Cytoplasmic Linker-associated Protein Research Articles

An unconventional TOG domain is required for CLASP localization

Cytoplasmic linker-associated proteins (CLASPs) form a conserved family of microtubule-associated proteins (MAPs) that maintain microtubules in a growing state by promoting rescue while suppressing catastrophe.1 CLASP function involves an ordered array of tumor overexpressed gene (TOG) domains and binding to multiple protein partners via a conserved C-terminal domain (CTD).2,3 In migrating cells, CLASPs concentrate at the cortex near focal adhesions as part of cortical microtubule stabilization complexes (CMSCs), via binding of their CTD to the focal adhesion protein PHLDB2/LL5β.4,5 Cortical CLASPs also stabilize a subset of microtubules, which stimulate focal adhesion turnover and generate a polarized microtubule network toward the leading edge of migrating cells. CLASPs are also recruited to the trans-Golgi network (TGN) via an interaction between their CTD and the Golgin protein GCC185.6 This allows microtubule growth toward the leading edge of migrating cells, which is required for Golgi organization, polarized intracellular transport, and cell motility.7 In dividing cells, CLASPs are essential at kinetochores for efficient chromosome segregation and anaphase spindle integrity.8,9 Both CENP-E and ASTRIN bind and target CLASPs to kinetochores,10,11 although the CLASP domain required for this interaction is not known. Despite its high evolutionary conservation, the CTD remains structurally uncharacterized. Here, we find that the CTD can be structurally modeled as a TOG domain. We identify a surface-exposed and conserved arginine residue essential for CLASP CTD interaction with partner proteins. Together, our results provide a structural mechanism by which the CLASP CTD directs diverse sub-cellular localizations throughout the cell cycle.

CLASPs stabilize the pre-catastrophe intermediate state between microtubule growth and shrinkage.

Cytoplasmic linker-associated proteins (CLASPs) regulate microtubules in fundamental cellular processes. CLASPs stabilize dynamic microtubules by suppressing microtubule catastrophe and promoting rescue, the switch-like transitions between growth and shrinkage. How CLASPs specifically modulate microtubule transitions is not understood. Here, we investigate the effects of CLASPs on the pre-catastrophe intermediate state of microtubule dynamics, employing distinct microtubule substrates to mimic the intermediate state. Surprisingly, we find that CLASP1 promotes the depolymerization of stabilized microtubules in the presence of GTP, but not in the absence of nucleotide. This activity is also observed for CLASP2 family members and a minimal TOG2-domain construct. Conversely, we find that CLASP1 stabilizes unstable microtubules upon tubulin dilution in the presence of GTP. Strikingly, our results reveal that CLASP1 drives microtubule substrates with vastly different inherent stabilities into the same slowly depolymerizing state in a nucleotide-dependent manner. We interpret this state as the pre-catastrophe intermediate state. Therefore, we conclude that CLASPs suppress microtubule catastrophe by stabilizing the intermediate state between growth and shrinkage.

CLASP2 facilitates dynamic actin filament organization along the microtubule lattice.

Coordination between the microtubule and actin networks is essential for cell motility, neuronal growth cone guidance, and wound healing. Members of the CLASP (cytoplasmic linker-associated protein) family of proteins have been implicated in the cytoskeletal crosstalk between microtubules and actin networks, however, the molecular mechanisms underlying CLASPs role in cytoskeletal coordination are unclear. Here, we investigate CLASP2α's crosslinking function with microtubules and F-actin. Our results demonstrate that CLASP2α crosslinks F-actin to the microtubule lattice in vitro. We find that the crosslinking ability is retained by L-TOG2-S, a minimal construct containing the TOG2 domain and serine-arginine rich region of CLASP2α. Furthermore, CLASP2α promotes the accumulation of multiple actin filaments along the microtubule, supporting up to 11 F-actin landing events on a single microtubule lattice region. CLASP2α also facilitates dynamic organization of polymerizing actin filaments templated by the microtubule network, with F-actin forming bridges between individual microtubules. Finally, we find that depletion of CLASPs in vascular smooth muscle cells results in disorganized actin fibers and reduced co-alignment of actin fibers with microtubules, suggesting that CLASP and microtubules contribute to higher-order actin structures. Taken together, our results indicate that CLASP2α can directly crosslink F-actin to microtubules, and that this microtubule-CLASP-actin interaction may influence overall cytoskeletal organization in cells.

CLASP2 recognizes tubulins exposed at the microtubule plus-end in a nucleotide state-sensitive manner.

CLASPs (cytoplasmic linker-associated proteins) are ubiquitous stabilizers of microtubule dynamics, but their molecular targets at the microtubule plus-end are not understood. Using DNA origami-based reconstructions, we show that clusters of human CLASP2 form a load-bearing bond with terminal non-GTP tubulins at the stabilized microtubule tip. This activity relies on the unconventional TOG2 domain of CLASP2, which releases its high-affinity bond with non-GTP dimers upon their conversion into polymerization-competent GTP-tubulins. The ability of CLASP2 to recognize nucleotide-specific tubulin conformation and stabilize the catastrophe-promoting non-GTP tubulins intertwines with the previously underappreciated exchange between GDP and GTP at terminal tubulins. We propose that TOG2-dependent stabilization of sporadically occurring non-GTP tubulins represents a distinct molecular mechanism to suppress catastrophe at the freely assembling microtubule ends and to promote persistent tubulin assembly at the load-bearing tethered ends, such as at the kinetochores in dividing cells.

CLASP2 facilitates dynamic actin filament organization along the microtubule lattice.

Coordination between the microtubule and actin networks is essential for cell motility, neuronal growth cone guidance, and wound healing. Members of the CLASP (cytoplasmic linker-associated protein) family of proteins have been implicated in the cytoskeletal cross-talk between microtubules and actin networks; however, the molecular mechanisms underlying the role of CLASP in cytoskeletal coordination are unclear. Here, we investigate CLASP2α's cross-linking function with microtubules and F-actin. Our results demonstrate that CLASP2α cross-links F-actin to the microtubule lattice in vitro. We find that the cross-linking ability is retained by L-TOG2-S, a minimal construct containing the TOG2 domain and serine-arginine-rich region of CLASP2α. Furthermore, CLASP2α promotes the accumulation of multiple actin filaments along the microtubule, supporting up to 11 F-actin landing events on a single microtubule lattice region. CLASP2α also facilitates the dynamic organization of polymerizing actin filaments templated by the microtubule network, with F-actin forming bridges between individual microtubules. Finally, we find that depletion of CLASPs in vascular smooth muscle cells results in disorganized actin fibers and reduced coalignment of actin fibers with microtubules, suggesting that CLASP and microtubules contribute to higher-order actin structures. Taken together, our results indicate that CLASP2α can directly cross-link F-actin to microtubules and that this microtubule-CLASP-actin interaction may influence overall cytoskeletal organization in cells.

Drosophila CLASP regulates microtubule orientation and dendrite pruning by suppressing Par-1 kinase.

The evolutionarily conserved CLASPs (cytoplasmic linker-associated proteins) are microtubule-associated proteins that inhibit microtubule catastrophe and promote rescue. CLASPs can regulate axonal elongation and dendrite branching in growing neurons. However, their roles in microtubule orientation and neurite pruning in remodeling neurons remain unknown. Here, we identify the Drosophila CLASP homolog Orbit/MAST, which is required for dendrite pruning in ddaC sensory neurons during metamorphosis. Orbit is important for maintenance of the minus-end-out microtubule orientation in ddaC dendrites. Our structural analysis reveals that the microtubule lattice-binding TOG2 domain is required for Orbit to regulate dendritic microtubule orientation and dendrite pruning. In a genetic modifier screen, we further identify the conserved Par-1 kinase as a suppressor of Orbit in dendritic microtubule orientation. Moreover, elevated Par-1 function impairs dendritic microtubule orientation and dendrite pruning, phenocopying orbit mutants. Overall, our study demonstrates that Drosophila CLASP governs dendritic microtubule orientation and dendrite pruning at least partly via suppressing Par-1 kinase.

Septins guide noncentrosomal microtubules to promote focal adhesion disassembly in migrating cells.

Endothelial cell migration is critical for vascular angiogenesis and is compromised to facilitate tumor metastasis. The migratory process requires the coordinated assembly and disassembly of focal adhesions (FA), actin, and microtubules (MT). MT dynamics at FAs deliver vesicular cargoes and enhance actomyosin contractility to promote FA turnover and facilitate cell advance. Noncentrosomal (NC) MTs regulate FA dynamics and are sufficient to drive cell polarity, but how NC MTs target FAs to control FA turnover is not understood. Here, we show that Rac1 induces the assembly of FA-proximal septin filaments that promote NC MT growth into FAs and inhibit mitotic centromere-associated kinesin (MCAK)-associated MT disassembly, thereby maintaining intact MT plus ends proximal to FAs. Septin-associated MT rescue is coupled with accumulation of Aurora-A kinase and cytoplasmic linker-associated protein (CLASP) localization to the MT between septin and FAs. In this way, NC MTs are strategically positioned to undergo MCAK- and CLASP-regulated bouts of assembly and disassembly into FAs, thereby regulating FA turnover and cell migration.

Articles of Significant Interest in This Issue

Human immunodeficiency virus type 1 (HIV-1) exploits specialized microtubule (MT) plus-end tracking proteins (commonly known as +TIPs) to induce the formation of stable MTs soon after virus entry and to promote early stages of infection. Mitra et al. (e00404-20) found that distinct domains in cytoplasmic linker-associated protein 2 (CLASP2), a +TIP that captures cortical MT plus ends to enable filament stabilization, mediate interactions with incoming HIV-1 particles, and facilitate MT stabilization and trafficking of incoming cores. These findings identify a new +TIP that acts as a host cofactor to promote early stages of viral infection.

HIV-1 Exploits CLASP2 To Induce Microtubule Stabilization and Facilitate Virus Trafficking to the Nucleus.

Human immunodeficiency virus type 1 (HIV-1) exploits a number of specialized microtubule (MT) plus-end tracking proteins (commonly known as +TIPs) to induce the formation of stable microtubules soon after virus entry and promote early stages of infection. However, given their functional diversity, the nature of the +TIPs involved and how they facilitate HIV-1 infection remains poorly understood. Here, we identify cytoplasmic linker-associated protein 2 (CLASP2), a +TIP that captures cortical MT plus ends to enable filament stabilization, as a host factor that enables HIV-1 to induce MT stabilization and promote early infection in natural target cell types. Using fixed- and live-cell imaging in human microglia cells, we further show that CLASP2 is required for the trafficking of incoming HIV-1 particles carrying wild-type (WT) envelope. Moreover, both WT CLASP2 and a CLASP2 mutant lacking its C-terminal domain, which mediates its interaction with several host effector proteins, bind to intact HIV-1 cores or in vitro-assembled capsid-nucleocapsid (CA-NC) complexes. However, unlike WT CLASP2, the CLASP2 C-terminal mutant is unable to induce MT stabilization or promote early HIV-1 infection. Our findings identify CLASP2 as a new host cofactor that utilizes distinct regulatory domains to bind incoming HIV-1 particles and facilitate trafficking of incoming viral cores through MT stabilization.IMPORTANCE While microtubules (MTs) have long been known to be important for delivery of incoming HIV-1 cores to the nucleus, how the virus engages and exploits these filaments remains poorly understood. Our previous work revealed the importance of highly specialized MT regulators that belong to a family called plus-end tracking proteins (+TIPs) in facilitating early stages of infection. These +TIPs perform various functions, such as engaging cargos for transport or engaging peripheral actin to stabilize MTs, suggesting several family members have the potential to contribute to infection in different ways. Here, we reveal that cytoplasmic linker-associated protein 2 (CLASP2), a key regulator of cortical capture and stabilization of MTs, interacts with incoming HIV-1 particles, and we identify a distinct C-terminal domain in CLASP2 that promotes both MT stabilization and early infection. Our findings identify a new +TIP acting as a host cofactor that facilitates early stages of viral infection.

Staphylococcus aureus–induced endothelial permeability and inflammation are mediated by microtubule destabilization

Staphylococcus aureus is a major etiological agent of sepsis and induces endothelial cell (EC) barrier dysfunction and inflammation, two major hallmarks of acute lung injury. However, the molecular mechanisms of bacterial pathogen-induced EC barrier disruption are incompletely understood. Here, we investigated the role of microtubules (MT) in the mechanisms of EC barrier compromise caused by heat-killed S. aureus (HKSA). Using a customized monolayer permeability assay in human pulmonary EC and MT fractionation, we observed that HKSA-induced barrier disruption is accompanied by MT destabilization and increased histone deacetylase-6 (HDAC6) activity resulting from elevated reactive oxygen species (ROS) production. Molecular or pharmacological HDAC6 inhibition rescued barrier function in HKSA-challenged vascular endothelium. The HKSA-induced EC permeability was associated with impaired MT-mediated delivery of cytoplasmic linker-associated protein 2 (CLASP2) to the cell periphery, limiting its interaction with adherens junction proteins. HKSA-induced EC barrier dysfunction was also associated with increased Rho GTPase activity via activation of MT-bound Rho-specific guanine nucleotide exchange factor-H1 (GEF-H1) and was abolished by HDAC6 down-regulation. HKSA activated the NF-κB proinflammatory pathway and increased the expression of intercellular and vascular cell adhesion molecules in EC, an effect that was also HDAC6-dependent and mediated, at least in part, by a GEF-H1/Rho-dependent mechanism. Of note, HDAC6 knockout mice or HDAC6 inhibitor-treated WT mice were partially protected from vascular leakage and inflammation caused by both HKSA or methicillin-resistant S. aureus (MRSA). Our results indicate that S. aureus-induced, ROS-dependent up-regulation of HDAC6 activity destabilizes MT and thereby activates the GEF-H1/Rho pathway, increasing both EC permeability and inflammation.

CLASP Facilitates Transitions between Cortical Microtubule Array Patterns.

Acentrosomal plant microtubule arrays form patterns at the cell cortex that influence cellular morphogenesis by templating the deposition of cell wall materials, but the molecular basis by which the microtubules form the cortical array patterns remains largely unknown. Loss of the Arabidopsis (Arabidopsis thaliana) microtubule-associated protein, CYTOPLASMIC LINKER ASSOCIATED PROTEIN (AtCLASP), results in cellular growth anisotropy defects in hypocotyl cells. We examined the microtubule array patterning in atclasp-1 null mutants and discovered a significant defect in the timing of transitions between array patterns but no substantive defect in the array patterns per se. Detailed analysis and computational modeling of the microtubule dynamics in two atclasp-1 fluorescent tubulin marker lines revealed marker-dependent effects on depolymerization and catastrophe frequency predicted to alter the steady-state microtubule population. Quantitative in vivo analysis of the underlying microtubule array architecture showed that AtCLASP is required to maintain the number of growing microtubule plus ends during transitions between array patterns. We propose that AtCLASP plays a critical role in cellular morphogenesis through actions on new microtubules that facilitate array transitions.

Identification and Functional Characterization of a Microtubule-Associated Protein, GhCLASP2, From Upland Cotton (Gossypium hirsutum L.).

Cytoplasmic linker-associated proteins (CLASPs) are microtubule-associated proteins (MAPs) involved in regulation of dynamics of microtubules (MTs) that play an important role in plant growth and development. In this study, we identified cotton CLASP genes and investigated the function of GhCLASP2. GhCLASP2 was mainly expressed in stem and developing fibers, especially in fibers of the secondary cell wall deposition stage. Ectopic expression of GhCLASP2 in Arabidopsis increased the branching number of leaf trichomes and rescued the defective phenotypes of clasp-1. In cotton, overexpression of GhCLASP2 increased fiber strength, probably related to enhanced expression levels of tubulin, cellulose synthase, and expansin genes. Suppression of GhCLASP2 caused shorter internodes and semi-dwarfism, abnormal flower stigma, aborted anthers without pollen grains, and sterility. These changed phenotypes were similar to those observed in the Arabidopsis clasp-1 mutant. GhCLASP2 was co-localized with MTs according to transient experiment. These results suggest that GhCLASP2 functions similarly as AtCLASP, acting as a MAP and controlling cotton growth and development by regulating MTs.

CLASP Suppresses Microtubule Catastrophes through a Single TOG Domain

SummaryThe dynamic instability of microtubules plays a key role in controlling their organization and function, but the cellular mechanisms regulating this process are poorly understood. Here, we show that cytoplasmic linker-associated proteins (CLASPs) suppress transitions from microtubule growth to shortening, termed catastrophes, including those induced by microtubule-destabilizing agents and physical barriers. Mammalian CLASPs encompass three TOG-like domains, TOG1, TOG2, and TOG3, none of which bind to free tubulin. TOG2 is essential for catastrophe suppression, whereas TOG3 mildly enhances rescues but cannot suppress catastrophes. These functions are inhibited by the C-terminal domain of CLASP2, while the TOG1 domain can release this auto-inhibition. TOG2 fused to a positively charged microtubule-binding peptide autonomously accumulates at growing but not shrinking ends, suppresses catastrophes, and stimulates rescues. CLASPs suppress catastrophes by stabilizing growing microtubule ends, including incomplete ones, preventing their depolymerization and promoting their recovery into complete tubes. TOG2 domain is the key determinant of these activities.

An isolated CLASP TOG domain suppresses microtubule catastrophe and promotes rescue.

Microtubules are heavily regulated dynamic polymers of αβ-tubulin that are required for proper chromosome segregation and organization of the cytoplasm. Polymerases in the XMAP215 family use arrayed TOG domains to promote faster microtubule elongation. Regulatory factors in the cytoplasmic linker associated protein (CLASP) family that reduce catastrophe and/or increase rescue also contain arrayed TOGs, but how CLASP TOGs contribute to activity is poorly understood. Here, using Saccharomyces cerevisiae Stu1 as a model CLASP, we report structural, biochemical, and reconstitution studies that clarify functional properties of CLASP TOGs. The two TOGs in Stu1 have very different tubulin-binding properties: TOG2 binds to both unpolymerized and polymerized tubulin, and TOG1 binds very weakly to either. The structure of Stu1-TOG2 reveals a CLASP-specific residue that likely confers distinctive tubulin-binding properties. The isolated TOG2 domain strongly suppresses microtubule catastrophe and increases microtubule rescue in vitro, contradicting the expectation that regulatory activity requires an array of TOGs. Single point mutations on the tubulin-binding surface of TOG2 ablate its anti-catastrophe and rescue activity in vitro, and Stu1 function in cells. Revealing that an isolated CLASP TOG can regulate polymerization dynamics without being part of an array provides insight into the mechanism of CLASPs and diversifies the understanding of TOG function.

Human CLASP2 specifically regulates microtubule catastrophe and rescue.

Cytoplasmic linker-associated proteins (CLASPs) are microtubule-associated proteins essential for microtubule regulation in many cellular processes. However, the molecular mechanisms underlying CLASP activity are not understood. Here, we use purified protein components and total internal reflection fluorescence microscopy to investigate the effects of human CLASP2 on microtubule dynamics in vitro. We demonstrate that CLASP2 suppresses microtubule catastrophe and promotes rescue without affecting the rates of microtubule growth or shrinkage. Strikingly, when CLASP2 is combined with EB1, a known binding partner, the effects on microtubule dynamics are strongly enhanced. We show that synergy between CLASP2 and EB1 is dependent on a direct interaction, since a truncated EB1 protein that lacks the CLASP2-binding domain does not enhance CLASP2 activity. Further, we find that EB1 targets CLASP2 to microtubules and increases the dwell time of CLASP2 at microtubule tips. Although the temporally averaged microtubule growth rates are unaffected by CLASP2, we find that microtubules grown with CLASP2 display greater variability in growth rates. Our results provide insight into the regulation of microtubule dynamics by CLASP proteins and highlight the importance of the functional interplay between regulatory proteins at dynamic microtubule ends.

Gibberellin DELLA signaling targets the retromer complex to redirect protein trafficking to the plasma membrane

The plant hormone gibberellic acid (GA) is a crucial regulator of growth and development. The main paradigm of GA signaling puts forward transcriptional regulation via the degradation of DELLA transcriptional repressors. GA has also been shown to regulate tropic responses by modulation of the plasma membrane incidence of PIN auxin transporters by an unclear mechanism. Here we uncovered the cellular and molecular mechanisms by which GA redirects protein trafficking and thus regulates cell surface functionality. Photoconvertible reporters revealed that GA balances the protein traffic between the vacuole degradation route and recycling back to the cell surface. Low GA levels promote vacuolar delivery and degradation of multiple cargos, including PIN proteins, whereas high GA levels promote their recycling to the plasma membrane. This GA effect requires components of the retromer complex, such as Sorting Nexin 1 (SNX1) and its interacting, microtubule (MT)-associated protein, the Cytoplasmic Linker-Associated Protein (CLASP1). Accordingly, GA regulates the subcellular distribution of SNX1 and CLASP1, and the intact MT cytoskeleton is essential for the GA effect on trafficking. This GA cellular action occurs through DELLA proteins that regulate the MT and retromer presumably via their interaction partners Prefoldins (PFDs). Our study identified a branching of the GA signaling pathway at the level of DELLA proteins, which, in parallel to regulating transcription, also target by a nontranscriptional mechanism the retromer complex acting at the intersection of the degradation and recycling trafficking routes. By this mechanism, GA can redirect receptors and transporters to the cell surface, thus coregulating multiple processes, including PIN-dependent auxin fluxes during tropic responses.

GSK3-mediated CLASP2 phosphorylation modulates kinetochore dynamics.

Error-free chromosome segregation requires dynamic control of microtubule attachment to kinetochores, but how kinetochore-microtubule interactions are spatially and temporally controlled during mitosis remains incompletely understood. In addition to the NDC80 microtubule-binding complex, other proteins with demonstrated microtubule-binding activities localize to kinetochores. One such protein is the cytoplasmic linker-associated protein 2 (CLASP2). Here, we show that global GSK3-mediated phosphorylation of the longest isoform, CLASP2α, largely abolishes CLASP2α-microtubule association in metaphase. However, it does not directly control localization of CLASP2α to kinetochores. Using dominant phosphorylation-site variants, we find that CLASP2α phosphorylation weakens kinetochore-microtubule interactions as evidenced by decreased tension between sister kinetochores. Expression of CLASP2α phosphorylation-site mutants also resulted in increased chromosome segregation defects, indicating that GSK3-mediated control of CLASP2α-microtubule interactions contributes to correct chromosome dynamics. Because of global inhibition of CLASP2α-microtubule interactions, we propose a model in which only kinetochore-bound CLASP2α is dephosphorylated, locally engaging its microtubule-binding activity.

Podosome dynamics and location in vascular smooth muscle cells require CLASP‐dependent microtubule bending

Extracellular matrix (ECM) remodeling during physiological processes is mediated by invasive protrusions called podosomes. Positioning and dynamics of podosomes define the extent of ECM degradation. Microtubules are known to be involved in podosome regulation, but the role of microtubule (MT) network configuration in podosome dynamics and positioning is not well understood. Here, we show that the arrangement of the microtubule network defines the pattern of podosome formation and relocation in vascular smooth muscle cells (VSMCs). We show that microtubule plus‐end targeting facilitates de novo formation of podosomes, in addition to podosome remodeling. Moreover, specialized bent microtubules with plus ends reversed towards the cell center promote relocation of podosomes from the cell edge to the cell center, resulting in an evenly distributed podosome pattern. Microtubule bending is induced downstream of protein kinase C (PKC) activation and requires microtubule‐stabilizing proteins known as cytoplasmic linker associated proteins (CLASPs) and retrograde actin flow. Similar to microtubule depolymerization, CLASP depletion by siRNA blocks microtubule bending and eliminates centripetal relocation of podosomes. Podosome relocation also coincides with translocation of podosome‐stimulating kinesin KIF1C, which is known to move preferentially along CLASP‐associated microtubules. These findings indicate that CLASP‐dependent microtubule network configuration is critical to the cellular location and distribution of KIF1C‐dependent podosomes. © 2016 Wiley Periodicals, Inc.

Podosome dynamics and location in vascular smooth muscle cells require CLASP-dependent microtubule bending.

Extracellular matrix (ECM) remodeling during physiological processes is mediated by invasive protrusions called podosomes. Positioning and dynamics of podosomes define the extent of ECM degradation. Microtubules are known to be involved in podosome regulation, but the role of microtubule (MT) network configuration in podosome dynamics and positioning is not well understood. Here, we show that the arrangement of the microtubule network defines the pattern of podosome formation and relocation in vascular smooth muscle cells (VSMCs). We show that microtubule plus-end targeting facilitates de novo formation of podosomes, in addition to podosome remodeling. Moreover, specialized bent microtubules with plus ends reversed towards the cell center promote relocation of podosomes from the cell edge to the cell center, resulting in an evenly distributed podosome pattern. Microtubule bending is induced downstream of protein kinase C (PKC) activation and requires microtubule-stabilizing proteins known as cytoplasmic linker associated proteins (CLASPs) and retrograde actin flow. Similar to microtubule depolymerization, CLASP depletion by siRNA blocks microtubule bending and eliminates centripetal relocation of podosomes. Podosome relocation also coincides with translocation of podosome-stimulating kinesin KIF1C, which is known to move preferentially along CLASP-associated microtubules. These findings indicate that CLASP-dependent microtubule network configuration is critical to the cellular location and distribution of KIF1C-dependent podosomes. © 2016 Wiley Periodicals, Inc.

Diacylglycerol Guides the Hopping of Clathrin-Coated PITs along Microtubules for Exo-Endocytosis Coupling

Many receptor-mediated endocytic processes are mediated by constitutively budding of clathrin-coated pits (CCPs) at spatially randomized sites before slowly pinching-off from the plasma membrane (60∼100 s). In contrast, clathrin-mediated endocytosis (CME) coupled with regulated exocytosis in excitable cells occurs at peri-exocytic sites shortly after vesicle fusion (∼10 s). The molecular mechanism underlying this spatiotemporal coupling remains elusive. We show that coupled endocytosis makes use of pre-formed CCPs, which hop to nascent fusion sites nearby following vesicle exocytosis. A dynamic cortical microtubular network, anchored at the cell surface by the cytoplasmic linker-associated protein on microtubules and the LL5β/ELKS complex on the plasma membrane, provides the track for CCP hopping. Local diacylglycerol gradients generated upon exocytosis guide the direction of hopping. Overall, the CCP-cytoskeleton-lipid interaction demonstrated here mediates exocytosis-coupled fast recycling of both plasma membrane and vesicular proteins, and is required for the sustained exocytosis during repetitive stimulations.