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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Strukturforschung / Elektronenmikroskopie

Humboldt-Universität zu Berlin | Faculty of Mathematics and Natural Sciences | Department of Physics | Strukturforschung / Elektronenmikroskopie | Publications | Template-free detection and classification of heterogeneous membrane-bound complexes in cryo-electron tomograms

Antonio Martinez-Sanchez, Zdravko Kochovski, Ulrike Laugks, Johannes Meyer zum Alten Borgloh, Stefan Pfeffer, Wolfgang Baumeister, and Vladan Lucic (2018)

Template-free detection and classification of heterogeneous membrane-bound complexes in cryo-electron tomograms

bioRxiv.

With faithful sample preservation and direct imaging of fully hydrated biological material, cryo-electron tomography (cryo-ET) provides an accurate representation of heterogeneous cellular constituents in their native environment. However, detection and precise localization of complexes within a cellular environment is aggravated by their large number and heterogeneous nature. We developed a template-free image processing procedure that allows an accurate tracing of complex networks of biological densities, as well as a comprehensive and automated detection, and an unsupervised classification of heterogeneous membrane-bound complexes in cryo-electron tomograms. Applying this procedure to tomograms of membrane vesicles from rough endoplasmic reticulum (ER), we detected and classified small protein complexes like the ER protein translocons, which were not detected by other methods. This classification provided sufficiently homogeneous particle sets for further processing by the currently available subtomogram averaging methods. Furthermore, we present structural evidence that different ribosome-free translocon species are present at the ER membrane, determine their 3D structures, and show that they have different localization patterns and form nanodomains.