The role of FtsZ treadmilling and torsional stress for bacterial cytokinesis: an in vitro study

In dieser Dissertation studierte ich die „Treadmilling“-Dynamik der GTPase FtsZ in vitro. FtsZ gehört zu den häufigsten Proteinen, die an der bakteriellen Zellteilung beteiligt sind. Da FtsZ in vivo an der Lipid Membran verankert ist und eine ringartige Struktur formt, wurde vermutet, dass FtsZ für den mechanischen Prozess der Zytokinese Einschnürungskräfte ausübt. Mehr noch als zu testen, ob FtsZ Kräfte ausübt, die hinreichend für die bakterielle Zellteilung sind, soll die hier präsentierte Studie vielmehr Eigenschaften von FtsZ Polymeren aufzeigen um den physikalischen Prozess der Einschnürung zu verste- hen. Zuerst habe ich vorherige Konzepte über die Rolle von FtsA als natürliches Ankerprotein in Frage gestellt nach Rekonstituierung von FtsZ auf „Supported Lipid Membranes“ (SLBs). Anders als erwartet konnte ich zeigen, dass die FtsZ-YFP-mts Chimäre autonom an die Membran binden kann und die Proteine sich auf der Membran zu einer ringartigen Struktur zusammensetzen, abhängig von der Dichte der Proteine auf der Membranoberfläche. Dieser Ring weist „treadmilling“ Verhalten auf, welches sich in einer anscheinenden Rotation im Uhrzeigersinn äußert. Bei intermediären Proteindichten von FtsZ-YFP-mts formten sich Ringe und bei hohen Proteindichten formten sich parallel zueinander liegende Filamente in nematischer Anordnung. Ferner scheint die Entstehung der ringförmigen Strukturen, ähnlich der in vivo Situation, eher von der GTPase Aktivität abzuhängen als von spezifischen Protein-Anker Interaktionen. Um den Einfluß von FtsZ auf deformierbare Oberflächen zu untersuchen, habe ich Ver- fahren entwickelt, um FtsZ Ringe an der Außenseite von gigantischen unilamellaren Vesikeln (GUVs) zu rekonstituieren. Nachdem die Vesikel einem osmotischen Schock ausgesetzt wurden um sie deformierbar zu machen, induzierten die FtsZ Ringe durch „Bohr-Kräfte“ nach innen gerichtete Membraneinstülpungen („Membran-Kegel“). Mit den nach innen gerichteten Membraneinstülpungen zeigte FtsZ „treadmilling“ Richtungen sowohl im Uhrzeigersinn, als auch gegen den Uhrzeigersinn. Ferner, um den Einfluß von FtsZ auf zylin- drische Geometrien zu untersuchen, habe ich ein neues Verfahren entwickelt, um längliche, weiche, Membranausstülpungen aus GUVs mit Hilfe optischer Pinzetten zu ziehen. Bei GUVs mit FtsZ Ringen an der Membranaußenseite und nach innen gerichteten Membraneinstülpungen, wurden die weichen mit der optischen Pinzette gezogenen „Lipid-Röhren“ bemerkenswerterweise durch FtsZ in eine 3D-helikale, federartige Struktur transformiert. Diese helikalen Deformationen lassen sich durch Verdrehen eines elastischen Stabes erklären. Zusätzlich verursacht die GTPase Aktivität einen „super-verdrehten“ Zustand der ausgezogenen Lipid Membran, die eine Feder-Kompression verursacht mit Kräften um die 0.6 − 1 pN. Diese GTPase abhängige Feder-Kompression entspricht den Einschnürungen, wenn FtsZ innerhalb von GUVs rekonstituiert wurde. Daher schlage ich vor, dass FtsZ bedingte Einschnürungen durch Torsion erzeugt werden.In this thesis, I studied the treadmilling-dynamic properties of GTP-consuming FtsZ filaments in vitro. FtsZ is the most abundant protein in bacteria cell-division machinery. Since FtsZ filaments are anchored to the lipid membrane to form a ring-like structure, it has been suggested that FtsZ exerts constriction-forces driving the mechanical process of cytokinesis. More than determining whether FtsZ generate forces that suffice bacteria cell division, the here presented in vitro study sheds light on FtsZ polymer properties aiming to understand the physical process of constriction. First, I challenged previous conceptions about the role of FtsA, a natural FtsZ membrane anchor, when FtsZ was reconstituted on Supported-Lipid Membranes (SLBs). Contrary to expected, I found that an autonomous membrane binding chimera FtsZ-YFP-mts self-assembles into treadmilling (clockwise-rotation) ring-like structures as a function of the surface protein density. At intermediate surface densities, FtsZ-YFP-mts formed rings whereas, at high surface densities, filaments exhibited a parallel-nematic arrangement. Moreover, rather than an specific protein-anchor interaction, FtsZ GTPase activity determined the emergence of treadmilling rings as also shown in vivo. To explore the impact of FtsZ on deformable surfaces, I found the conditions to reconstitute FtsZ rings outside Giant Unilamellar Vesicles (GUVs). Once vesicles were deflated (deformable), rings induced drilling-like forces shaping inwards cones. In this cone-geometry, FtsZ showed clockwise/anticlockwise treadmilling directions. Moreover, to investigate the impact of FtsZ on tubular geometries, I implemented a novel technique to pull soft lipid tubes (from GUVs) using optical tweezers. Then, soft tubes were pulled from GUVs displaying (outside) FtsZ rings and inwards deformation. Strikingly, FtsZ transformed the lipid tube into a 3D helical spring-like structure. These helical deformations can be rationalized by twisting an elastic rod. In addition, GTPase activity triggered a super-twisted state causing spring compression and delivering forces around 0.6 - 1 pN. This GTPase dependent spring-compression resulted equivalent to the formation of constriction necks when FtsZ was reconstituted inside GUVs. Therefore, I suggest that FtsZ makes constriction via torsional stress

Magnetism in curved geometries

Curvature impacts physical properties across multiple length scales, ranging from the macroscopic scale, where the shape and size vary drastically with the curvature, to the nanoscale at interfaces and inhomogeneities in materials with structural, chemical, electronic, and magnetic short-range order. In quantum materials, where correlations, entanglement, and topology dominate, the curvature opens the path to novel characteristics and phenomena that have recently emerged and could have a dramatic impact on future fundamental and applied studies of materials. Particularly, magnetic systems hosting non-collinear and topological states and 3D magnetic nanostructures strongly benefit from treating curvature as a new design parameter to explore prospective applications in the magnetic field and stress sensing, microrobotics, and information processing and storage. This Perspective gives an overview of recent progress in synthesis, theory, and characterization studies and discusses future directions, challenges, and application potential of the harnessing curvature for 3D nanomagnetism

Optical and Thermal Analysis of a Heteroconical Tubular Cavity Solar Receiver

The principal objective of this study is to develop, investigate and optimise the Heteroconical Tubular Cavity receiver for a parabolic trough reflector. This study presents a three-stage development process which allowed for the development, investigation and optimisation of the Heteroconical receiver. The first stage of development focused on the investigation into the optical performance of the Heteroconical receiver for different geometric configurations. The effect of cavity geometry on the heat flux distribution on the receiver absorbers as well as on the optical performance of the Heteroconical cavity was investigated. The cavity geometry was varied by varying the cone angle and cavity aperture width of the receiver. This investigation led to identification of optical characteristics of the Heteroconical receiver as well as an optically optimised geometric configuration for the cavity shape of the receiver. The second stage of development focused on the thermal and thermodynamic performance of the Heteroconical receiver for different geometric configurations. This stage of development allowed for the investigation into the effect of cavity shape and concentration ratio on the thermal performance of the Heteroconical receiver. The identification of certain thermal characteristics of the receiver further optimised the shape of the receiver cavity for thermal performance during the second stage of development. The third stage of development and optimisation focused on the absorber tubes of the Heteroconical receiver. This enabled further investigation into the effect of tube diameter on the total performance of the Heteroconical receiver and led to an optimal inner tube diameter for the receiver under given operating conditions. In this work, the thermodynamic performance, conjugate heat transfer and fluid flow of the Heteroconical receiver were analysed by solving the computational governing Equations set out in this work known as the Reynolds-Averaged Navier-Stokes (RANS) Equations as well as the energy Equation by utilising the commercially available CFD code, ANSYS FLUENT®. The optical model of the receiver which modelled the optical performance and produced the nonuniform actual heat flux distribution on the absorbers of the receiver was numerically modelled by solving the rendering Equation using the Monte-Carlo ray tracing method. SolTrace - a raytracing software package developed by the National Renewable Energy Laboratory (NREL), commonly used to analyse CSP systems, was utilised for modelling the optical response and performance of the Heteroconical receiver. These actual non-uniform heat flux distributions were applied in the CFD code by making use of user-defined functions for the thermal model and analysis of the Heteroconical receiver. The numerical model was applied to a simple parabolic trough receiver and reflector and validated against experimental data available in the literature, and good agreement was achieved. It was found that the Heteroconical receiver was able to significantly reduce the amount of reradiation losses as well as improve the uniformity of the heat flux distribution on the absorbers. The receiver was found to produce thermal efficiencies of up to 71% and optical efficiencies of up to 80% for practically sized receivers. The optimal receiver was compared to a widely used parabolic trough receiver, a vacuum tube receiver. It was found that the optimal Heteroconical receiver performed, on average, 4% more efficiently than the vacuum tube receiver across the temperature range of 50-210℃. In summary, it was found that the larger a Heteroconical receiver is the higher its optical efficiency, but the lower its thermal efficiency. Hence, careful consideration needs to be taken when determining cone angle and concentration ratio of the receiver. It was found that absorber tube diameter does not have a significant effect on the performance of the receiver, but its position within the cavity does have a vital role in the performance of the receiver. The Heteroconical receiver was found to successfully reduce energy losses and was found to be a successfully high performance solar thermal tubular cavity receiver