Ring coupler moving via the 'forced walk' mechanism [Microtubule depolymerization as a biological machine]

Abstract

This video shows the microtubule-depolymerization dependent motions of a ring coupler. Although the plane of the ring oscillates slightly, it remains motionlessly on the microtubule wall until the shortening end comes by (thermal energy is not sufficient to cause Brownian 'random walks' with any appreciable frequency). Bending protofilaments, however, can push on the linkers, forcing the ring to walk in front of the protofilaments' flare. This directed motion is highly deterministic, but the exact pathway of the ring's transitions between successive minimum energy configurations is stochastic and varies in repeated calculations. A strongly bound ring retards the rate with which the protofilaments bend, so it slows the rate of microtubule shortening. Furthermore, such tight binding reduces the useful work that can be performed by the microtubule, e.g. in moving a cargo, but it ensures a stable ring's attachment to the microtubule end, even if the flared protofilaments shorten or the microtubule begins to polymerize. It has been therefore suggested that a reasonable compromise between a reduced efficiency of force transduction and an increased strength of attachment might be appropriate for the coupler in an organism like S. cerevisiae, where a kinetochore is stably attached to only one microtubule and the chromosomes do not move far during Anaphase AComponente Curricular::Educação Superior::Ciências Biológicas::Morfologi