retracts from the region of the nuclear periphery not surrounding chromatin to locate more exclusively around mitotic chromatin (Fig 2A, compare merged image 0” to 7’30” and time points in between). After Src1 locates around the condensed chromatin, the chromatin then undergoes decondensation indicating exit from the mitotic state (Fig 2A 7’30”– 13’30”).


The Inner Nuclear Membrane Protein Src1 Is Required for Stable Post-Mitotic Progression into G1 in Aspergillus nidulans.

Liu HL, Osmani AH, Osmani SA - PLoS ONE (2015)

Src1 preferentially locates around chromatin during mitotic exit in the absence of mitotic spindle function.Cells expressing the indicated pairs of tagged protein were treated with the microtubule poison benomyl and imaged during spindle independent mitotic exit (SIME) [17]. (A) Time-lapse images of Src1-GFP with the DNA marker H1-chRFP during the transition from a SAC imposed mitotic arrest and SIME, the start of which is indicated as time 0. Src1 can be observed to retract from the nuclear region to the right and locate preferentially around the chromatin located in the left side of the nucleus. When this process is completed (time 7’30”) the chromatin begins to undergo decondensation. (B) A similar time course as A but following the nucleolar marker Bop1 and Src1 to show that it is the nucleolus from which Src1 retracts when it preferentially locates to mitotic chromatin during mitotic exit. The arrow at time 7’ 30” indicates completion of the removal of Src1 from around the nucleolus. Scale bar 5μm.
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pone.0132489.g002: Src1 preferentially locates around chromatin during mitotic exit in the absence of mitotic spindle function.Cells expressing the indicated pairs of tagged protein were treated with the microtubule poison benomyl and imaged during spindle independent mitotic exit (SIME) [17]. (A) Time-lapse images of Src1-GFP with the DNA marker H1-chRFP during the transition from a SAC imposed mitotic arrest and SIME, the start of which is indicated as time 0. Src1 can be observed to retract from the nuclear region to the right and locate preferentially around the chromatin located in the left side of the nucleus. When this process is completed (time 7’30”) the chromatin begins to undergo decondensation. (B) A similar time course as A but following the nucleolar marker Bop1 and Src1 to show that it is the nucleolus from which Src1 retracts when it preferentially locates to mitotic chromatin during mitotic exit. The arrow at time 7’ 30” indicates completion of the removal of Src1 from around the nucleolus. Scale bar 5μm.
Mentions: In A. nidulans several mitotic events can occur in the absence of the mitotic spindle including exclusion of the nucleolus from the nucleus, its subsequent disassembly in the cytoplasm, and reassembly within nuclei [17, 46]. During such spindle independent mitotic exit (SIME) core NPC proteins, including Nup170, relocate from around the entire nuclear periphery to preferentially locate around the mitotic chromatin leaving the nucleolus outside the nucleus. Dispersed peripheral NPC proteins, such as Nup49, are also preferentially recruited back to the NE surrounding chromatin during SIME. We therefore determined if Src1 might similarly display a preferential redistribution to chromatin during SIME. During mitotic arrest, due to spindle assembly checkpoint (SAC) activation (entry into mitosis after addition of benomyl to depolymerize microtubules), chromatin condensed and Src1 remained around the nuclear periphery, surrounding both the condensed chromatin and the nucleolus (Fig 2A, compare the Interphase and Mitosis +Benomyl panels). After a defined period of time (~45 minutes) the SAC is inactivated in a regulated manner allowing exit from mitosis in the absence of spindle function [46]. During exit from SAC arrest there was a clear transition in the location of Src1; it retracts from the region of the nuclear periphery not surrounding chromatin to locate more exclusively around mitotic chromatin (Fig 2A, compare merged image 0” to 7’30” and time points in between). After Src1 locates around the condensed chromatin, the chromatin then undergoes decondensation indicating exit from the mitotic state (Fig 2A 7’30”– 13’30”).

Bottom Line: How membranes and associated proteins of the nuclear envelope (NE) are assembled specifically and inclusively around segregated genomes during exit from mitosis is incompletely understood.We suggest the term "reboot regulation" to define this mode of cell cycle regulation.The findings are discussed in relationship to recent studies showing the Cdk1 master oscillator can entrain subservient oscillators that when uncoupled cause cell cycle transitions to be repeated.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Genetics, Ohio State University, Columbus, Ohio 43210, United States of America.

ABSTRACT
How membranes and associated proteins of the nuclear envelope (NE) are assembled specifically and inclusively around segregated genomes during exit from mitosis is incompletely understood. Inner nuclear membrane (INM) proteins play key roles by providing links between DNA and the NE. In this study we have investigated the highly conserved INM protein Src1 in Aspergillus nidulans and have uncovered a novel cell cycle response during post mitotic formation of G1 nuclei. Live cell imaging indicates Src1 could have roles during mitotic exit as it preferentially locates to the NE abscission points during nucleokinesis and to the NE surrounding forming daughter G1 nuclei. Deletion analysis further supported this idea revealing that although Src1 is not required for interphase progression or mitosis it is required for stable post-mitotic G1 nuclear formation. This conclusion is based upon the observation that in the absence of Src1 newly formed G1 nuclei are structurally unstable and immediately undergo architectural modifications typical of mitosis. These changes include NPC modifications that stop nuclear transport as well as disassembly of nucleoli. More intriguingly, the newly generated G1 nuclei then cycle between mitotic- and interphase-like states. The findings indicate that defects in post-mitotic G1 nuclear formation caused by lack of Src1 promote repeated failed attempts to generate stable G1 nuclei. To explain this unexpected phenotype we suggest a type of regulation that promotes repetition of defective cell cycle transitions rather than preventing progression past the defective cell cycle transition. We suggest the term "reboot regulation" to define this mode of cell cycle regulation. The findings are discussed in relationship to recent studies showing the Cdk1 master oscillator can entrain subservient oscillators that when uncoupled cause cell cycle transitions to be repeated.

No MeSH data available.


Related in: MedlinePlus