To power dynamic processes in cells, the actin and microtubule cytoskeletons organize into complex structures. Although it is known that cytoskeletal coordination is vital for cell function, the ...mechanisms by which cross-linking proteins coordinate actin and microtubule activities remain poorly understood. In particular, it is unknown how the distinct mechanical properties of different actin architectures modulate the outcome of actin-microtubule interactions. To address this question, we engineered the protein TipAct, which links growing microtubule ends via end-binding proteins to actin filaments. We show that growing microtubules can be captured and guided by stiff actin bundles, leading to global actin-microtubule alignment. Conversely, growing microtubule ends can transport, stretch and bundle individual actin filaments, thereby globally defining actin filament organization. Our results provide a physical basis to understand actin-microtubule cross-talk, and reveal that a simple cross-linker can enable a mechanical feedback between actin and microtubule organization that is relevant to diverse biological contexts.
Microtubules (MTs) are highly dynamic polymers, which control many aspects of cellular architecture. Growing MT plus ends accumulate a specific set of evolutionary conserved factors, the so‐called MT ...plus‐end‐tracking proteins (+TIPs). +TIPs regulate MT dynamics and the reciprocal interactions of MTs with the cell cortex, mitotic kinetochores or different cellular organelles. Most +TIPs can directly bind to MTs, but the molecular mechanisms of their specific targeting to the growing plus ends remain poorly understood. Recent studies suggest that the members of one particular +TIP family, EB1 and its homologues, are present in all eucaryotic kingdoms, interact directly with the majority of other known plus‐end‐associated proteins and may be responsible for their specific accumulation at the MT tips.
Microtubules control different aspects of cell polarization. In cells with a radial microtubule system, a pivotal role in setting up asymmetry is attributed to the relative positioning of the ...centrosome and the nucleus. Here, we show that centrosome loss had no effect on the ability of endothelial cells to polarize and move in 2D and 3D environments. In contrast, non-centrosomal microtubules stabilized by the microtubule minus-end-binding protein CAMSAP2 were required for directional migration on 2D substrates and for the establishment of polarized cell morphology in soft 3D matrices. CAMSAP2 was also important for persistent endothelial cell sprouting during in vivo zebrafish vessel development. In the absence of CAMSAP2, cell polarization in 3D could be partly rescued by centrosome depletion, indicating that in these conditions the centrosome inhibited cell polarity. We propose that CAMSAP2-protected non-centrosomal microtubules are needed for establishing cell asymmetry by enabling microtubule enrichment in a single-cell protrusion.
Microtubules are cytoskeletal polymers with two structurally and functionally distinct ends, the plus- and the minus-end. Here, we focus on the mechanisms underlying the regulation of microtubule ...minus-ends by the CAMSAP/Nezha/Patronin protein family. We show that CAMSAP2 is required for the proper organization and stabilization of interphase microtubules and directional cell migration. By combining live-cell imaging and in vitro reconstitution of microtubule assembly from purified components with laser microsurgery, we demonstrate that CAMSAPs regulate microtubule minus-end growth and are specifically deposited on the lattice formed by microtubule minus-end polymerization. This process leads to the formation of CAMSAP-decorated microtubule stretches, which are stabilized from both ends and serve as sites of noncentrosomal microtubule outgrowth. The length of the stretches is regulated by the microtubule-severing protein katanin, which interacts with CAMSAPs. Our data thus indicate that microtubule minus-end assembly drives the stabilization of noncentrosomal microtubules and that katanin regulates this process.
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•CAMSAP2 is required for microtubule organization and directional cell migration•CAMSAP2 and CAMSAP3 are deposited on growing microtubule minus-ends•CAMSAP2 and CAMSAP3 autonomously stabilize microtubule lattice from both ends•Stabilization of noncentrosomal microtubules is coupled to minus-end growth
Jiang et al. show that mammalian CAMSAP/Nezha/Patronin protein family members control noncentrosomal microtubule formation by decorating and stabilizing the lattice formed by minus-end polymerization, forming stretches that are sites of microtubule outgrowth. The microtubule-severing protein katanin counteracts this process. These data illuminate the tight regulation of minus-end growth.
The microtubule cytoskeleton regulates cell polarity by spatially organizing membrane trafficking and signaling processes. In epithelial cells, microtubules form parallel arrays aligned along the ...apico-basal axis, and recent work has demonstrated that the members of CAMSAP/Patronin family control apical tethering of microtubule minus ends. Here, we show that in mammalian intestinal epithelial cells, the spectraplakin ACF7 (also known as MACF1) specifically binds to CAMSAP3 and is required for the apical localization of CAMSAP3-decorated microtubule minus ends. Loss of ACF7 but not of CAMSAP3 or its homolog CAMSAP2 affected the formation of polarized epithelial cysts in three-dimensional cultures. In short-term epithelial polarization assays, knockout of CAMSAP3, but not of CAMSAP2, caused microtubule re-organization into a more radial centrosomal array, redistribution of Rab11-positive (also known as Rab11A) endosomes from the apical cell surface to the pericentrosomal region and inhibition of actin brush border formation at the apical side of the cell. We conclude that ACF7 is an important regulator of apico-basal polarity in mammalian intestinal cells and that a radial centrosome-centered microtubule organization can act as an inhibitor of epithelial polarity.
Abstract
Small molecule inhibitors are prime reagents for studies in microtubule cytoskeleton research, being applicable across a range of biological models and not requiring genetic engineering. ...However, traditional chemical inhibitors cannot be experimentally applied with spatiotemporal precision suiting the length and time scales inherent to microtubule-dependent cellular processes. We have synthesised photoswitchable paclitaxel-based microtubule stabilisers, whose binding is induced by photoisomerisation to their metastable state. Photoisomerising these reagents in living cells allows optical control over microtubule network integrity and dynamics, cell division and survival, with biological response on the timescale of seconds and spatial precision to the level of individual cells within a population. In primary neurons, they enable regulation of microtubule dynamics resolved to subcellular regions within individual neurites. These azobenzene-based microtubule stabilisers thus enable non-invasive, spatiotemporally precise modulation of the microtubule cytoskeleton in living cells, and promise new possibilities for studying intracellular transport, cell motility, and neuronal physiology.
The position of the mitotic spindle determines the plane of cell cleavage, and thereby daughter cell location, size, and content. Spindle positioning is driven by dynein-mediated pulling forces ...exerted on astral microtubules, which requires an evolutionarily conserved complex of Gα∙GDP, GPR-1/2
, and LIN-5
proteins. To examine individual functions of the complex components, we developed a genetic strategy for light-controlled localization of endogenous proteins in
embryos. By replacing Gα and GPR-1/2 with a light-inducible membrane anchor, we demonstrate that Gα∙GDP, Gα∙GTP, and GPR-1/2 are not required for pulling-force generation. In the absence of Gα and GPR-1/2, cortical recruitment of LIN-5, but not dynein itself, induced high pulling forces. The light-controlled localization of LIN-5 overruled normal cell-cycle and polarity regulation and provided experimental control over the spindle and cell-cleavage plane. Our results define Gα∙GDP-GPR-1/2
as a regulatable membrane anchor, and LIN-5
as a potent activator of dynein-dependent spindle-positioning forces.
End-binding proteins (EBs) are the core components of microtubule plus end tracking protein complexes, but it is currently unknown whether they are essential for mammalian microtubule organization. ...Here, by using CRISPR/Cas9-mediated knockout technology, we generated stable cell lines lacking EB2 and EB3 and the C-terminal partner-binding half of EB1. These cell lines show only mild defects in cell division and microtubule polymerization. However, the length of CAMSAP2-decorated stretches at noncentrosomal microtubule minus ends in these cells is reduced, microtubules are detached from Golgi membranes, and the Golgi complex is more compact. Coorganization of microtubules and Golgi membranes depends on the EB1/EB3-myomegalin complex, which acts as membrane-microtubule tether and counteracts tight clustering of individual Golgi stacks. Disruption of EB1 and EB3 also perturbs cell migration, polarity, and the distribution of focal adhesions. EB1 and EB3 thus affect multiple interphase processes and have a major impact on microtubule minus end organization.
In neurons, the distinct molecular composition of axons and dendrites is established through polarized targeting mechanisms, but it is currently unclear how nonpolarized cargoes, such as ...mitochondria, become uniformly distributed over these specialized neuronal compartments. Here, we show that TRAK family adaptor proteins, TRAK1 and TRAK2, which link mitochondria to microtubule-based motors, are required for axonal and dendritic mitochondrial motility and utilize different transport machineries to steer mitochondria into axons and dendrites. TRAK1 binds to both kinesin-1 and dynein/dynactin, is prominently localized in axons, and is needed for normal axon outgrowth, whereas TRAK2 predominantly interacts with dynein/dynactin, is more abundantly present in dendrites, and is required for dendritic development. These functional differences follow from their distinct conformations: TRAK2 preferentially adopts a head-to-tail interaction, which interferes with kinesin-1 binding and axonal transport. Our study demonstrates how the molecular interplay between bidirectional adaptor proteins and distinct microtubule-based motors drives polarized mitochondrial transport.
► Kinesin-1 and dynein drive mitochondrial transport to axons and dendrites ► TRAK1 binds to both kinesin-1 and dynein/dynactin and is localized in axons ► TRAK2 predominantly interacts with dynein/dynactin and is present in dendrites ► TRAK2 backfolding inhibits kinesin-1 binding and controls mitochondrial transport
van Spronsen et al. show that mitochondria utilize different machineries to steer their transport into axons and dendrites. The molecular interplay between mitochondrial adaptor protein family TRAK/Milton and distinct microtubule-based motors drives polarized mitochondrial transport.
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