Coordinated polymerization of actin filaments provides force for cell migration, morphogenesis, and endocytosis. Capping Protein (CP) is central regulator of actin dynamics in all eukaryotes. It binds actin filament (F-actin) barbed ends with high affinity and slow dissociation kinetics to prevent filament polymerization and depolymerization. In cells, however, CP displays remarkably rapid dynamics within F-actin networks, but the underlying mechanism has remained enigmatic. We report that a conserved cytoskeletal regulator, twinfilin, is responsible for CP's rapid dynamics and specific localization in cells. Depletion of twinfilin led to stable association of CP with cellular F-actin arrays and its treadmilling throughout leading-edge lamellipodium. These were accompanied by diminished F-actin disassembly rates. In vitro single filament imaging approaches revealed that twinfilin directly promotes dissociation of CP from filament barbed ends, while allowing subsequent filament depolymerization. These results uncover an evolutionary conserved bipartite mechanism that controls how actin cytoskeleton-mediated forces are generated in cells. Coordinated polymerization of actin filaments (F-actin) generatespushing force, which drives the protrusion of lamellipodia at the leading edge of migrating cells, and contributes to the formation of plasma membrane invaginations in endocytic processes [1][2][3][4] . A large array of actin-binding proteins regulates the dynamics and organization of actin filaments in these processes, but only few (<10) of them are conserved in evolution from protozoan parasites to animals 5 . Among these 'core' actin-binding proteins are the heterodimeric Capping Protein (CP) and twinfilin.Generation of membrane protrusions requires that a subset of actin filament barbed ends is capped to funnel the assemblycompetent actin monomers to a limited number of growing barbed ends [6][7][8] . CP is the most prominent actin filament barbed end capping protein in most organisms and cell types 9,10 . It is an essential component of in vitro reconstituted actin-based motility system 11 and actin-based processes in cells [12][13][14] . Moreover, its inhibition in animal non-muscle cells disturbs actin-based lamellipodial morphology, protrusion and cell migration [15][16][17][18] . In addition, CP plays an important role in controlling the length and the density of branches within actin filament networks nucleated by the Arp2/3 complex 19 .The activities of CP are controlled by several proteins 9,10 . V-1/ myotrophin binds and sequesters CP with nanomolar affinity and inhibits its capping activity [20][21][22][23] . Moreover, the capping protein interaction (CPI)-motif containing proteins, such as CARMILs (capping protein, Arp2/3 and myosin-I linker protein) 24 , interact with CP to reduce its affinity to both actin filament barbed ends [25][26][27] and to V-1 22 through an allosteric mechanism 25,28 . Depletion of CARMILs and other CPI motif containing proteins disrupts the subcellular localization of CP [29]...

Coordinated polymerization of actin filaments provides force for cell migration, morphogenesis, and endocytosis. Capping Protein (CP) is central regulator of actin dynamics in all eukaryotes. It binds actin filament (F-actin) barbed ends with high affinity and slow dissociation kinetics to prevent filament polymerization and depolymerization. In cells, however, CP displays remarkably rapid dynamics within F-actin networks, but the underlying mechanism has remained enigmatic. We report that a conserved cytoskeletal regulator, twinfilin, is responsible for CP's rapid dynamics and specific localization in cells. Depletion of twinfilin led to stable association of CP with cellular F-actin arrays and its treadmilling throughout leading-edge lamellipodium. These were accompanied by diminished F-actin disassembly rates. In vitro single filament imaging approaches revealed that twinfilin directly promotes dissociation of CP from filament barbed ends, while allowing subsequent filament depolymerization. These results uncover an evolutionary conserved bipartite mechanism that controls how actin cytoskeleton-mediated forces are generated in cells.Coordinated polymerization of actin filaments (F-actin) generates pushing force, which drives the protrusion of lamellipodia at the leading edge of migrating cells, and contributes to the formation of plasma membrane invaginations in endocytic processes 1-4 . A large array of actin-binding proteins regulates the dynamics and organization of actin filaments in these processes, but only few (<10) of them are conserved in evolution from protozoan parasites to animals 5 . Among these 'core' actin-binding proteins are the heterodimeric Capping Protein (CP) and twinfilin.Generation of membrane protrusions requires that a subset of actin filament barbed ends is capped to funnel the assemblycompetent actin monomers to a limited number of growing barbed ends 6-8 . CP is the most prominent actin filament barbed end capping protein in most organisms and cell types 9,10 . It is an essential component of in vitro reconstituted actin-based motility system 11 and actin-based processes in cells [12][13][14] . Moreover, its inhibition in animal non-muscle cells disturbs actin-based lamellipodial morphology, protrusion and cell migration [15][16][17][18] . In addition, CP plays an important role in controlling the length and the density of branches within actin filament networks nucleated by the Arp2/3 complex 19 .The activities of CP are controlled by several proteins 9,10 . V-1/ myotrophin binds and sequesters CP with nanomolar affinity and inhibits its capping activity 20-23 . Moreover, the capping protein interaction (CPI)-motif containing proteins, such as CARMILs (capping protein, Arp2/3 and myosin-I linker protein) 24 , interact with CP to reduce its affinity to both actin filament barbed ends [25][26][27] and to V-1 22 through an allosteric mechanism 25,28 . Depletion of CARMILs and other CPI motif containing proteins disrupts the subcellular localization of CP 29-33 . Thus, it was suggested...

During embryonic development, regeneration, and homeostasis, cells have to migrate and physically integrate into the target tissues where they ultimately execute their function. While much is known about the biochemical pathways driving cell migration in vivo, we are only beginning to understand the mechanical interplay between migrating cells and their surrounding tissue. Here, we reveal that multiciliated cell precursors in the Xenopus embryo use filopodia to pull at the vertices of the overlying epithelial sheet. This pulling is effectively used to sense vertex stiffness and identify the preferred positions for cell integration into the tissue. Notably, we find that pulling forces equip multiciliated cells with the ability to remodel the epithelial junctions of the neighboring cells, enabling them to generate a permissive environment that facilitates integration. Our findings reveal the intricate physical crosstalk at the cell-tissue interface and uncover previously unknown functions for mechanical forces in orchestrating cell integration.

During embryonic development, regeneration and homeostasis, cells have to physically integrate into their target tissues, where they ultimately execute their function. Despite a significant body of research on how mechanical forces instruct cellular behaviors within the plane of an epithelium, very little is known about the mechanical interplay at the interface between migrating cells and their surrounding tissue, which has its own dynamics, architecture and identity. Here, using quantitative in vivo imaging and molecular perturbations, together with a theoretical model, we reveal that multiciliated cell (MCC) precursors in the Xenopus embryo form dynamic filopodia that pull at the vertices of the overlying epithelial sheet to probe their stiffness and identify the preferred positions for their integration into the tissue. Moreover, we report a novel function for a structural component of vertices, the lipolysis-stimulated lipoprotein receptor (LSR), in filopodia dynamics and show its critical role in cell intercalation. Remarkably, we find that pulling forces equip the MCCs to remodel the epithelial junctions of the neighboring tissue, enabling them to generate a permissive environment for their integration. Our findings reveal the intricate physical crosstalk at the cell-tissue interface and uncover previously unknown functions for mechanical forces in orchestrating cell integration.