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Structural and biophysical studies of cytoskeletal filaments: actin and septins

le 21 juin 2023

Séminaire d'Aurélie Bertin

To dissect and decouple the specific functions of proteins from other factors, we mostly use cell-free bottom-up assays and more particularly cryo-electron microscopy. My talk will be divided in two  independent sections. First, I will focus my presentation on the conformation of formins at the barbed ends of actin. Second, I will present how curvature sensitive septins can induce membrane reshaping.
Formins are known to regulate actin polymerization by interacting with their polymerizing barbed ends. Indirect evidences suggested a two steps mechanism with an alternation of open and close conformation. To directly test for this hypothesis, we have used electron microscopy and single particle analysis. We have first optimized our sample to maximize the number of barbed ends visible on a field of view. After image analysis, we were able to directly visualize the so called open and close conformations of formins and discriminate between two competing models. Besides, the open conformation would actually display a continuum of conformations. Interestingly, formins were also visualized within the core of actin filaments. This unexpected observation thus suggest that formins might be able to translocate from the barbed ends [1].
Septins are ubiquitous cytoskeletal proteins essential for cell division which participate in the formation of diffusion barrier and might be involved in membrane deformation and rigidity. They polymerize into filamentous structures associated with the inner plasma membrane. We have used a combination of in vitro bottom approaches and theory to analyze how the curvature sensitivity of budding yeast and human septins could induce membrane deformations [2,3]. We have compared how purified septin complexes arrange differently on positive or negative curvatures using scanning Electron Microscopy imaging on undulated PDMS substrates, cryo-electron microscopy or fluorescence imaging. Within an in vitro reconstituted systems, we observe that human septins tend to organize within filamentous orthogonal arrays while yeast septins assemble into parallel arrays of filaments. Specific filamentous patterns are able to restrict the diffusion of membrane bound components and tune the mechanical properties of membranes.

[1] Julien Maufront, Bérengère Guichard, Lu-Yan Cao, Aurélie Di Cicco, Antoine Jégou, Guillaume Romet-Lemonne, Aurélie Bertin, Direct observation of the conformational states of formin mDia1 at actin filament barbed ends and along the filament, MBOC, 2022, 34, 1,
[2] Alexandre Beber, Cyntia Taveneau, Manuela Nania, Feng-Ching Tsai, Aurelie Di Cicco, Patricia Bassereau, Daniel Lévy, João T. Cabral, Hervé Isambert, Stéphanie Mangenot, Aurélie Bertin. Membrane reshaping by micrometric curvature sensitive septin filaments, Nat Commun. 2019 Jan 24;10(1):420. doi: 10.1038/s41467-019-08344-5.
[3] Koyomi Nakazawa, Gaurav Kumar, Brieuc Chauvin, Aurélie Di Cicco, Luca Pellegrino, Michael Trichet, Bassam Hajj, João Cabral, Anirban Sain, Stéphanie Mangenot,  Aurélie Bertin, A specific mesh-like organization of human septin octameric complex drives membrane reshaping and curvature sensitivity, Biorxiv 2022,, under revision, JCS.

Aurélie Bertin est invitée par Laurence Serre et Isabelle Arnal.

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Amphi Kampf
Mise à jour le 24 avril 2023

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