Mechanisms and regulation underlying membraneless organelle plasticity control

Evolution has enabled living cells to adopt their structural and functional complexity by organizing intricate cellular compartments,such as membrane-bound and membraneless organelles(MLOs),for spatiotemporal catalysis of physiochemical reactions essential for cell plasticity control.Emerging evidence and view support the notion that MLOs are built by multivalent interactions of biomolecules via phase separation and transition mechanisms.In healthy cells,dynamic chemical modifications regulate MLO plasticity,and reversible phase separation is essential for cell homeostasis.Emerging evidence revealed that aberrant phase separation results in numerous neurodegenerative disorders,cancer,and other diseases.In this review,we provide molecular underpinnings on(i)mechanistic understanding of phase separation,(ii)unifying structural and mechanistic principles that underlie this phenomenon,(iii)various mechanisms that are used by cells for the regulation of phase separation,and(iv)emerging therapeutic and other applications.

The septin complex links the catenin complex to the actin cytoskeleton for establishing epithelial cell polarity

Cell polarity is essential for spatially regulating of physiological processes in metazoans by which hormonal stimulation‒secretion coupling is precisely coupled for tissue homeostasis and organ communications.However,the molecular mechanisms underlying epithelial cell polarity establishment remain elusive.Here,we show that septin cytoskeleton interacts with catenin complex to organize a functional domain to separate apical from basal membranes in polarized epithelial cells.Using polarized epithelial cell monolayer as a model system with transepithelial electrical resistance as functional readout,our studies show that septins are essential for epithelial cell polarization.Our proteomic analyses discovered a novel septin‒catenin complex during epithelial cell polarization.The functional relevance of septin‒catenin complex was then examined in three-dimensional(3D)culture in which suppression of septins resulted in deformation of apical lumen in cysts,a hallmark seen in polarity-deficient 3D cultures and animals.Mechanistically,septin cytoskeleton stabilizes the association of adherens catenin complex with actin cytoskeleton,and depletion or disruption of septin cytoskeleton liberates adherens junction and polarity complexes into the cytoplasm.Together,these findings reveal a previously unrecognized role for septin cytoskeleton in the polarization of the apical‒basal axis and lumen formation in polarized epithelial cells.

SKAP interacts with Aurora B to guide end-on capture of spindle microtubules via phase separation

Chromosome segregation in mitosis is orchestrated by the dynamic interactions between the kinetochore and spindle microtubules.Our recent studies show that mitotic motor CENP-E cooperates with SKAP and forms a link between kinetochore core MIS13 complexand spindle microtubule plus-ends to achieve accurate chromosome alignment in mitosis. However, it remains elusive how SKAP regulates kinetochore attachment from lateral association to end-on attachment during metaphase alignment. Here, we identify a novel interaction between Aurora B and SKAP that orchestrates accurate interaction between the kinetochore and dynamic spindle microtubules. Interestingly, SKAP spontaneously phase-separates in vitro via weak, multivalent interactions into droplets with fast internaldynamics. SKAP and Aurora B form heterogeneous coacervates in vitro, which recapitulate the dynamics and behavior of SKAP cometsin vivo. Importantly, SKAP interaction with Aurora B via phase separation is essential for accurate chromosome segregation and alignment. Based on those findings, we reason that SKAP–Aurora B interaction via phase separation constitutes a dynamic pool of Aurora Bactivity during the lateral to end-on conversion of kinetochore–microtubule attachments to achieve faithful cell division.