Biology subjects

Kotak, S.

Publications and source records attributed to Kotak, S..

5 recordsLinked to original sources

bioRxiv2025

Phase transition of spindle pole localized protein orchestrates nuclear organization at mitotic exit

AbstractAnimal cells dismantle their nuclear envelope (NE) at the beginning and reconstruct it at the end of mitosis. This process is closely coordinated with spindle pole organization: poles enlarge at mitotic onset and reduce size as mitosis concludes. The significance of this coordination remains unknown. Here, we demonstrate that Aurora A maintains a pole-localized protein NuMA in a dynamic state during anaphase. Without Aurora A, NuMA shifts from a dynamic to a solid phase, abnormally accumulating at the poles, leading to chromosome bending and misshaped nuclei formation around poles. NuMA localization relies on interactions with dynein/dynactin, its coiled-coil domain, and intrinsically disordered region (IDR). Mutagenesis experiments revealed that cation-{pi} interactions within IDR are key for NuMA localization, while glutamine residues trigger its solid-state transition upon Aurora A inhibition. This study emphasizes the role of the physical properties of spindle poles in organizing the nucleus and genome post-mitosis.

cell biology↗

bioRxiv2023

PP2A-B55SUR-6 promotes nuclear envelope breakdown in C. elegans embryos

Nuclear envelope (NE) disassembly during mitosis is critical to ensure faithful segregation of the genetic material. NE disassembly is a phosphorylation-dependent process wherein mitotic kinases hyper-phosphorylate lamina and nucleoporins to initiate nuclear envelope breakdown (NEBD). In this study, we uncover an unexpected role of the PP2A phosphatase B55SUR-6 in NEBD during the first embryonic division of Caenorhabditis elegans embryo. B55SUR-6 depletion delays NE permeabilization and stabilizes lamina and nucleoporins. As a result, the merging of parental genomes and chromosome segregation is impaired. This NEBD defect upon B55SUR-6 depletion is not due to delayed mitotic onset or mislocalization of mitotic kinases. Importantly, we demonstrate that microtubule-dependent mechanical forces synergize with B55SUR-6 for efficient NE disassembly. Finally, our data suggest that the lamin LMN-1 is likely a bona fide target of PP2A-B55SUR-6. These findings establish a model highlighting biochemical cross-talk between kinases, PP2A-B55SUR-6 phosphatase, and microtubule-generated mechanical forces in timely NE dissolution.

cell biology↗

bioRxiv2022

Membrane compartmentalization of Ect2/Cyk4/Mklp1 and NuMA/dynein/dynactin is essential for cleavage furrow formation during anaphase

In animal cells, spindle elongation during anaphase is temporally coupled with cleavage furrow formation. Spindle elongation during anaphase is regulated by NuMA/dynein/dynactin complexes that occupy the polar region of the cell membrane and are excluded from the equatorial membrane. How NuMA/dynein/dynactin are excluded from the equatorial membrane and the biological significance of this exclusion remains unknown. Here, we show that the centralspindlin (Cyk4/Mklp1) and its interacting partner RhoGEF Ect2 are required for NuMA/dynein/dynactin exclusion from the equatorial cell membrane. The Ect2-based (Ect2/Cyk4/Mklp1) and NuMA-based (NuMA/dynein/dynactin) complexes occupy mutually exclusive membrane surfaces during anaphase. The equatorial membrane enrichment of Ect2-based complexes is essential for NuMA/dynein/dynactin exclusion and proper spindle elongation. Conversely, NuMA-based complexes at the polar region of the cell membrane ensure spatially confined localization of Ect2-based complexes and thus RhoA. Overall, our work establishes that membrane compartmentalization of NuMA-based and Ect2-based complexes at the two distinct cell surfaces restricts dynein/dynactin and RhoA for coordinating spindle elongation with cleavage furrow formation.

cell biology↗

bioRxiv2020

NuMA interaction with chromatin is vital for proper nuclear architecture in human cells

NuMA is an abundant long-coiled-coil protein that plays a prominent role in spindle organization during mitosis. In interphase, NuMA is localized to the nucleus and hypothesized to control gene expression and chromatin organization. However, because of the prominent mitotic phenotype upon NuMA loss, its precise function in the interphase nucleus remains elusive. Here, we report that NuMA is associated with chromatin in interphase and prophase but released upon nuclear envelope breakdown (NEBD) by the action of Cdk1. We uncovered that NuMA directly interacts with DNA via evolutionarily conserved sequences in its C-terminus. Notably, the expression of the DNA-binding mutant of NuMA affects chromatin decondensation at the mitotic exit, and nuclear shape in interphase. The impact on nuclear shape by mutant NuMA expression is due to its potential to polymerize into high-order fibrillar structures. This study links the chromatin binding ability of NuMA with the maintenance of nuclear shape and architecture, which has a well-studied role in regulating gene expression during development and diseases.

cell biology↗

bioRxiv2018

Centrosome Aurora A gradient ensures a single PAR-2 polarity axis by regulating RhoGEF ECT-2 localization in C. elegans embryos

The proper establishment of the cell polarity is essential for development and morphogenesis. In the Caenorhabditis elegans one-cell embryo, a centrosome localized signal provides spatial information that is responsible for generating a single polarity axis. It is hypothesized that such a signal causes local inhibition of cortical actomyosin network in the vicinity of the centrosome. This pivotal event initiates symmetry breaking to direct partitioning of the partition defective proteins (PARs) in the one-cell embryo. However, the molecular nature of the centrosome regulated signal that impinges on the posterior cortex to bring upon cortical anisotropy in the actomyosin network and to promote polarity establishment remains elusive. Here, we discover that Aurora A kinase (AIR-1 in C. elegans) is essential for proper cortical contractility in the one-cell embryo. Loss of AIR-1 causes pronounced cortical contractions on the entire embryo surface during polarity establishment phase, and this creates more than one PAR-2 polarity axis. Moreover, we show that in the absence of AIR-1, centrosome positioning becomes dispensable in dictating the PAR-2 polarity axis. Interestingly, we identify that Rho Guanine Exchange Factor (GEF) ECT-2 acts downstream to AIR-1 to control excess contractility and notably AIR-1 loss affects ECT-2 cortical localization and thereby polarity establishment. Overall, our study unravels a novel insight whereby an evolutionarily conserved kinase Aurora A inhibits promiscuous PAR-2 domain formation and ensures singularity in the polarity establishment axis.

cell biology↗