Fusion proteins containing the Src homology (SH)3 domains of Dictyostelium myosin IB (myoB) and IC (myoC) bind a 116-kD protein (p116), plus nine other proteins identified as the seven member Arp2/3 complex, and the α and β subunits of capping protein. Immunoprecipitation reactions indicate that myoB and myoC form a complex with p116, Arp2/3, and capping protein in vivo, that the myosins bind to p116 through their SH3 domains, and that capping protein and the Arp2/3 complex in turn bind to p116. Cloning of p116 reveals a protein dominated by leucine-rich repeats and proline-rich sequences, and indicates that it is a homologue of Acan 125. Studies using p116 fusion proteins confirm the location of the myosin I SH3 domain binding site, implicate NH2-terminal sequences in binding capping protein, and show that a region containing a short sequence found in several G-actin binding proteins, as well as an acidic stretch, can activate Arp2/3-dependent actin nucleation. p116 localizes along with the Arp2/3 complex, myoB, and myoC in dynamic actin-rich cellular extensions, including the leading edge of cells undergoing chemotactic migration, and dorsal, cup-like, macropinocytic extensions. Cells lacking p116 exhibit a striking defect in the formation of these macropinocytic structures, a concomitant reduction in the rate of fluid phase pinocytosis, a significant decrease in the efficiency of chemotactic aggregation, and a decrease in cellular F-actin content. These results identify a complex that links key players in the nucleation and termination of actin filament assembly with a ubiquitous barbed end–directed motor, indicate that the protein responsible for the formation of this complex is physiologically important, and suggest that previously reported myosin I mutant phenotypes in Dictyostelium may be due, at least in part, to defects in the assembly state of actin. We propose that p116 and Acan 125, along with homologues identified in Caenorhabditis elegans, Drosophila, mouse, and man, be named CARMIL proteins, for capping protein, Arp2/3, and myosin I linker.

SUMMARY Non-muscle myosin II (NM II) powers myriad developmental and cellular processes, including embryogenesis, cell migration, and cytokinesis [1]. To exert its functions, monomers of NM II assemble into bipolar filaments that produce a contractile force on the actin cytoskeleton. Mammalian cells express up to three isoforms of NM II (NM IIA, IIB and IIC), each of which possesses distinct biophysical properties and supports unique, as well as redundant, cellular functions [2-8]. Despite previous efforts [9-13], it remains unclear if NM II isoforms assemble in living cells to produce mixed (heterotypic) bipolar filaments, or if filaments consist entirely of a single isoform (homotypic). We addressed this question using fluorescently-tagged versions of NM IIA, IIB and IIC, isoform-specific immunostaining of the endogenous proteins, and two-color total internal reflection fluorescence structured-illumination microscopy, or TIRF-SIM, to visualize individual myosin II bipolar filaments inside cells. We show that NM II isoforms co-assemble into heterotypic filaments in a variety of settings, including various types of stress fibers, individual filaments throughout the cell, and the contractile ring. We also show that the differential distribution of NM IIA and NM IIB typically seen in confocal micrographs of well-polarized cells is reflected in the composition of individual bipolar filaments. Interestingly, this differential distribution is less pronounced in freshly-spread cells, arguing for the existence of sorting mechanism acting over time. Together, our work argues that individual NM II isoforms are potentially performing both isoform-specific and isoform-redundant functions while co-assembled with other NM II isoforms.

Cellular mechanisms governing non-muscle myosin 2 (NM2) filament assembly are largely unknown. Using EGFP-NM2A knock-in fibroblasts and multiple super-resolution imaging modalities, we characterized and quantified the sequential amplification of NM2 filaments within lamella, wherein filaments emanating from single nucleation events continuously partition, forming filament clusters that populate large-scale actomyosin structures deeper in the cell. Individual partitioning events coincide spatially and temporally with the movements of diverging actin fibers, suppression of which inhibits partitioning. These and other data indicate that NM2A filaments are partitioned by the dynamic movements of actin fibers to which they are bound. Finally, we showed that partition frequency and filament growth rate in the lamella depend on MLCK, and that MLCK is competing with centrally-active ROCK for a limiting pool of monomer with which to drive lamellar filament assembly. Together, our results provide new insights into the mechanism and spatio-temporal regulation of NM2 filament assembly in cells.

Nonmusclemyosin 2 (NM-2) powers cell motility and tissue morphogenesis by assembling into bipolar filaments that interact with actin. Although the enzymatic properties of purified NM-2 motor fragments have been determined, the emergent properties of filament ensembles are unknown. Using single myosin filament in vitro motility assays, we report fundamental differences in filaments formed of different NM-2 motors. Filaments consisting of NM2-B moved processively along actin, while under identical conditions, NM2-A filaments did not. By more closely mimicking the physiological milieu, either by increasing solution viscosity or by co-polymerization with NM2-B, NM2-A containing filaments moved processively. Our data demonstrate that both the kinetic and mechanical properties of these two myosins, in addition to the stochiometry of NM-2 subunits, can tune filament mechanical output. We propose altering NM-2 filament composition is a general cellular strategy for tailoring force production of filaments to specific functions, such as maintaining tension or remodeling actin.

Although capping protein (CP) terminates actin filament elongation, it promotes Arp2/3-dependent actin network assembly and accelerates actin-based motility both in vitro and in vivo. In vitro, capping protein Arp2/3 myosin I linker (CARMIL) antagonizes CP by reducing its affinity for the barbed end and by uncapping CPcapped filaments, whereas the protein V-1/myotrophin sequesters CP in an inactive complex. Previous work showed that CARMIL can readily retrieve CP from the CP:V-1 complex, thereby converting inactive CP into a version with moderate affinity for the barbed end. Here we further clarify the mechanism of this exchange reaction, and we demonstrate that the CP:CARMIL complex created by complex exchange slows the rate of barbed-end elongation by rapidly associating with, and dissociating from, the barbed end. Importantly, the cellular concentrations of V-1 and CP determined here argue that most CP is sequestered by V-1 at steady state in vivo. Finally, we show that CARMIL is recruited to the plasma membrane and only at cell edges undergoing active protrusion. Assuming that CARMIL is active only at this location, our data argue that a large pool of freely diffusing, inactive CP (CP:V-1) feeds, via CARMIL-driven complex exchange, the formation of weakcapping complexes (CP:CARMIL) at the plasma membrane of protruding edges. In vivo, therefore, CARMIL should promote Arp2/3-dependent actin network assembly at the leading edge by promoting barbed-end capping there.cell migration | VASP

Programs exist for searching protein sequences for potential membrane-penetrating segments (hydrophobic regions) and for lipid-binding sites with highly defined tertiary structures, such as PH, FERM, C2, ENTH, and other domains. However, a rapidly growing number of membrane-associated proteins (including cytoskeletal proteins, kinases, GTP-binding proteins, and their effectors) bind lipids through less structured regions. Here, we describe the development and testing of a simple computer search program that identifies unstructured potential membrane-binding sites. Initially, we found that both basic and hydrophobic amino acids, irrespective of sequence, contribute to the binding to acidic phospholipid vesicles of synthetic peptides that correspond to the putative membrane-binding domains of Acanthamoeba class I myosins. Based on these results, we modified a hydrophobicity scale giving Arg-and Lyspositive, rather than negative, values. Using this basic and hydrophobic scale with a standard search algorithm, we successfully identified previously determined unstructured membranebinding sites in all 16 proteins tested. Importantly, basic and hydrophobic searches identified previously unknown potential membrane-binding sites in class I myosins, PAKs and CARMIL (capping protein, Arp2/3, myosin I linker; a membrane-associated cytoskeletal scaffold protein), and synthetic peptides and protein domains containing these newly identified sites bound to acidic phospholipids in vitro.Recently, there has been considerable interest in characterizing protein domains, such as PH, FERM, C2, and ENTH, that are responsible for specific binding to membrane lipids (for review, see Ref. 1). These domains have highly defined tertiary structures comprising ␣-helices, ␤-sheets and loops, and there are multiple programs for recognizing them in protein sequences. However, a growing number of membrane-binding proteins, including cytoskeletal proteins (2), GTP-binding proteins (e.g. Rit), and GTP-binding protein effectors (e.g. some PAKs) (see references below), do not fit within these categories and bind membrane lipids through much less structured regions. Here, we describe the development and testing of a simple computer search program that identifies such potential membrane-binding sites. This novel search program evolved from our previous effort (3) to identify the membrane-binding site in the heavy chain of Acanthamoeba myosin IC (AMIC) 2 (4), which was known to bind to acidic phospholipids (3, 5, 6) and cell membranes (7-10) through its 220-residue basic region (5).We had found (3) that AMIC binds nonspecifically to acidic phospholipid vesicles in proportion to their negative charge. Prompted by the report (11) that several proteins bind to acidic phospholipids through a basic-hydrophobic-basic (BHB) region consisting of two small clusters of basic amino acids separated by hydrophobic residues, we identified, by visual inspection, a 13-residue BHB sequence, KVKPFLYVLKRR, within the basic region of the AMIC heavy chain (3). A synthetic ...

It has been brought to our attention that the version of Movie S2 that originally accompanied this article was not of sufficient quality. The purpose of Movie S2 is to demonstrate that the actomyosin II contractile ring of LLC-Pk1 cells can be imaged using TIRF-SIM. Movie S2 has been replaced online with an updated version that is the exact same dataset stored at higher resolution. The authors apologize for any inconvenience.

Bulk solution assays have shown that the isolated CARMIL homology 3 (CAH3) domain from mouse and Acanthamoeba CARMIL rapidly and potently restores actin polymerization when added to actin filaments previously capped with capping protein (CP). To demonstrate this putative uncapping activity directly, we used total internal reflection microscopy to observe single, CP-capped actin filaments before and after the addition of the CAH3 domain from mouse CARMIL-1 (mCAH3). The addition of mCAH3 rapidly restored the polymerization of individual capped filaments, consistent with uncapping. To verify uncapping, filaments were capped with recombinant mouse CP tagged with monomeric green fluorescent protein (mGFP-CP). Restoration of polymerization upon the addition of mCAH3 was immediately preceded by the complete dissociation of mGFP-CP from the filament end, confirming the CAH3-driven uncapping mechanism. Quantitative analyses showed that the percentage of capped filaments that uncapped increased as the concentration of mCAH3 was increased, reaching a maximum of ϳ90% at ϳ250 nM mCAH3. Moreover, the time interval between mCAH3 addition and uncapping decreased as the concentration of mCAH3 increased, with the half-time of CP at the barbed end decreasing from ϳ30 min without mCAH3 to ϳ10 s with a saturating amount of mCAH3. Finally, using mCAH3 tagged with mGFP, we obtained direct evidence that the complex of CP and mCAH3 has a small but measurable affinity for the barbed end, as inferred from previous studies and kinetic modeling. We conclude that the isolated CAH3 domain of CARMIL (and presumably the intact molecule as well) possesses the ability to uncap CP-capped actin filaments. Capping protein (CP)2 is a highly conserved, ubiquitously expressed, heterodimeric actin-binding protein that plays a major role in limiting the duration of actin filament elongation within cells (1, 2). CP accomplishes this task by binding with high affinity (K d ϳ 0.1 nM) to the barbed end of the actin filament, thereby blocking the further association (and dissociation) of actin monomers at the fast growing end. CP is one of five proteins that are required for the reconstitution of actin polymerization-driven motility in vitro (3) Moreover, biochemical, cell biological, and modeling studies all suggest that rapid filament capping by CP in vivo is required in order to generate an Arp2/3-dependent dendritic actin network at the leading edge that is sufficiently branched to push the cell forward (4 -7). Consistent with these studies, cells that lack CP or in which CP levels have been reduced exhibit profound alterations in the assembly of their actin cytoskeleton (8 -12). Together, these observations highlight the necessity of identifying physiological regulators of CP function. A second compelling reason to search for such regulators stems from the very long half-time of CP at the barbed end in vitro (ϳ30 min for vertebrate CP) (13,14), because this half-time seems incompatible with the dynamics of actin in vivo, where large regions of F-actin ca...