74 research outputs found
Kinetic Monte Carlo (KMC) Algorithm for Nanocrystals
This thesis uses the kinetic Monte Carlo (KMC) algorithm to examine the
growth morphology and structure of nanocrystals. Crystal growth in a
supersaturated gas of atoms and in an undercooled binary melt is investigated.
First, in the gas phase, the interplay of the deposition and surface
diffusion rates is studied. Then, the KMC algorithm is refined by including
solidification events and finally, by adding diffusion in the surrounding
liquid.
A new algorithm is developed for modelling solidification from an undercooled melt.
This algorithm combines the KMC method, which models
the change in shape of the crystal during growth, with a macroscopic continuum
method that tracks the diffusion of material through solution towards
the crystal. For small length and time scales, this approach provides
simple, effective front tracking with fully resolved atomistic detail of the
crystal-melt interface. Anisotropy is included in the model as a surface
diffusion process and the growth rate of the crystal is found to increase
monotonically with increase in the surface anisotropy value. The method
allows for the study of multiple crystal nuclei and Ostwald ripening. This
method will aid researchers to explain why certain crystal shapes form
under particular conditions during growth, and may enable nanotechnologists
to design techniques for growing nanocrystals with specific shapes
for a variety of applications, from catalysis to the medicine field and electronics
industry. This will lead to a better understanding of the atomistic
process of crystal growth at the nanoscale
Myosin-5, kinesin-1 and myosin-17 cooperate in secretion of fungal chitin synthase
The interactions between cytoskeletal motors are here explored in the context of polarized secretion in fungi, revealing unexpected complexity in the regulation of vesicle transport to and docking at the hyphal tip
Dynamic Behavior of Salmonella-Induced Membrane Tubules in Epithelial Cells
Salmonella Typhimurium is a facultative intracellular pathogen that causes acute gastroenteritis in man. Intracellular Salmonella survive and replicate within a modified phagosome known as the Salmonella-containing vacuole (SCV). The onset of intracellular replication is accompanied by the appearance of membrane tubules, called Salmonella-induced filaments (Sifs), extending from the SCV. Sifs are enriched in late endosomal/lysosomal membrane proteins such as lysosome-associated membrane protein 1, but their formation and ability to interact with endosomal compartments are not characterized. In this study, we use live cell imaging techniques to define the dynamics of Sif formation in infected epithelial cells. At early time-points, Sifs are simple tubules extending from the surface of SCVs. These tubules are highly dynamic and exhibit bidirectional, microtubule-dependent movement. At the distal ends of individual Sif tubules, furthest from the SCV, a distinct ‘leader’ domain was often observed. At later times, Sifs develop into highly complex tubular networks that extend throughout the cell and appear less dynamic than nascent Sifs; however, individual tubules continue to display bidirectional dynamics. Sifs can acquire endocytic content by fusion, indicating a sustained interaction with the endocytic pathway. Together, these results show that these Salmonella-induced tubules form a highly dynamic network that involves both microtubule-dependent motility and interactions with endosomal compartments
Aspergillus Myosin-V Supports Polarized Growth in the Absence of Microtubule-Based Transport
In the filamentous fungus Aspergillus nidulans, both microtubules and actin filaments are important for polarized growth at the hyphal tip. Less clear is how different microtubule-based and actin-based motors work together to support this growth. Here we examined the role of myosin-V (MYOV) in hyphal growth. MYOV-depleted cells form elongated hyphae, but the rate of hyphal elongation is significantly reduced. In addition, although wild type cells without microtubules still undergo polarized growth, microtubule disassembly abolishes polarized growth in MYOV-depleted cells. Thus, MYOV is essential for polarized growth in the absence of microtubules. Moreover, while a triple kinesin null mutant lacking kinesin-1 (KINA) and two kinesin-3s (UNCA and UNCB) undergoes hyphal elongation and forms a colony, depleting MYOV in this triple mutant results in lethality due to a severe defect in polarized growth. These results argue that MYOV, through its ability to transport secretory cargo, can support a significant amount of polarized hyphal tip growth in the absence of any microtubule-based transport. Finally, our genetic analyses also indicate that KINA (kinesin-1) rather than UNCA (kinesin-3) is the major kinesin motor that supports polarized growth in the absence of MYOV
Roles of Dynein and Dynactin in Early Endosome Dynamics Revealed Using Automated Tracking and Global Analysis
Microtubule-dependent movement is crucial for the spatial organization of endosomes in most eukaryotes, but as yet there has been no systematic analysis of how a particular microtubule motor contributes to early endosome dynamics. Here we tracked early endosomes labeled with GFP-Rab5 on the nanometer scale, and combined this with global, first passage probability (FPP) analysis to provide an unbiased description of how the minus-end microtubule motor, cytoplasmic dynein, supports endosome motility. Dynein contributes to short-range endosome movement, but in particular drives 85–98% of long, inward translocations. For these, it requires an intact dynactin complex to allow membrane-bound p150Glued to activate dynein, since p50 over-expression, which disrupts the dynactin complex, inhibits inward movement even though dynein and p150Glued remain membrane-bound. Long dynein-dependent movements occur via bursts at up to ∼8 µms−1 that are linked by changes in rate or pauses. These peak speeds during rapid inward endosome movement are still seen when cellular dynein levels are 50-fold reduced by RNAi knock-down of dynein heavy chain, while the number of movements is reduced 5-fold. Altogether, these findings identify how dynein helps define the dynamics of early endosomes
Moteurs moléculaires et rôle de l'actine dans le transport intracellulaire des compartiments d'endocytose
La fonction des compartiments d endocytose est liée à leur localisation et à leur dynamique, qui sont dépendantes des microtubules, de l actine et des moteurs moléculaires associés. Nous avons mis au point un test de motilité in vitro utilisant des organites isolés par un processus original basé sur l internalisation de nanoparticules magnétiques. Combiné à des analyses biochimiques et de vidéo-microscopie in vivo, ce test nous a permis de montrer que les compartiments précoces et tardifs d endocytose ont des équipements en kinésines et dynéine qui diffèrent, et que la dynéine ne contrôle la localisation des lysosomes que de façon indirecte en régulant l organisation de l actine. D autre part, nous avons montré que les endosomes et les lysosomes interagissent dans les cellules avec des structures d actine. Enfin nous avons observé que les endosomes et lysosomes peuvent eux-mêmes induire la polymérisation d actine à leur surface in vitro en l absence d extrait cellulaire. Les résultats présentés ici approfondissent la compréhension des rôles joués par le cytosquelette dans le contrôle de la dynamique des compartiments d endocytosePARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF
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