Limits...
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 concentrates to chromatin during mitotic exit.(A) Protein domain structure of human MAN1, budding yeast Src1 (also named Heh1) compared to A. nidulans Src1 indicting the relative positions of the N-terminal LEM domain PF12949, the internal Man1-Src1p-C-terminal (MSC) domain PF09402, and the predicted transmembrane domains. (B-D) Images from live cell spinning disc confocal microscopy of mitosis in cells expressing the indicated tagged proteins. (B) Src1 associates around mitotic chromatin (marked by histone H1-chRFP) in a punctate pattern and also to two foci (white arrowheads 3’ 45”) at sites corresponding to NE abscissions during anaphase-telophase. (C) Src1 locates preferentially to the chromatin of reforming daughter nuclei in a punctate pattern and not around the nucleolus as marked by the nucleolar protein Bop1-chRFP. (D) Two nuclei shown at G2, M, and G1. The ER/NE marker Erg24 locates around both forming daughter nuclei and the nucleolus during anaphase-telophase [17]. However, at this stage of mitosis Src1 preferentially locates around the reforming daughter nuclei in a punctate pattern and not around the nucleolus (merged mitotic panel). Src1 also concentrates at the two NE abscission points indicated by the pairs of arrowheads. Scale bar 5μm.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4492595&req=5

pone.0132489.g001: Src1 concentrates to chromatin during mitotic exit.(A) Protein domain structure of human MAN1, budding yeast Src1 (also named Heh1) compared to A. nidulans Src1 indicting the relative positions of the N-terminal LEM domain PF12949, the internal Man1-Src1p-C-terminal (MSC) domain PF09402, and the predicted transmembrane domains. (B-D) Images from live cell spinning disc confocal microscopy of mitosis in cells expressing the indicated tagged proteins. (B) Src1 associates around mitotic chromatin (marked by histone H1-chRFP) in a punctate pattern and also to two foci (white arrowheads 3’ 45”) at sites corresponding to NE abscissions during anaphase-telophase. (C) Src1 locates preferentially to the chromatin of reforming daughter nuclei in a punctate pattern and not around the nucleolus as marked by the nucleolar protein Bop1-chRFP. (D) Two nuclei shown at G2, M, and G1. The ER/NE marker Erg24 locates around both forming daughter nuclei and the nucleolus during anaphase-telophase [17]. However, at this stage of mitosis Src1 preferentially locates around the reforming daughter nuclei in a punctate pattern and not around the nucleolus (merged mitotic panel). Src1 also concentrates at the two NE abscission points indicated by the pairs of arrowheads. Scale bar 5μm.

Mentions: Interestingly, many INM proteins, including lamins and the lamin-associated proteins Emerin, Otefin, BAF, Lap1 and Lap2, are only found in metazoans [31, 32]. On the other hand the LEM2/MAN1-like group of the LEM (Lap2-Emerin-MAN1) family of integral inner nuclear membrane proteins is conserved [33]. All LEM2/MAN1-like proteins contain two transmembrane domains and a MSC (MAN1/Src1p/C-terminal) motif (see Fig 1A) and help mediate NE-chromatin interactions. For example, the Man1 homolog in S. cerevisiae, Src1 (also termed Heh1) is an INM protein that preferentially interacts with subtelomeric chromatin and the ribosomal DNA (rDNA) locus [34–36] as also found for Man1 in S. pombe [37, 38]. Src1 also plays a role in sister chromatid segregation during budding yeast mitosis [39] and is involved in the assembly of NPCs, potentially through the formation of a luminal bridge with the transmembrane Nup Pom152 [40]. In mammalian cells MAN1 has been implicated in a redundant manner with other INM chromatin binding proteins during nuclear reassembly after mitosis [30]. In C. elegans the LEM domain proteins Ce-Man1 and Ce-Emerin are required for successful exit from mitosis and in their absence lagging chromosome bridges are formed which fail to lose their mitotic characteristics [41]. Interestingly in the fission yeast S. japonicus, which partially disassembles its NE during mitosis [42, 43], Man1 is required as a tether for even distribution of NPCs to daughter G1 nuclei by linking NPCs to segregating chromatin and is required for normal disassembly and inheritance of nucleoli [44].


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 concentrates to chromatin during mitotic exit.(A) Protein domain structure of human MAN1, budding yeast Src1 (also named Heh1) compared to A. nidulans Src1 indicting the relative positions of the N-terminal LEM domain PF12949, the internal Man1-Src1p-C-terminal (MSC) domain PF09402, and the predicted transmembrane domains. (B-D) Images from live cell spinning disc confocal microscopy of mitosis in cells expressing the indicated tagged proteins. (B) Src1 associates around mitotic chromatin (marked by histone H1-chRFP) in a punctate pattern and also to two foci (white arrowheads 3’ 45”) at sites corresponding to NE abscissions during anaphase-telophase. (C) Src1 locates preferentially to the chromatin of reforming daughter nuclei in a punctate pattern and not around the nucleolus as marked by the nucleolar protein Bop1-chRFP. (D) Two nuclei shown at G2, M, and G1. The ER/NE marker Erg24 locates around both forming daughter nuclei and the nucleolus during anaphase-telophase [17]. However, at this stage of mitosis Src1 preferentially locates around the reforming daughter nuclei in a punctate pattern and not around the nucleolus (merged mitotic panel). Src1 also concentrates at the two NE abscission points indicated by the pairs of arrowheads. Scale bar 5μm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4492595&req=5

pone.0132489.g001: Src1 concentrates to chromatin during mitotic exit.(A) Protein domain structure of human MAN1, budding yeast Src1 (also named Heh1) compared to A. nidulans Src1 indicting the relative positions of the N-terminal LEM domain PF12949, the internal Man1-Src1p-C-terminal (MSC) domain PF09402, and the predicted transmembrane domains. (B-D) Images from live cell spinning disc confocal microscopy of mitosis in cells expressing the indicated tagged proteins. (B) Src1 associates around mitotic chromatin (marked by histone H1-chRFP) in a punctate pattern and also to two foci (white arrowheads 3’ 45”) at sites corresponding to NE abscissions during anaphase-telophase. (C) Src1 locates preferentially to the chromatin of reforming daughter nuclei in a punctate pattern and not around the nucleolus as marked by the nucleolar protein Bop1-chRFP. (D) Two nuclei shown at G2, M, and G1. The ER/NE marker Erg24 locates around both forming daughter nuclei and the nucleolus during anaphase-telophase [17]. However, at this stage of mitosis Src1 preferentially locates around the reforming daughter nuclei in a punctate pattern and not around the nucleolus (merged mitotic panel). Src1 also concentrates at the two NE abscission points indicated by the pairs of arrowheads. Scale bar 5μm.
Mentions: Interestingly, many INM proteins, including lamins and the lamin-associated proteins Emerin, Otefin, BAF, Lap1 and Lap2, are only found in metazoans [31, 32]. On the other hand the LEM2/MAN1-like group of the LEM (Lap2-Emerin-MAN1) family of integral inner nuclear membrane proteins is conserved [33]. All LEM2/MAN1-like proteins contain two transmembrane domains and a MSC (MAN1/Src1p/C-terminal) motif (see Fig 1A) and help mediate NE-chromatin interactions. For example, the Man1 homolog in S. cerevisiae, Src1 (also termed Heh1) is an INM protein that preferentially interacts with subtelomeric chromatin and the ribosomal DNA (rDNA) locus [34–36] as also found for Man1 in S. pombe [37, 38]. Src1 also plays a role in sister chromatid segregation during budding yeast mitosis [39] and is involved in the assembly of NPCs, potentially through the formation of a luminal bridge with the transmembrane Nup Pom152 [40]. In mammalian cells MAN1 has been implicated in a redundant manner with other INM chromatin binding proteins during nuclear reassembly after mitosis [30]. In C. elegans the LEM domain proteins Ce-Man1 and Ce-Emerin are required for successful exit from mitosis and in their absence lagging chromosome bridges are formed which fail to lose their mitotic characteristics [41]. Interestingly in the fission yeast S. japonicus, which partially disassembles its NE during mitosis [42, 43], Man1 is required as a tether for even distribution of NPCs to daughter G1 nuclei by linking NPCs to segregating chromatin and is required for normal disassembly and inheritance of nucleoli [44].

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