Contractility kits promote assembly of the mechanoresponsive cytoskeletal network

Abstract

Cellular contractility is governed by a control system of proteins that integrates internal and external cues to drive diverse shape change processes. This contractility controller includes myosin II motors, actin crosslinkers and protein scaffolds, which exhibit robust and cooperative mechanoaccumulation. However, the biochemical interactions and feedback mechanisms that drive the controller remain unknown. Here, we use a proteomics approach to identify direct interactors of two key nodes of the contractility controller in the social amoeba Dictyostelium discoideum: the actin crosslinker cortexillin I and the scaffolding protein IQGAP2. We highlight several unexpected proteins that suggest feedback from metabolic and RNA-binding proteins on the contractility controller. Quantitative in vivo biochemical measurements reveal direct interactions between myosin II and cortexillin I, which form the core mechanosensor. Furthermore, IQGAP1 negatively regulates mechanoresponsiveness by competing with IQGAP2 for binding the myosin II-cortexillin I complex. These myosin II-cortexillin I-IQGAP2 complexes are pre-assembled into higher-order mechanoresponsive contractility kits (MCKs) that are poised to integrate into the cortex upon diffusional encounter coincident with mechanical inputs. This article has an associated First Person interview with the first author of the paper.

Interplay between myosin II and actin dynamics in chemotactic amoeba

“…Further experiments with cells that are simultaneously labeled for MHCKA and F-actin should shed more light on the factors determining the delay time τ AM in the model. Besides MHCKA, so called contractility kits composed of cortexillinI and the scaffolding protein IQGAP2 that are preassembled in the cytosol of D. discoideumand regulate myosin activity [68], might be relevant for the first, rapid response of the actomyosin machinery to stimulation, as the quick reshuffling within the first 10s upon stimulation (i.e. between phases 1 and 2), which we have identified.…”

How the mechanobiome drives cell behavior, viewed through the lens of control theory

“…IQGAP1 binds cortexillin I with a 2:1 stoichiometry, while IQGAP2 only interacts at a 1:1 stoichiometry with cortexillin I. Thus, IQGAP1 may sequester cortexillin I, and likely myosin II, preventing their mechanoresponsiveness, and IQGAP2 then competes off IQGAP1, freeing cortexillin I and myosin II (Kee et al, 2012;Srivastava and Robinson, 2015;Kothari et al, 2019). This crossantagonism and feedback between the signaling module, the IQGAP proteins and the contractile machinery regulates the setpoint of the control system.…”

“…More recently, control theory has been used to understand various biological processes (see Iglesias and Ingalls, 2010 and the references therein). Proteins are modeled after existing PDB structures and reflect approximate concentrations, numbers, and lengths of crosslinkers and filaments from Dictyostelium (Condeelis et al, 1984;Goldmann and Isenberg, 1993;Faix et al, 1996;Mahajan and Pardee, 1996;Reichl et al, 2008;Robinson et al, 2012;Luo et al, 2013;Kothari et al, 2019). Scale bar: 100 nm.…”

“…In this system, myosin II and cortexillin I (the contractile machinery) serve as both sensors and actuators. Feedback is mediated by IQGAP2, which antagonizes the binding between IQGAP1 and the contractile machinery and allows for the formation of mechanoresponsive contractility kits (Kothari et al, 2019). (D) Schematic illustration of myosin II assembly from a monomer to a bipolar thick filament (BTF).…”

“…Cells have evolved to respond to mechanical stimuli, including stretch, compression and shear forces, as well as internally generated tension and strain. We define the mechanobiome as the set of macromolecules that allows cells to generate, sense and respond to such forces Kothari et al, 2019). The mechanobiome incorporates many components of the cytoskeleton, including myosin II motor proteins, actin crosslinkers, scaffolding proteins, actin polymerization and depolymerization factors, small GTPases and other cytoskeletal polymer systems (Wu et al, 2008;Robinson et al, 2012;Blanchoin et al, 2014;Zaidel-Bar et al, 2015;Chugh and Paluch, 2018;Dogterom and Koenderink, 2019).…”