An atomic model for the human septin hexamer by cryo-EM. Issue 15 (23rd July 2021)

• Record Type: Journal Article
• Authors: Mendonça, Deborah C.; Guimarães, Samuel L.; Pereira, Humberto D'Muniz; Pinto, Andressa A.; de Farias, Marcelo A.; de Godoy, Andre S.; Araujo, Ana P.U.; van Heel, Marin; Portugal, Rodrigo V.; Garratt, Richard C.
• Is Part Of: Journal of molecular biology. Volume 433:Issue 15(2021)
• Publication Date: 2021-07-23
• Subjects: septins -- hetero-oligomer complex -- protein–protein interfaces -- flexibility -- cryo-electron microscopy, local resolution; Molecular biology -- Periodicals; Biology -- Periodicals; Biochemistry -- Periodicals; Bacteriology -- Periodicals; Molecular Biology -- Periodicals; Biochemistry -- Periodicals; Biologie moléculaire -- Périodiques; Biologie -- Périodiques; Biochimie -- Périodiques; Moleculaire biologie; Biochemistry; Biology; Molecular biology; Periodicals; 572.805
• DOI: 10.1016/j.jmb.2021.167096
• Languages: English
• ISSNs: 0022-2836

Abstract

Graphical abstract: Highlights: We provide a first cryo-EM structure of a human septin oligomer as a free particle. The NC-interface between SEPT6 and SEPT7 houses a large internal cavity. The α0 helix is an integral part of this interface and sequesters a polybasic region. The basic residues of α0 form specific interactions with the neighbouring subunit. Flexibility of the particle reveals a preferential bending direction. Abstract: In order to form functional filaments, human septins must assemble into hetero-oligomeric rod-like particles which polymerize end-to-end. The rules governing the assembly of these particles and the subsequent filaments are incompletely understood. Although crystallographic approaches have been successful in studying the separate components of the system, there has been difficulty in obtaining high resolution structures of the full particle. Here we report a first cryo-EM structure for a hexameric rod composed of human septins 2, 6 and 7 with a global resolution of ~3.6 Å and a local resolution of between ~3.0 Å and ~5.0 Å. By fitting the previously determined high-resolution crystal structures of the component subunits into the cryo-EM map, we are able to provide an essentially complete model for the particle. This exposes SEPT2 NC-interfaces at the termini of the hexamer and leaves internal cavities between the SEPT6-SEPT7 pairs. The floor of the cavity is formed by the two α0 helices including their polybasic regions. These are locked into place between the two subunits by interactions made with the α5 and α6 helices of the neighbouring monomer together with its polyacidic region. The cavity may serve to provide space allowing the subunits to move with respect to one another. The elongated particle shows a tendency to bend at its centre where two copies of SEPT7 form a homodimeric G-interface. Such bending is almost certainly related to the ability of septin filaments to recognize and even induce membrane curvature.