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1. Disassembly of bundled F-actin and cellular remodeling via an interplay of Mical, cofilin, and F-actin crosslinkers

   https://doi.org/10.1073/pnas.2309955120  

   Rajan, Sudeepa ; Yoon, Jimok ; Wu, Heng ; Srapyan, Sargis ; Baskar, Raju ; Ahmed, Giasuddin ; Yang, Taehong ; Grintsevich, Elena E. ; Reisler, Emil ; Terman, Jonathan R. ( September 2023 , Proceedings of the National Academy of Sciences)

   Cellular form and function are controlled by the assembly and stability of actin cytoskeletal structures—but disassembling/pruning these structures is equally essential for the plasticity and remodeling that underlie behavioral adaptations. Importantly, the mechanisms of actin assembly have been well-defined—including that it is driven by actin’s polymerization into filaments (F-actin) and then often bundling by crosslinking proteins into stable higher-order structures. In contrast, it remains less clear how these stable bundled F-actin structures are rapidly disassembled. We now uncover mechanisms that rapidly and extensively disassemble bundled F-actin. Using biochemical, structural, and imaging assays with purified proteins, we show that F-actin bundled with one of the most prominent crosslinkers, fascin, is extensively disassembled by Mical, the F-actin disassembly enzyme. Furthermore, the product of this Mical effect, Mical-oxidized actin, is poorly bundled by fascin, thereby further amplifying Mical’s disassembly effects on bundled F-actin. Moreover, another critical F-actin regulator, cofilin, also affects fascin-bundled filaments, but we find herein that it synergizes with Mical to dramatically amplify its disassembly of bundled F-actin compared to the sum of their individual effects. Genetic and high-resolution cellular assays reveal that Mical also counteracts crosslinking proteins/bundled F-actin in vivo to control cellular extension, axon guidance, and Semaphorin/Plexin cell-cell repulsion. Yet, our results also support the idea that fascin-bundling serves to dampen Mical’s F-actin disassembly in vitro and in vivo—and that physiologically relevant cellular remodeling requires a fine-tuned interplay between the factors that build bundled F-actin networks and those that disassemble them.

    

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2. Plakophilin-3 Binds the Membrane and Filamentous Actin without Bundling F-Actin

   https://doi.org/10.3390/ijms24119458  

   Gupta, Jyoti ; Rangarajan, Erumbi S. ; Troyanovsky, Regina B. ; Indra, Indrajyoti ; Troyanovsky, Sergey M. ; Izard, Tina ( June 2023 , International Journal of Molecular Sciences)

   Plakophilin-3 is a ubiquitously expressed protein found widely in epithelial cells and is a critical component of desmosomes. The plakophilin-3 carboxy-terminal domain harbors nine armadillo repeat motifs with largely unknown functions. Here, we report the 5 Å cryogenic electron microscopy (cryoEM) structure of the armadillo repeat motif domain of plakophilin-3, one of the smaller cryoEM structures reported to date. We find that this domain is a monomer or homodimer in solution. In addition, using an in vitro actin co-sedimentation assay, we show that the armadillo repeat domain of plakophilin-3 directly interacts with F-actin. This feature, through direct interactions with actin filaments, could be responsible for the observed association of extra-desmosomal plakophilin-3 with the actin cytoskeleton directly attached to the adherens junctions in A431 epithelial cells. Further, we demonstrate, through lipid binding analyses, that plakophilin-3 can effectively be recruited to the plasma membrane through phosphatidylinositol-4,5-bisphosphate-mediated interactions. Collectively, we report on novel properties of plakophilin-3, which may be conserved throughout the plakophilin protein family and may be behind the roles of these proteins in cell–cell adhesion. 

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3. Dissociation Mechanisms of G-actin Subunits Govern Deformation Response of Actin Filament

   https://doi.org/10.1021/acs.biomac.0c01602  

   Jaswandkar, Sharad V. ; Faisal, H M ; Katti, Kalpana S. ; Katti, Dinesh R. ( February 2021 , Biomacromolecules)

   null (Ed.)
4. Actin filament sliding in CaMKII-actin bundles

   https://doi.org/10.1016/j.bpj.2022.11.1051  

   Basurco, Carlos A. ; Schafer, Nicholas P. ; Waxham, Neal ; Cheung, Margaret S. ; Wolynes, Peter G. ( February 2023 , Biophysical Journal)
5. Plant KASH proteins SINE1 and SINE2 have synergistic and antagonistic interactions with actin branching and actin bundling factors.

   Moser, M ; Groves NR ; Meier I ( October 2023 , Journal of experimental botany)

   Linker of nucleoskeleton and cytoskeleton complexes consist of outer nuclear membrane KASH proteins, interacting in the nuclear envelope lumen with inner nuclear membrane SUN proteins and connecting nucleus and cytoskeleton. The paralogous Arabidopsis KASH proteins SINE1 and SINE2 function during stomatal dynamics induced by light-dark transitions and ABA, which requires F-actin reorganization. SINE2 influences actin depolymerization and SINE1 actin repolymerization. The actin-related protein 2/3 (ARP2/3) complex, an actin nucleator, and the plant actin bundling and stabilizing factor SCAB1 are involved in stomatal aperture control. Here, we have tested the genetic interaction of SINE1 and SINE2 with SCAB1 and the ARP2/3 complex. We show that SINE1 and the ARP2/3 complex function in the same pathway during ABA-induced stomatal closure, while SINE2 and the ARP2/3 complex play opposing roles. The actin repolymerization defect observed in sine1-1 is partially rescued in scab1-2 sine1-1, while SINE2 is epistatic to SCAB1. In addition, SINE1 and ARP2/3 act synergistically in lateral root development. Absence of SINE2 renders trichome development independent of the ARP2/3 complex. Together, these data reveal complex and differential interactions of the two KASH proteins with the actin-remodeling apparatus and add evidence to the proposed differential role of SINE1 and SINE2 in actin dynamics. 

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