Instruct-ERIC Events

Title: C- and D-type lateral interactions weaken the microtubule lattice

Abstract

Tubulin subunits (alpha, beta) are usually considered to engage two types of lateral interactions within microtubules, heterotypic (a-b, b-a) or homotypic (a-a, b-b) of the A- and B-type, respectively. These lateral interactions follow left-handed, 2- to 4-start monomeric helical paths in microtubules with varying protofilament numbers. Using a combination of cryo-electron microscopy and tomography, we describe two new types of lateral interactions that are shifted about half the tubulin monomer repeat along the protofilament axis from the classical ones, giving rise to 3.5-start lateral monomer helices. The two protofilaments involved in these C- and D-type lateral interactions rotate substantially towards each other and engage weaker contacts than the A- and B-types. They allow microtubules to adopt protofilament skew angles intermediate to those observed in 3- and 4-start microtubules for a same protofilament number, consistent with a relaxation mechanism in response to high protofilament skew. They occur at a high frequency in the presence of the slowly hydrolysable GTP-analogues GDP-BeF3-, GTPgS, and GDP-AlFx, and appear at a low frequency with GMPCPP only at high tubulin concentration. They form at the onset of tubulin self-assembly in the presence of GTP to disappear rapidly at the beginning of the sigmoidal assembly phase, suggesting that they are not stable enough to remain trapped in the microtubule lattice in dynamic assembly conditions. By analogy with defects found in materials, these weaker interactions must impact microtubule's mechanical properties and their response to stress, and as a consequence their stability and dynamic properties.

Speaker : Leandro Estrozi, IBS, Grenoble

Title: Helical Symmetry Determination for 3D Cryo-Electron Microscopy

Abstract

Determining the symmetry of helical assemblies using 3D cryo-electron microscopy is a complex and challenging task. Despite involving only two real parameters - the helical twist angle and the axial rise distance - converging to an incorrect symmetry is a well-documented issue in the field. Experts generally agree that no fully satisfactory theoretical solution exists, leaving trial-and-error as the primary approach. However, correctly identifying the symmetry can significantly improve the resolution of 3D reconstructions.
In this webinar we will show how to use the online tool HELIXPLORER (http://rico.ibs.fr/helixplorer) to rationally and visually define a finite list of plausible symmetries or parameter ranges to be tested against cryo-EM data.

Please register here : https://us06web.zoom.us/webinar/register/WN_IH_Q_Kg5TXOhW085A0Ev0Q

Marie-Christine for the organizing committee