In vitro reconstitution of the functional interplay between MCAK and EB3 at microtubule plus ends

Depolymerizing kinesins Kip3 and MCAK shape cellular microtubule architecture by differential control of catastrophe

Microtubules are dynamic filaments whose ends alternate between periods of slow growth and rapid shortening as they explore intracellular space and move organelles. A key question is how regulatory proteins modulate catastrophe, the conversion from growth to shortening. To study this process, we reconstituted microtubule dynamics in the absence and presence of the kinesin-8 Kip3 and the kinesin-13 MCAK. Surprisingly, we found that, even in the absence of the kinesins, the microtubule catastrophe frequency depends on the age of the microtubule, indicating that catastrophe is a multistep process. Kip3 slowed microtubule growth in a length-dependent manner and increased the rate of aging. In contrast, MCAK eliminated the aging process. Thus, both kinesins are catastrophe factors; Kip3 mediates fine control of microtubule length by narrowing the distribution of maximum lengths prior to catastrophe, whereas MCAK promotes rapid restructuring of the microtubule cytoskeleton by making catastrophe a first-order random process.

Interaction of mammalian end binding proteins with CAP-Gly domains of CLIP-170 and p150(glued)

End binding proteins (EBs) track growing microtubule ends and play a master role in organizing dynamic protein networks. Mammalian cells express up to three different EBs (EB1, EB2, and EB3). Besides forming homodimers, EB1 and EB3 also assemble into heterodimers. One group of EB-binding partners encompasses proteins that harbor CAP-Gly domains. The binding properties of the different EBs towards CAP-Gly proteins have not been systematically investigated. This information is, however, important to compare and contrast functional differences. Here we analyzed the interactions between CLIP-170 and p150(glued) CAP-Gly domains with the three EB homodimers and the EB1-EB3 heterodimer. Using isothermal titration calorimetry we observed that some EBs bind to the individual CAP-Gly domains with similar affinities while others interact with their targets with pronounced differences. We further found that the two types of CAP-Gly domains use alternative mechanisms to target the C-terminal domains of EBs. We succeeded to solve the crystal structure of a complex composed of a heterodimer of EB1 and EB3 C-termini together with the CAP-Gly domain of p150(glued). Together, our results provide mechanistic insights into the interaction properties of EBs and offer a molecular framework for the systematic investigation of their functional differences in cells.

Shaping microtubules into diverse patterns: molecular connections for setting up both ends

Microtubules serve as rails for intracellular trafficking and their appropriate organization is critical for the generation of cell polarity, which is a foundation of cell differentiation, tissue morphogenesis, ontogenesis and the maintenance of homeostasis. The microtubule array is not just a static railway network; it undergoes repeated collapse and reassembly in diverse patterns during cell morphogenesis. In the last decade much progress has been made toward understanding the molecular mechanisms governing complex microtubule patterning. This review first revisits the basic principle of microtubule dynamics, and then provides an overview of how microtubules are arranged in highly shaped and functional patterns in cells changing their morphology by factors controlling the fate of microtubule ends.

FORMIN a link between kinetochores and microtubule ends

The mammalian diaphanous-related (mDia) formin proteins are well known for their actin-nucleation and filament-elongation activities in mediating actin dynamics. They also directly bind to microtubules and regulate microtubule stabilization at the leading edge of the cell during cell migration. Recently, the formin mDia3 was shown to associate with the kinetochore and to contribute to metaphase chromosome alignment, a process in which kinetochores form stable attachments with growing and shrinking microtubules. We suggest that the formin mDia3 could contribute to the regulation of kinetochore-bound microtubule dynamics, in coordination with attachment via its own microtubule-binding activity, as well as via its interaction with the tip-tracker EB1 (end-binding protein 1).

Insights into EB1 structure and the role of its C-terminal domain for discriminating microtubule tips from the lattice

End-binding proteins (EBs) comprise a conserved family of microtubule plus end-tracking proteins. The concerted action of calponin homology (CH), linker, and C-terminal domains of EBs is important for their autonomous microtubule tip tracking, regulation of microtubule dynamics, and recruitment of numerous partners to microtubule ends. Here we report the detailed structural and biochemical analysis of mammalian EBs. Small-angle X-ray scattering, electron microscopy, and chemical cross-linking in combination with mass spectrometry indicate that EBs are elongated molecules with two interacting CH domains, an arrangement reminiscent of that seen in other microtubule- and actin-binding proteins. Removal of the negatively charged C-terminal tail did not affect the overall conformation of EBs; however, it increased the dwell times of EBs on the microtubule lattice in microtubule tip-tracking reconstitution experiments. An even more stable association with the microtubule lattice was observed when the entire negatively charged C-terminal domain of EBs was replaced by a neutral coiled-coil motif. In contrast, the interaction of EBs with growing microtubule tips was not significantly affected by these C-terminal domain mutations. Our data indicate that long-range electrostatic repulsive interactions between the C-terminus and the microtubule lattice drive the specificity of EBs for growing microtubule ends.

A complex of Kif18b and MCAK promotes microtubule depolymerization and is negatively regulated by Aurora kinases

INTRODUCTION

Spindle assembly requires tight control of microtubule (MT) dynamics. This is dependent on a variety of MT binding proteins and their upstream regulators. The Aurora kinases have several well-described functions during cell division, but it remains unclear whether they control global spindle microtubule dynamics.

RESULTS

Here, we find that simultaneous inhibition of Aurora A and B results in a dramatic decrease in spindle MT stability, and we identify the uncharacterized kinesin-8 Kif18b as a mediator of this effect. In interphase, Kif18b is nuclear, but upon nuclear envelope breakdown, Kif18b binds to astral MT plus ends through an interaction with EB1. Surprisingly, Kif18b also binds to the kinesin-13 motor MCAK, and this interaction is required for robust MT depolymerization. Furthermore, the Kif18b-MCAK interaction is negatively regulated by Aurora kinases through phosphorylation of MCAK, indicating that Aurora kinases regulate MT plus-end stability in mitosis through control of Kif18b-MCAK complex formation.

CONCLUSION

Together, these results uncover a novel role for Aurora kinases in regulating spindle MT dynamics through Kif18b-MCAK and suggest that the Kif18b-MCAK complex constitutes the major MT plus-end depolymerizing activity in mitotic cells.

The molecular basis of anaphase A in animal cells

The mechanisms that move chromatids poleward during anaphase A have fascinated researchers for decades. There is now growing evidence that this movement is tightly linked to the active depolymerization of both ends of kinetochore-associated microtubules, a mechanism we refer to as "Pacman-Flux." Contemporary data suggest that this is catalyzed by the integration of multiple enzymatic activities including (1) microtubule-end depolymerases housed at the pole or kinetochore, (2) microtubule-severing enzymes used to uncap the ends of kinetochore-associated microtubules, and (3) molecular motors which drive tubulins towards the pole or into kinetochores.

Localized Aurora B activity spatially controls non-kinetochore microtubules during spindle assembly

Efficient spindle assembly involves the generation of spatial cues around chromosomes that locally stabilize microtubule (MT) plus-ends. In addition to the small GTPase Ran, there is evidence that Aurora B kinase might also generate a spatial cue around chromosomes but direct proof for this is still lacking. Here, we find that the Aurora B substrate MCAK localizes to MT plus-ends throughout the mitotic spindle, but its accumulation is strongly reduced on MT plus-ends near chromatin, suggesting that a signal emanating from chromosomes negatively regulates MCAK plus-end binding. Indeed, we show that Aurora B is the kinase responsible for producing this chromosome-derived signal. These results are the first to visualize spatially restricted Aurora B kinase activity around chromosomes on an endogenous substrate and explain how Aurora B could spatially control the dynamics of non-kinetochore MTs during spindle assembly.

Kif18B interacts with EB1 and controls astral microtubule length during mitosis

Kif18B is a newly discovered plus-tip-tracking protein that is enriched on astral microtubule (MT) ends during early mitosis. Kif18B binds directly to EB1, and this interaction is required for proper localization of Kif18B and to control astral MT length.

Regulation of microtubule (MT) dynamics is essential for proper spindle assembly and organization. Kinesin-8 family members are plus-end-directed motors that modulate plus-end MT dynamics by acting as MT depolymerases or as MT plus-end capping proteins. In this paper, we show that the human kinesin-8 Kif18B functions during mitosis to control astral MT organization. Kif18B is a MT plus-tip-tracking protein that localizes to the nucleus in interphase and is enriched at astral MT plus ends during early mitosis. Knockdown of Kif18B caused spindle defects, resulting in an increased number and length of MTs. A yeast two-hybrid screen identified an interaction of the C-terminal domain of Kif18B with the plus-end MT-binding protein EB1. EB1 knockdown disrupted Kif18B targeting to MT plus ends, indicating that EB1/Kif18B interaction is physiologically important. This interaction is direct, as the far C-terminal end of Kif18B is sufficient for binding to EB1 in vitro. Overexpression of this domain is sufficient for plus-end MT targeting in cells; however, targeting is enhanced by the motor domain, which cooperates with the tail to achieve proper Kif18B localization at MT plus ends. Our results suggest that Kif18B is a new MT dynamics regulatory protein that interacts with EB1 to control astral MT length.

Microtubule dynamics: Patronin, protector of the minus end

It has long been surmised that cellular microtubules are capped at the minus ends to prevent their depolymerization. A recent study provides the first definitive identification of a minus-end-specific capping protein, termed Patronin, which protects the microtubule arrays of both mitotic and interphase cells.

SLAIN2 links microtubule plus end-tracking proteins and controls microtubule growth in interphase

SLAIN2’s interactions with multiple different microtubule plus end–tracking proteins stimulate processive microtubule polymerization and ensure proper microtubule organization.

The ends of growing microtubules (MTs) accumulate a set of diverse factors known as MT plus end–tracking proteins (+TIPs), which control microtubule dynamics and organization. In this paper, we identify SLAIN2 as a key component of +TIP interaction networks. We showed that the C-terminal part of SLAIN2 bound to end-binding proteins (EBs), cytoplasmic linker proteins (CLIPs), and CLIP-associated proteins and characterized in detail the interaction of SLAIN2 with EB1 and CLIP-170. Furthermore, we found that the N-terminal part of SLAIN2 interacted with ch-TOG, the mammalian homologue of the MT polymerase XMAP215. Through its multiple interactions, SLAIN2 enhanced ch-TOG accumulation at MT plus ends and, as a consequence, strongly stimulated processive MT polymerization in interphase cells. Depletion or disruption of the SLAIN2–ch-TOG complex led to disorganization of the radial MT array. During mitosis, SLAIN2 became highly phosphorylated, and its interaction with EBs and ch-TOG was inhibited. Our study provides new insights into the molecular mechanisms underlying cell cycle–specific regulation of MT polymerization and the organization of the MT network.

Drosophila katanin is a microtubule depolymerase that regulates cortical-microtubule plus-end interactions and cell migration

Regulation of microtubule dynamics at the cell cortex is important for cell motility, morphogenesis and division. Here we show that the Drosophila katanin Dm-Kat60 functions to generate a dynamic cortical-microtubule interface in interphase cells. Dm-Kat60 concentrates at the cell cortex of S2 Drosophila cells during interphase, where it suppresses the polymerization of microtubule plus-ends, thereby preventing the formation of aberrantly dense cortical arrays. Dm-Kat60 also localizes at the leading edge of migratory D17 Drosophila cells and negatively regulates multiple parameters of their motility. Finally, in vitro, Dm-Kat60 severs and depolymerizes microtubules from their ends. On the basis of these data, we propose that Dm-Kat60 removes tubulin from microtubule lattice or microtubule ends that contact specific cortical sites to prevent stable and/or lateral attachments. The asymmetric distribution of such an activity could help generate regional variations in microtubule behaviours involved in cell migration.

Cytoskeletons by the sea

The ESF-EMBO meeting, 'Emergent Properties of the Cytoskeleton: Molecules to Cells', took place in October 2010 in San Feliu dex Guíxols on the eastern coast of Spain. It brought together a diverse group of international cytoskeletal researchers who gave presentations on topics from structural biology and biophysical analyses of the cytoskeleton and its motors, to studies of the role of cytoskeletal proteins in multicellular development.

How to measure microtubule dynamics?

Microtubules are one of the most spectacular features in the cell: long, fairly rigid tubules that provide physical strength while at the same time serving as tracks of the intracellular transport network. In addition, they are the main constituents of the cell division machinery, and guide axonal growth and the direction of cell migration. To be able to fulfil such diverse functions, microtubules have to be arranged into suitable patterns and remodelled according to extra- and intracellular cues. Moreover, the delicate regulation of microtubule dynamics and the dynamic interactions with subcellular structures, such as kinetochores or cell adhesion sites, appear to be of crucial importance to microtubule functions. It is, therefore, important to understand microtubule dynamics and its spatiotemporal regulation at the molecular level. In this chapter, I introduce the concept of microtubule dynamics and discuss the techniques that can be employed to study microtubule dynamics in vitro and in cells, for many of which detailed protocols can be found in this volume. Microtubule dynamics is traditionally assessed by the four parameters of dynamic instability: growth and shrinkage rates, rescue and catastrophe frequencies, sometimes supplemented by pause duration. I discuss emerging issues with and alternatives to this parameter description of microtubule dynamics.