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Mol Biol Cell. 2014 Apr;25(7):1111-26. doi: 10.1091/mbc.E13-12-0712. Epub 2014 Feb 12.

ER sheet persistence is coupled to myosin 1c-regulated dynamic actin filament arrays.

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Cell and Molecular Biology Program, Institute of Biotechnology, 00014 University of Helsinki, Helsinki, Finland Electron Microscopy Unit, Institute of Biotechnology, 00014 University of Helsinki, Helsinki, Finland Faculty of Life Sciences, University of Manchester, Manchester M13 9PL, United Kingdom.


The endoplasmic reticulum (ER) comprises a dynamic three-dimensional (3D) network with diverse structural and functional domains. Proper ER operation requires an intricate balance within and between dynamics, morphology, and functions, but how these processes are coupled in cells has been unclear. Using live-cell imaging and 3D electron microscopy, we identify a specific subset of actin filaments localizing to polygons defined by ER sheets and tubules and describe a role for these actin arrays in ER sheet persistence and, thereby, in maintenance of the characteristic network architecture by showing that actin depolymerization leads to increased sheet fluctuation and transformations and results in small and less abundant sheet remnants and a defective ER network distribution. Furthermore, we identify myosin 1c localizing to the ER-associated actin filament arrays and reveal a novel role for myosin 1c in regulating these actin structures, as myosin 1c manipulations lead to loss of the actin filaments and to similar ER phenotype as observed after actin depolymerization. We propose that ER-associated actin filaments have a role in ER sheet persistence regulation and thus support the maintenance of sheets as a stationary subdomain of the dynamic ER network.

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