Direct observation of branching MT nucleation in living animal cells

Centrosome-mediated microtubule (MT) nucleation has been well characterized; however, numerous noncentrosomal MT nucleation mechanisms exist. The branching MT nucleation pathway envisages that the γ-tubulin ring complex (γ-TuRC) is recruited to MTs by the augmin complex to initiate nucleation of new MTs. While the pathway is well conserved at a molecular and functional level, branching MT nucleation by core constituents has never been directly observed in animal cells. Here, multicolor TIRF microscopy was applied to visualize and quantitatively define the entire process of branching MT nucleation in dividing Drosophila cells during anaphase. The steps of a stereotypical branching nucleation event entailed augmin binding to a mother MT and recruitment of γ-TuRC after 15 s, followed by nucleation 16 s later of a daughter MT at a 36° branch angle. Daughters typically remained attached throughout their ∼40-s lifetime unless the mother depolymerized past the branch point. Assembly of branched MT arrays, which did not require Drosophila TPX2, enhanced localized RhoA activation during cytokinesis.

Direct observation of branching MT nucleation in living animal cells

ABSTRACTCentrosome-mediated microtubule (MT) nucleation has been well-characterized; however, numerous non-centrosomal MT nucleation mechanisms exist. The branching MT nucleation pathway envisages that the γ-tubulin ring complex (γ-TuRC) is recruited to MTs by the augmin complex to initiate nucleation of new MTs. While the pathway is well-conserved at a molecular and functional level, branching MT nucleation by core constituents has never been directly observed in animal cells. Here, multi-color TIRF microscopy was applied to visualize and quantitatively define the entire process of branching MT nucleation in dividing Drosophila cells. The stereotypical branching nucleation event entailed augmin first binding to a “mother” MT, recruitment of γ-TuRC after 16s, followed by nucleation 15s later of a “daughter” MT at a 36° branch angle. Daughters typically remained attached throughout their ~40s lifetime unless the mother depolymerized past the branch point. Assembly of branched MT arrays, which did not require D-TPX2 (Drosophila TPX2) or evident regulation by a RanGTP gradient, enhanced localized RhoA activation during cytokinesis.

A network theory of the origin of life

The origin of life may have been a low probability event that involved both RNA and polypeptides. While such an event would occur with low probability, the probability is not too low in the context of the number of nucleation opportunities on the primitive Earth. The probability for the nucleation event is too small if needed components occur as frequently as disruptive components do, without specific interactions between the two classes. However, if many of the needed components contribute by inhibiting otherwise disruptive components, a high probability for a complex nucleation as the beginning of life event emerges. RNA interference and long non-coding RNAs are ascribed roles in inhibiting components that would otherwise be disruptive in the context of a nucleation event. The theory likewise provides an inhibitory role for the large fraction of proteins (35% to 40%) that have no known function in both prokaryotes and eukaryotes. It is suggested that volcanic ash could provide the large number of random shapes needed as the basis for the nucleation event. Experiments based on that hypothesis are proposed.

Mechanical Modification of Al-6%Si By Semisolid Process

In this study by semisolid processing of Al-6%Si alloy, it has been found that small additions of Ti-6Al-4V shift the liquidus temperature up and the recalescence decreases. The nucleation event takes place at higher temperatures. Furthermore, the rate at which growth temperature increases is less than that of nucleation temperature and therefore more nuclei form with less potential for growth. In this processing refiner increases the α-Al percentage and reduces globule size. Improvement of mechanical property and microstructure characterization are the main advantages of this study.

Solidification of Sn-3Ag-0.5Cu and Sn-0.7Cu-0.05Ni Solders

The solidification of Sn-3Ag-0.5Cu and Sn-0.7Cu-0.05Ni are overviewed and compared. In joints on Cu substrates, both solders begin solidification with primary Cu6Sn5 growing in the bulk liquid prior to tin nucleation. In freestanding balls and joints, SAC305 generally solidifies with a single tin nucleation event and exhibits a mutually-twinned tin grain structure. In contrast, SN100C BGA balls and joints exhibit multiple independent tin grains that grow as a columnar array in joints.

Martensite Spread: Analytical Modeling and Computer Simulation

In bulk polycrystals, the first nucleation event in a single grain may induce transformationin neighboring grains, resulting in a cluster of partially transformed grains. This cluster of partiallytransformed grains is normally designated as a ‘spread event’. The collection of these single spreadevents can be defined as the ‘spread’or ‘martensite spread’. In this paper we show how the martensitespreadmay be well described by formal kinetics. Spread taking place as a result of athermalmartensitetransformation, isothermal martensite transformation and even martensite burst are analyzed with thehelp of such a methodology. Formal kinetics is essentially based on a stochastic geometry approach ofthe transformations considered as nucleation and growth (or birth-and-growth ) processes. Therefore,the success of such an analysis strongly suggests the possibility of a computer simulation of the spreadentirely based on probabilistic considerations. Such a simulation has been carried out and the resultsare discussed in this paper with reference to the results obtained by the analytical methods.