Self-organization of actin networks by a monomeric myosin

Abstract

The organization of actomyosin networks lies at the center of many types of cellular motility, including cell polarization and collective cell migration during development and morphogenesis. Myosin-IXa is critically involved in these processes. Using total internal reflection fluorescence microscopy, we resolved actin bundles assembled by myosin-IXa. Electron microscopic data revealed that the bundles consisted of highly ordered lattices with parallel actin polarity. The myosin-IXa motor domains aligned across the network, forming crosslinks at a repeat distance of precisely 36 nm, matching the helical repeat of actin. Single-particle image processing resolved three distinct conformations of myosin-IXa in the absence of nucleotide. Using crosscorrelation of a modeled actomyosin crystal structure, we identified sites of additional mass, which can only be accounted for by the large insert in loop 2 exclusively found in the motor domain of class IX myosins. We show that the large insert in loop 2 binds calmodulin and creates two coordinated actin-binding sites that constrain the actomyosin interactions generating the actin lattices. The actin lattices introduce orientated tracks at specific sites in the cell, which might install platforms allowing Rho-GTPase-activating protein (RhoGAP) activity to be focused at a definite locus. In addition, the lattices might introduce a myosin-related, force-sensing mechanism into the cytoskeleton in cell polarization and collective cell migration.electron microscopy | unconventional myosin | actin network T he actin-based cytoskeleton, composed of actin filaments and actin-binding partners, including a large variety of myosin motor proteins, is responsible for highly diverse forms of cellular motility. The actomyosin cytoskeleton continuously undergoes major structural reorganizations in the lamellipodium of migrating cells and during polarization in epithelium cells in morphogenesis (1-4). The molecular mechanical mechanisms of these local actomyosin networks, however, are still largely unknown.Myosin class IX, a monomeric myosin with two mammalian isoforms, has been shown to play an important role in these processes. Whereas myosin-IXb is found in migrating cells of the immune system, myosin-IXa is abundantly expressed in the brain and testis and at lower levels in the kidney, adrenal gland, lung, and spleen and has been shown to play a critical role in epithelial differentiation and morphology (5-8). The N-terminal catalytic domain of myosin-IX binds actin and hydrolyses ATP, whereas the following light-chain binding-neck region acts as a lever arm to amplify the small conformational changes in the catalytic motor domain into nanometer displacements at the end of the lever. The C-terminal tail domain comprises one or two C1 zinc-binding domains and a Rho-GTPase-activating protein (RhoGAP) domain that inactivates the small GTPase Rho (5). Interestingly, the motor head of class IX myosins features a unique domain not found in any other class of myosin, namely a very large inser...

Periodic propagating waves coordinate RhoGTPase network dynamics at the leading and trailing edges during cell migration

“…Here, we present a core model of the signaling RhoA – Rac1 system, which captures the formation of RhoA-Rac1 periodic propagating waves that coordinate different signaling dynamics at the cell trailing and leading edges. In our core network, intertwined regulatory connections from RhoGTPase effectors to the GEFs and GAPs can be induced not only by phosphorylation or the formation of protein complexes but can also be mediated by cytoskeletal proteins ( Banerjee and Wedegaertner, 2004 ; Lovelace et al, 2017 ; Mitin et al, 2012 ; Ren et al, 1998 ; Saczko-Brack et al, 2016 ). We hypothesize that crosstalk interactions of this core signaling network with cytoskeleton proteins generate actomyosin waves and the cytoskeletal dynamics required for cell migration ( Saha et al, 2018 ).…”

Modulation of Agrin and RhoA Pathways Ameliorates Movement Defects and Synapse Morphology in MYO9A-Depleted Zebrafish

“…There may be a requirement for fine modulation of dosage for fasudil throughout development due to its widespread action and the likely differing contribution of the RhoA/ROCK pathway at different developmental stages. It could also be that effects of Myo9aa/ab at the NMJ are not mediated by the ROCK pathway as development proceeds, and instead are linked to other functions of this protein such as cross-linking actin filaments, or as yet unidentified interactions with other pathways of impact on the NMJ [36]. Nevertheless, the movement benefits due to fasudil treatment suggest there may be modest improvements in release/functionality at the NMJ that are not detectable by the morphological changes measured here.…”

A Salutary Role of Reactive Oxygen Species in Intercellular Tunnel-Mediated Communication

“…In this case, ERK promotes the lamellipodia protrusion by directly phosphorylating the WAVE2 Regulatory Complex (WRC), which then activates the Arp2/3 complex for actin assembly together with activated WAVE2 and Abi1 (activation by the ERK phosphorylation as well) (Mendoza et al, 2011 ). The mobilizing force to drive actin network remodeling may come from the myosin motor (Nambiar et al, 2010 ) that has been widely shown in the recent reports (Bishai et al, 2013 ; Lou et al, 2015 ; Yochelis et al, 2015 ; Saczko-Brack et al, 2016 ), so what factor, if there is any, is directing the force to this process? Interestingly, a recent report has pointed out that the ERK signaling could provide a clue in which ERK promotes lamellipodia protrusion by lifting the sequestration of myosin 1E by SH3P2 via phosphorylating its Ser 202, thus resulting in myosin-actin association at the leading edge of the human MKN1 tumor cell (Tanimura et al, 2016 ; Tanimura and Takeda, 2017 ).…”