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Force generation in small ensembles of Brownian motors
KTH, School of Engineering Sciences (SCI), Theoretical Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics.
Department of Medical Biochemistry and Microbiology, Uppsala Biomedical Center, Uppsala University.
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.ORCID iD: 0000-0003-1164-0831
2006 (English)In: Physical Review E, ISSN 1539-3755, Vol. 74, no 2, 021908-1-021908-8 p.Article in journal (Refereed) Published
Abstract [en]

The motility of certain gram-negative bacteria is mediated by retraction of type IV pili surface filaments, which are essential for infectivity. The retraction is powered by a strong molecular motor protein, PilT, producing very high forces that can exceed 150 pN. The molecular details of the motor mechanism are still largely unknown, while other features have been identified, such as the ring-shaped protein structure of the PilT motor. The surprisingly high forces generated by the PilT system motivate a model investigation of the generation of large forces in molecular motors. We propose a simple model, involving a small ensemble of motor subunits interacting through the deformations on a circular backbone with finite stiffness. The model describes the motor subunits in terms of diffusing particles in an asymmetric, time-dependent binding potential (flashing ratchet potential), roughly corresponding to the ATP hydrolysis cycle. We compute force-velocity relations in a subset of the parameter space and explore how the maximum force (stall force) is determined by stiffness, binding strength, ensemble size, and degree of asymmetry. We identify two qualitatively different regimes of operation depending on the relation between ensemble size and asymmetry. In the transition between these two regimes, the stall force depends nonlinearly on the number of motor subunits. Compared to its constituents without interactions, we find higher efficiency and qualitatively different force-velocity relations. The model captures several of the qualitative features obtained in experiments on pilus retraction forces, such as roughly constant velocity at low applied forces and insensitivity in the stall force to changes in the ATP concentration.

Place, publisher, year, edition, pages
2006. Vol. 74, no 2, 021908-1-021908-8 p.
Keyword [en]
molecular motors, pilus-retraction, twitching motility, ratchet model, iv pilus, transport, protein
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-15963DOI: 10.1103/PhysRevE.74.021908ISI: 000240238100094ScopusID: 2-s2.0-33746882496OAI: diva2:334005
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2010-08-20Bibliographically approved
In thesis
1. Stochastic modeling of motor proteins
Open this publication in new window or tab >>Stochastic modeling of motor proteins
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Motor proteins are microscopic biological machines that convert chemical energy into mechanical motion and work. They power a diverse range of biological processes, for example the swimming and crawling motion of bacteria, intracellular transport, and muscle contraction. Understanding the physical basis of these processes is interesting in its own right, but also has an interesting potential for applications in medicine and nanotechnology.

The ongoing rapid developments in single molecule experimental techniques make it possible to probe these systems on the single molecule level, with increasing temporal and spatial resolution. The work presented in this thesis is concerned with physical modeling of motor proteins on the molecular scale, and with theoretical challenges in the interpretation of single molecule experiments.

First, we have investigated how a small groups of elastically coupled motors collaborate, or fail to do so, when producing strong forces. Using a simple model inspired by the motor protein PilT, we find that the motors counteract each other if the density becomes higher than a certain threshold, which depends on the asymmetry of the system.

Second, we have contributed to the interpretation of experiments in which the stepwise motion of a motor protein is followed in real time. Such data is naturally interpreted in terms of first passage processes. Our main conclusions are (1) Contrary to some earlier suggestions, the stepping events do not correspond to the cycle completion events associated with the work of Hill and co-workers. We have given a correct formulation. (2) Simple kinetic models predict a generic mechanism that gives rise to correlations in step directions and waiting times. Analysis of stepping data from a chimaeric flagellar motor was consistent with this prediction. (3) In the special case of a reversible motor, the chemical driving force can be extracted from statistical analysis of stepping trajectories.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. vii, 71 p.
Trita-FYS, ISSN 0280-316X ; 2008:9
molecular motor, motor protein, Markov process, fluctuations, statistical mechanics, microscopic reversibility
National Category
Physical Sciences
urn:nbn:se:kth:diva-4664 (URN)978-91-7178-897-9 (ISBN)
Public defence
2008-03-28, FA32, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00
QC 20100820Available from: 2008-03-07 Created: 2008-03-07 Last updated: 2010-08-20Bibliographically approved

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