Bending actin filaments: twists of fate

Abstract

In many cellular contexts, intracellular actomyosin networks must generate directional forces to carry out cellular tasks such as migration and endocytosis, which play important roles during normal developmental processes. A number of different actin binding proteins have been identified that form linear or branched actin, and that regulate these filaments through activities such as bundling, crosslinking, and depolymerization to create a wide variety of functional actin assemblies. The helical nature of actin filaments allows them to better accommodate tensile stresses by untwisting, as well as to bend to great curvatures without breaking. Interestingly, this latter property, the bending of actin filaments, is emerging as an exciting new feature for determining dynamic actin configurations and functions. Indeed, recent studies using in vitro assays have found that proteins including IQGAP, Cofilin, Septins, Anillin, α-Actinin, Fascin, and Myosins-alone or in combination-can influence the bending or curvature of actin filaments. This bending increases the number and types of dynamic assemblies that can be generated, as well as the spectrum of their functions. Intriguingly, in some cases, actin bending creates directionality within a cell, resulting in a chiral cell shape. This actin-dependent cell chirality is highly conserved in vertebrates and invertebrates and is essential for cell migration and breaking L-R symmetry of tissues/organs. Here, we review how different types of actin binding protein can bend actin filaments, induce curved filament geometries, and how they impact on cellular functions.

Keywords: Anillin; Cofilin; Cytoskeleton; Fascin; IQGAP; L-R asymmetry; Septins; actin; actin bending; actin helicity; chirality; unconventional myosins; α-Actinin.

Figure 1.. Actin filament formation and helicity.

(A) Schematic diagram showing de novo linear actin filament formation through nucleation promoting factors such as Formins and Spire. Formins act via actin dimer stabilization and processive movement with the elongating filament (fast-growing) barbed end, whereas Spire forms a prenucleation complex containing up to four actin monomers. Actin crosslinkers bind to the sides of actin filaments to aid in the formation of filament bundles. Adapted from Hui et al. (2022), Cells 11(18): 2777; https://doi.org/10.3390/cells11182777. (B) Schematic diagram showing de novo branched actin filament formation via the Arp2/3 complex and Wiskott-Aldrich Syndrome (WAS) family proteins. Arp2/3 binds to the sides of preexisting linear actin filaments and nucleates branched actin networks upon activation by WAS family proteins. Adapted from Hui et al. (2022), Cells 11(18): 2777; https://doi.org/10.3390/cells11182777. (C) Actin filaments are helical. Schematic diagram of an actin filament that is generated from actin monomers (spheres with black dots showing orientation of monomers). The double helix actin filament has a pitch of 72nm. The filament can be seen as either a left-handed, one-start, single helix (orange line) or a right-handed, two-start, double helix (red line). Reprinted from Jegou et al. (2020), Semin. Cell Dev. Biol. 102:65-72. doi: 10.1016/j.semcdb.2019.10.018, with permission from Elsevier.

Figure 2.. Actin bending by different types of actin binding proteins.

(A) Actin filaments polymerized by formin bend in the presence of the curly region (1-189 aa) of IQGAP. Scale bar: 1 μm. Reprinted from Palani et al. (2021), eLife 10:e61078. doi: 10.7554/eLife.61078. (B) Single slice image of Primary E18 rat hippocampal neurons labeled with actin (green) and Cofilin (red)(a). Maximum intensity projections of lower and upper dashed square areas in a, respectively (b-c). Time-lapse images showing different types of actin conformation (Kink, Wave, and Bend) in a (d-f). Scale bars: 5 μm (a and b), 500 nm (d). Reprinted from Hylton et al. (2022). Nat Commun. 13(1):2439. doi: 10.1038/s41467-022-30116-x. (C) in vitro polymerized F-actin co-incubated with no protein, Drosophila Septins, human Septins, or Fascin. Both Drosophila and human Septins bundle and bend F-actin. Scale bars: 5 μm. Reprinted from Mavrakis et al. (2014). Nat. Cell Biol. 16(4):322–34. doi: 10.1038/ncb2921, with permission from Springer Nature. (D) Schematic diagram showing TIRF microscopy for F-actin and Anillin imaging (a). Time-lapse images showing actin ring contraction by Anillin (b). Reprinted from Kučera et al. (2021). Nat. Commun. 12(1):4595. doi: 10.1038/s41467-021-24474-1. (E-F) Schematic diagram showing spaces and shapes of bundled actin by Fascin and α-Actinin (E) and in vitro polymerized F-actin co-incubate with Fascin and α-Actinin showing that Fascin and α-Actinin localization is segregated (F). Reprinted from Winkelman et al. (2016) Curr. Biol. 26(20):2697–2706. doi: 10.1016/j.cub.2016.07.080, with permission from Elsevier. (G) Time-lapse images for F-actin buckling. Reprinted from Murrell et al. (2012), Proc. Natl. Acad. Sci. USA 109(51): 20820–20825. doi: 10.1073.pnas, with permission from PNAS. (H) Schematic diagram showing F-actin and membrane-tethered myosins in the myosin gliding assays (a). Maximum intensity projections from 10 min (b) and 20 min (c) time-lapse movies. Green and orange actin filaments show later and initial time points, respectively. Myo1C induces anti-clockwise actin filament swirling, but Myo1A does not. Scale bar: 5 μm. Reprinted from Pyrpassopoulos et al. (2012) Curr. Biol. 22(18):1688–92. doi: 10.1016/j.cub.2012.06.069, with permission from Elsevier.