The motor domain of the kinesin Kip2 promotes microtubule polymerization at microtubule tips.

Kinesins are microtubule-dependent motor proteins, some of which moonlight as microtubule polymerases, such as the yeast protein Kip2. Here, we show that the CLIP-170 ortholog Bik1 stabilizes Kip2 at microtubule ends where the motor domain of Kip2 promotes microtubule polymerization. Live-cell imaging and mathematical estimation of Kip2 dynamics reveal that disrupting the Kip2-Bik1 interaction aborts Kip2 dwelling at microtubule ends and abrogates its microtubule polymerization activity. Structural modeling and biochemical experiments identify a patch of positively charged residues that enables the motor domain to bind free tubulin dimers alternatively to the microtubule shaft. Neutralizing this patch abolished the ability of Kip2 to promote microtubule growth both in vivo and in vitro without affecting its ability to walk along microtubules. Our studies suggest that Kip2 utilizes Bik1 as a cofactor to track microtubule tips, where its motor domain then recruits free tubulin and catalyzes microtubule assembly.

Self-repair protects microtubules from destruction by molecular motors.

Microtubule instability stems from the low energy of tubulin dimer interactions, which sets the growing polymer close to its disassembly conditions. Molecular motors use ATP hydrolysis to produce mechanical work and move on microtubules. This raises the possibility that the mechanical work produced by walking motors can break dimer interactions and trigger microtubule disassembly. We tested this hypothesis by studying the interplay between microtubules and moving molecular motors in vitro. Our results show that molecular motors can remove tubulin dimers from the lattice and rapidly destroy microtubules. We also found that dimer removal by motors was compensated for by the insertion of free tubulin dimers into the microtubule lattice. This self-repair mechanism allows microtubules to survive the damage induced by molecular motors as they move along their tracks. Our study reveals the existence of coupling between the motion of molecular motors and the renewal of the microtubule lattice.

Effects of α-tubulin acetylation on microtubule structure and stability.

Acetylation of K40 in α-tubulin is the sole posttranslational modification to mark the luminal surface of microtubules. It is still controversial whether its relationship with microtubule stabilization is correlative or causative. We have obtained high-resolution cryo-electron microscopy (cryo-EM) reconstructions of pure samples of αTAT1-acetylated and SIRT2-deacetylated microtubules to visualize the structural consequences of this modification and reveal its potential for influencing the larger assembly properties of microtubules. We modeled the conformational ensembles of the unmodified and acetylated states by using the experimental cryo-EM density as a structural restraint in molecular dynamics simulations. We found that acetylation alters the conformational landscape of the flexible loop that contains αK40. Modification of αK40 reduces the disorder of the loop and restricts the states that it samples. We propose that the change in conformational sampling that we describe, at a location very close to the lateral contacts site, is likely to affect microtubule stability and function.

Microtubules acquire resistance from mechanical breakage through intralumenal acetylation.

Eukaryotic cells rely on long-lived microtubules for intracellular transport and as compression-bearing elements. We considered that long-lived microtubules are acetylated inside their lumen and that microtubule acetylation may modify microtubule mechanics. Here, we found that tubulin acetylation is required for the mechanical stabilization of long-lived microtubules in cells. Depletion of the tubulin acetyltransferase TAT1 led to a significant increase in the frequency of microtubule breakage. Nocodazole-resistant microtubules lost upon removal of acetylation were largely restored by either pharmacological or physical removal of compressive forces. In in vitro reconstitution experiments, acetylation was sufficient to protect microtubules from mechanical breakage. Thus, acetylation increases mechanical resilience to ensure the persistence of long-lived microtubules.

Tubulin acetylation protects long-lived microtubules against mechanical ageing.

Long-lived microtubules endow the eukaryotic cell with long-range transport abilities. While long-lived microtubules are acetylated on Lys40 of α-tubulin (αK40), acetylation takes place after stabilization and does not protect against depolymerization. Instead, αK40 acetylation has been proposed to mechanically stabilize microtubules. Yet how modification of αK40, a residue exposed to the microtubule lumen and inaccessible to microtubule-associated proteins and motors, could affect microtubule mechanics remains an open question. Here we develop FRET-based assays that report on the lateral interactions between protofilaments and find that αK40 acetylation directly weakens inter-protofilament interactions. Congruently, αK40 acetylation affects two processes largely governed by inter-protofilament interactions, reducing the nucleation frequency and accelerating the shrinkage rate. Most relevant to the biological function of acetylation, microfluidics manipulations demonstrate that αK40 acetylation enhances flexibility and confers resilience against repeated mechanical stresses. Thus, unlike deacetylated microtubules that accumulate damage when subjected to repeated stresses, long-lived microtubules are protected from mechanical ageing through their acquisition of αK40 acetylation. In contrast to other tubulin post-translational modifications that act through microtubule-associated proteins, motors and severing enzymes, intraluminal acetylation directly tunes the compliance and resilience of microtubules.

An Actin Network Dispatches Ciliary GPCRs into Extracellular Vesicles to Modulate Signaling.

Signaling receptors dynamically exit cilia upon activation of signaling pathways such as Hedgehog. Here, we find that when activated G protein-coupled receptors (GPCRs) fail to undergo BBSome-mediated retrieval from cilia back into the cell, these GPCRs concentrate into membranous buds at the tips of cilia before release into extracellular vesicles named ectosomes. Unexpectedly, actin and the actin regulators drebrin and myosin 6 mediate ectosome release from the tip of cilia. Mirroring signal-dependent retrieval, signal-dependent ectocytosis is a selective and effective process that removes activated signaling molecules from cilia. Congruently, ectocytosis compensates for BBSome defects as ectocytic removal of GPR161, a negative regulator of Hedgehog signaling, permits the appropriate transduction of Hedgehog signals in Bbs mutants. Finally, ciliary receptors that lack retrieval determinants such as the anorexigenic GPCR NPY2R undergo signal-dependent ectocytosis in wild-type cells. Our data show that signal-dependent ectocytosis regulates ciliary signaling in physiological and pathological contexts.

Micropatterning microtubules.

The following protocol describes a method to control the orientation and polarity of polymerizing microtubules (MTs). Reconstitution of specific geometries of dynamic MT networks is achieved using a ultraviolet (UV) micropatterning technique in combination with stabilized MT microseeds. The process is described in three main parts. First, the surface is passivated to avoid the non-specific absorption of proteins, using different polyethylene glycol (PEG)-based surface treatment. Second, specific adhesive surfaces (the micropatterns) are imprinted through a photomask using deep UVs. Lastly, MT microseeds are adhered to the micropatterns followed by MT polymerization.

Microtubule-sliding activity of a kinesin-8 promotes spindle assembly and spindle-length control.

Molecular motors play critical roles in the formation of mitotic spindles, either through controlling the stability of individual microtubules, or by crosslinking and sliding microtubule arrays. Kinesin-8 motors are best known for their regulatory roles in controlling microtubule dynamics. They contain microtubule-destabilizing activities, and restrict spindle length in a wide variety of cell types and organisms. Here, we report an antiparallel microtubule-sliding activity of the budding yeast kinesin-8, Kip3. The in vivo importance of this sliding activity was established through the identification of complementary Kip3 mutants that separate the sliding activity and microtubule-destabilizing activity. In conjunction with Cin8, a kinesin-5 family member, the sliding activity of Kip3 promotes bipolar spindle assembly and the maintenance of genome stability. We propose a slide-disassemble model where the sliding and destabilizing activity of Kip3 balance during pre-anaphase. This facilitates normal spindle assembly. However, the destabilizing activity of Kip3 dominates in late anaphase, inhibiting spindle elongation and ultimately promoting spindle disassembly.

MAP65 coordinate microtubule growth during bundle formation.

Microtubules (MTs) are highly dynamical structures that play a crucial role in cell physiology. In cooperation with microtubule-associated proteins (MAPs), MTs form bundles endowing cells with specific mechanisms to control their shape or generate forces. Whether the dynamics of MTs is affected by the lateral connections that MAPs make between MTs during bundle formation is still under debate. Using in vitro reconstitution of MT bundling, we analyzed the dynamics of MT bundles generated by two plant MAP65 (MAP65-1/4), MAP65-1 being the plant ortholog of vertebrate PRC1 and yeast Ase1. MAP65-1/4 limit the amplitude of MT bundle depolymerization and increase the elongation phases. The subsequent sustained elongation of bundles is governed by the coordination of MT growth, so that MT ends come in close vicinity. We develop a model based on the assumption that both MAP65-1/4 block MT depolymerization. Model simulations reveal that rescue frequencies are higher between parallel than between anti-parallel MTs. In consequence the polarity of bundled MTs by MAP65 controls the amplitude of bundle's growth. Our results illustrate how MAP-induced MT-bundling, which is finely tuned by MT polarity, robustly coordinates MT elongation within bundles.

Quantification of MAP and molecular motor activities on geometrically controlled microtubule networks.

The spatial organization of the microtubule (MT) network directs cell polarity and mitosis. It is finely regulated by hundreds of different types of microtubule-associated proteins and molecular motors whose specific functions are difficult to investigate directly in cells. Here, we have investigated their functions using geometrically controlled MT networks in vitro in cell-free assay. This was achieved by developing a new method to spatially define MT nucleation using MT microseeds adsorbed on a micropatterned glass substrate. This method could be used to control MT growth and the induction of complex MT networks. We selected the interaction of two radial arrays of dynamic and polarized MTs to analyze the formation of the central antiparallel MT bundle. We investigated the effects of the MT cross-linker anaphase spindle elongation 1 (Ase1) and the kinesin motor Klp2, which are known to regulate MT organization in the spindle midzone. We thus identified the respective roles of each protein and revealed their synergy on the establishment of stable antiparallel MT bundles by quantifying MT interactions over hundreds of comparable MT networks.

Selective transmigration of monocyte-associated HIV-1 across a human cervical monolayer and its modulation by seminal plasma.

OBJECTIVE: To analyse the transmigration of immune cells infected by HIV-1 across the epithelial monolayer using the endometrial human endometrial carcinoma (HEC)-1A cell line and to study the influence of seminal plasma in this process. DESIGN: After sexual intercourse involving a male partner infected by HIV-1, a selection process has been shown to lead to a predominant transmission of the R5 phenotype despite the presence of X4 and R5 strains in semen. Transmigration of HIV-infected monocytes present in semen may represent a pertinent mechanism that could explain this tropism selection. METHODS: Epithelial monolayer crossing was studied by using HEC-1A epithelial cells cultured on permeable support and monocyte-enriched or lymphocyte-enriched populations of cells infected or not by HIV R5 or X4 strains. Transmigrating cells were quantified and analysed for their ability to transmit HIV infection to immune target cells. The effect of HIV-negative seminal plasma on cell transmigration was analysed. RESULTS: A preferential passage of the R5 strain associated with monocyte-enriched populations was observed together with the ability of this strain to transmit infection. Seminal plasma was found able to decrease the epithelial crossing of immune cells by enhancing transepithelial resistance and by increasing the adherence of immune cells to the monolayer. CONCLUSION: The preferential transmigration of HIV R5 strains associated with monocytes across the endocervical monolayer may explain the predominant transmission of the R5 strains after sexual intercourse. By its capacity to modulate the tightness of the epithelial structure, seminal plasma reinforces this selection process.