Cytoskeleton club Maya Guerri (eq. Le Clainche, BioCell dpt) and Thennavan Ulaganathan (eq. Charbonnier/Zinn-Justin, B3S dpt)

Maya Guerri (eq. Le Clainche, BioCell dpt) and Thennavan Ulaganathan (eq. Charbonnier/Zinn-Justin, B3S dpt)

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Speaker1: Maya Guerri 

Title1: Role of myosin II and α-actinin-1 in the generation of actin contractile bundles

Abstract1:
Many cellular processes rely on the forces produced by the actin protrusive and contractile structures. In non-muscle migrating cells, contractile stress fibers (SF) responsible for pulling the cell body, are made of anti-parallel actin bundles cross-linked by α-actinin-1 and are contracted by myosin II motor activity. Recent works have highlighted that myosin II is not just a contractile agent but it also plays a role in the regulation of actin turnover, presumably by contributing to the disassembly of the branched actin network forming the lamellipodium at the cell leading edge. However, it is unclear if myosin II can reorganize those disassembled actin structures into contractile SFs or if it even plays a role in the assembly of SFs. Moreover, if SFs are thought to result either from mDia1-nucleated actin filaments or from the coalescence of pre-existing SFs, it has been shown that myosin II is involved in the formation of specific SFs through the reorganization of the actin cortex. To this aim, we are using kinetics assays and single-filaments observations to study myosin II role in the generation and organization of actin contractile bundles, as well as the synergy between its motor activity and α-actinin-1 cross-linking activity in the reorganization of linear and branched actin networks into SFs. We also address this question in structures more similar to lamellipodia through in vitro reconstitution observed in confocal microscopy.

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Speaker2: Thennavan Ulaganathan 

Title2: Multivalency of KIF2C interactions leads to formation of biomolecular condensates during mitosis

Abstract2: Mitosis is where a cell undergoes equal division of duplicated chromosomes and ensures transmission of the genetic information to the daughter cells. The spindle microtubule attaches to the kinetochore at the centromere of the chromosomes. Successful bipolar attachment leads to proper chromosomal segregation; however, errors often happen. These errors are rectified by the KIF2C, kinesin-13 superfamily, which binds at the plus ends of the microtubule and depolymerise them by inducing the curvature of the protofilaments. Various cancer cell lines overexpress KIF2C, and a recent report from Pao et al. (2025) highlights that KIF2C overexpression causes chemoresistance to paclitaxel. We recently reported the formation of condensates by KIF2C depending on phosphorylation by PLK1. The interactions contributing to condensation are mediated by the folded domain at the N-terminus of KIF2C, which we termed as the phospho-barrel (PB) domain. Deletion of PB completely disrupts condensation in cells. We identified the potential cellular partners of KIF2C by Biotin proximity labelling approach (TurboID) in cells arrested in mitosis. Using 1H-15N SOFAST-HSQC NMR and ITC experiments, we confirmed the direct phospho-dependent binding of the PB domain to several of these partners and determined the binding motif of the PB domain: pT-{P}-X-[IVL]. We noticed that KIF2C PB domain binds to several of these motifs in its mitotic partners Sgo2, CEP170 and CEP170b. We propose that this contributes to the multivalency of the phospho-dependent KIF2C interaction network. Moreover, KIF2C is a dimer. We demonstrated that a construct named NMC containing the neck, motor, and C-terminal coiled coil domain forms a stable dimer despite its NTP state: Apo, AMPPNP, ATP, and/or salt conditions: 150 mM and 300 mM concentrations of NaCl. Next, we will focus on solving the structure of this dimer by cryo-electron microscopy to determine the essential amino acids involved in the dimerization. My results suggest that both PB interactions and NMC dimerization contribute to KIF2C condensation in mitosis. The structural insights of the KIF2C will be utilised to design specific compounds disrupting condensates and thus counter the chemoresistance in cancer cells.