Concerning the efficiency of the treadmilling phenomenon with microtubules

Structures of TOG1 and TOG2 from the human microtubule dynamics regulator CLASP1

Tubulin-binding TOG domains are found arrayed in a number of proteins that regulate microtubule dynamics. While much is known about the structure and function of TOG domains from the XMAP215 microtubule polymerase family, less in known about the TOG domain array found in animal CLASP family members. The animal CLASP TOG array promotes microtubule pause, potentiates rescue, and limits catastrophe. How structurally distinct the TOG domains of animal CLASP are from one another, from XMAP215 family TOG domains, and whether a specific order of structurally distinct TOG domains in the TOG array is conserved across animal CLASP family members is poorly understood. We present the x-ray crystal structures of Homo sapiens (H.s.) CLASP1 TOG1 and TOG2. The structures of H.s. CLASP1 TOG1 and TOG2 are distinct from each other and from the previously determined structure of Mus musculus (M.m.) CLASP2 TOG3. Comparative analyses of CLASP family TOG domain structures determined to date across species and paralogs supports a conserved CLASP TOG array paradigm in which structurally distinct TOG domains are arrayed in a specific order. H.s. CLASP1 TOG1 bears structural similarity to the free-tubulin binding TOG domains of the XMAP215 family but lacks many of the key tubulin-binding determinants found in XMAP215 family TOG domains. This aligns with studies that report that animal CLASP family TOG1 domains cannot bind free tubulin or microtubules. In contrast, animal CLASP family TOG2 and TOG3 domains have reported microtubule-binding activity but are structurally distinct from the free-tubulin binding TOG domains of the XMAP215 family. H.s. CLASP1 TOG2 has a convex architecture, predicted to engage a hyper-curved tubulin state that may underlie its ability to limit microtubule catastrophe and promote rescue. M.m. CLASP2 TOG3 has unique structural elements in the C-terminal half of its α-solenoid domain that our modeling studies implicate in binding to laterally-associated tubulin subunits in the microtubule lattice in a mode similar to, yet distinct from those predicted for the XMAP215 family TOG4 domain. The potential ability of the animal CLASP family TOG3 domain to engage lateral tubulin subunits may underlie the microtubule rescue activity ascribed to the domain. These findings highlight the structural diversity of TOG domains within the CLASP family TOG array and provide a molecular foundation for understanding CLASP-dependent effects on microtubule dynamics.

Thermodynamics and kinetics of DNA nanotube polymerization from single-filament measurements

Single-filament measurement of the thermodynamic and kinetic parameters of DNA nanotube assembly supports a polymerization/depolymerization model sharing common features with cytoskeletal polymer models.

DNA nanotubes provide a programmable architecture for molecular self-assembly and can serve as model systems for one-dimensional biomolecular assemblies. While a variety of DNA nanotubes have been synthesized and employed as models for natural biopolymers, an extensive investigation of DNA nanotube kinetics and thermodynamics has been lacking. Using total internal reflection microscopy, DNA nanotube polymerization was monitored in real time at the single filament level over a wide range of free monomer concentrations and temperatures. The measured polymerization rates were subjected to a global nonlinear fit based on polymerization theory in order to simultaneously extract kinetic and thermodynamic parameters. For the DNA nanotubes used in this study, the association rate constant is (5.99 ± 0.15) × 10⁵ M^–1 s^–1, the enthalpy is 87.9 ± 2.0 kcal mol^–1, and the entropy is 0.252 ± 0.006 kcal mol^–1 K^–1. The qualitative and quantitative similarities between the kinetics of DNA nanotubes, actin filaments, and microtubules polymerization highlight the prospect of building complex dynamic systems from DNA molecules inspired by biological architecture.

Dynamics of microtubules visualized by darkfield microscopy: treadmilling and dynamic instability

Individual microtubules undergoing treadmilling in vitro were visualized by darkfield light microscopy, and the relationship between treadmilling and dynamic instability was studied as a function of microtubule-associated proteins (MAPs). In order to demonstrate treadmilling directly by real-time observation, we constructed three-block microtubules, the center-block of which was decorated with Tetrahymena dynein. The decorated block can easily be distinguished from undecorated blocks in the darkfield microscope because the decorated one appears much thicker. At steady-state conditions, the length of an undecorated block at one end increased and that at another end decreased, while the decorated center-block did not change in its length. The results from these direct observations show that calf brain 3X-microtubules exhibit a treadmilling flux of 0.9 micron/h. Using a similar microscopy technique, we previously demonstrated that phosphocellulose PC-microtubules existed in either the growing or the shortening phase and alternated quite frequently at steady-state conditions (dynamic instability). How does treadmilling relate to dynamic instability? An image recording of individual 3X-microtubules containing MAPs revealed that the microtubules undergo treadmilling and do not exhibit any dynamic instability. This evidence shows that MAPs suppress the dynamic instability of microtubules. That is, treadmilling can take place in the steady state only after microtubules have been stabilized by MAPs.

On the relationship between nucleotide hydrolysis and microtubule assembly: studies with a GTP-regenerating system

The assembly of pure tubulin dimer has been studied in two buffer systems (containing low and high glycerol/Mg), using a regeneration system protocol to assess the amount of GDP-tubulin in the assembling polymer. For both assembly systems studied, the GDP content is effectively stoichiometric with tubulin throughout assembly. This indicates a high degree of coupling between assembly and GTP-hydrolysis, giving a hydrolysis rate at least 10-fold faster than previously deduced. The steady state GTP hydrolysis rate is quantitatively consistent with this finding. We conclude that the extent of any GTP-tubulin cap is below the detectable limit, both during elongation and at steady state.

Location of the guanosine triphosphate (GTP) hydrolysis site in microtubules

The rate for GTP hydrolysis remains approximately constant during microtubule assembly from microtubular protein. This indicates that GTP hydrolysis does not accompany tubulin-GTP subunit addition to microtubule ends. We suggest that GTP, within tubulin-GTP subunits that are incorporated into microtubules, is hydrolyzed predominantly at one or both microtubule ends at an interface of a cap of tubulin-GTP subunits and a core of tubulin-GDP subunits.

Treadmilling, diffusional exchange and cytoplasmic structures

Microfilaments and microtubules exchange monomers from solution by at least two mechanisms; treadmilling and diffusional exchange. Refined kinetic analysis of both mechanisms shows that this exchange may be nonlinear under certain conditions. The two mechanisms of exchange differ in some of their predictions for the behaviour of cytoplasmic structures. Studies of assembly of cytoplasmic structures in vivo suggest that diffusional exchange is probably predominant for steady-state structures and further suggest that additional mechanisms may be operating in the cell.

Formation of microtubules at low temperature by tubulin from antarctic fish

Tubulin was isolated from two species of antarctic fish, Pagothenia borchgrevinki and Dissostichus mawsoni, by cycles of temperature-dependent assembly, centrifugation, disassembly, and centrifugation. The preparations were found to consist almost entirely of tubulin and to contain negligibly small amounts of microtubule-associated proteins. This tubulin polymerized to make microtubules of ordinary dimensions. The formed microtubules appear to be in labile equilibrium with free tubulin dimer at all temperatures observed. In a buffer consisting of 0.1 M 1,4-piperazinediethanesulfonic acid, 2 mM dithioerythritol, 1 mM MgSO4, 2 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and 1 mM guanosine 5'-triphosphate, pH 6.9, the tubulin of P. borchgrevinki has a critical concentration for assembly of 0.046 (+/- 0.008) mg/mL at 35 degrees C and 0.74 (+/- 0.15) mg/mL at the habitat temperature of the fish, -1.8 degrees C. The critical concentration measured at the lower temperature is quite small relative to the critical concentration for formation of mammalian microtubules from pure tubulin at the same temperature, which must be at least 2 orders of magnitude larger. The antarctic fish microtubules may thus be called "cold stable" by comparison with mammalian microtubules. They do not fully dissociate at temperatures near 0 degree C because they are composed of tubulin that assembles more readily at these temperatures than does mammalian tubulin. There is no evidence for the presence of a cold-stabilizing factor in association with the tubulin. These findings suggest that alteration of tubulin may be a means by which some poikilotherms can adapt to a cold environment.(ABSTRACT TRUNCATED AT 250 WORDS)