Tue, 13/01/2015 - 14:15
,
Campus Saarbr├╝cken, Geb. E2 6, Raum E.04

Alexandre Mamane
(
Host: Prof. Dr. Heiko Rieger
)
Institute Curie, Paris

Collective behavior of molecular motors within cellular functions: extraction of membrane tubes and forcing of cytoplasmic streaming

In the 1st part of the presentation we focus on modeling the cooperation between Myo1b non processive catch-bond motors to extract membrane tubes : Myosin 1b is a single-headed membrane-associated motor that binds to actin filaments with a catch-bond behaviour in response to load. In vivo, myosin 1b is required to form membrane tubules at both endosomes and the trans-Golgi network. To establish the link between these two fundamental properties, here we investigate the capacity of myosin 1b to extract membrane tubes along bundled actin filaments in a minimal reconstituted system. We show that single-headed non-processive myosin 1b can extract membrane tubes at a biologically relevant low density. In contrast to kinesins we do not observe motor accumulation at the tip, suggesting that the underlying mechanism for tube formation is different. In our theoretical model, myosin 1b catch-bond properties facilitate tube extraction under conditions of increasing membrane tension by reducing the density of myo1b required to pull tubes.
In the 2nd part of the presentation we focus on modeling the emergence and reversal of meiotic cytoplasmic streaming in C. elegans zygotes by ER-mediated alignment of microtubules : Cytoplasmic streaming is present in plant and animal cell. Its functions can be to to mix the cytoplasm to increase the diffusion of big particles (organelles, macromolecules), to increase transport through membranes, or to generate a deterministic flow that transports organelles. In the meiotic C. elegans embryo, a kinesin-dependent meiotic cytoplasmic streaming (MCS) develops immediately after fertilization. Experimentalists found that not only vesicular organelles but also cortical ER network flows orthogonal to the long axis of the embryo. The flow orientation stochastically reverses. Quantitative analyses of the orientation of microtubules and of the flow direction of ER revealed a correlation between them. When ER network was experimentally fragmented, the flow and the polarization of the microtubules vanished. Experimentalists discovered a function for the MSC, it increases cortical granule exocytosis. In our model, we assume that kinesins bind cortical microtubules and ER tubules to shear the ER. The microtubules interact through the ER and polarize in the direction of the flow. The cytoplasm is forced to flow by the ER movement. Our theoretical model verifies these hypotheses, reproduces the features of the streaming, and predicts the stabilization of flow with longer microtubules, which was experimentally confirmed.

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