scholarly journals The N-terminal tropomyosin- and actin-binding sites are important for leiomodin 2’s function

2016 ◽  
Vol 27 (16) ◽  
pp. 2565-2575 ◽  
Author(s):  
Thu Ly ◽  
Natalia Moroz ◽  
Christopher T. Pappas ◽  
Stefanie M. Novak ◽  
Dmitri Tolkatchev ◽  
...  
Keyword(s):  
Binding Sites ◽  
Actin Polymerization ◽  
Nemaline Myopathy ◽  
Actin Binding ◽  
Thin Filaments ◽  
Capping Protein ◽  
New Findings ◽  
Actin Nucleator ◽  
Pointed End ◽  
First Time

Leiomodin is a potent actin nucleator related to tropomodulin, a capping protein localized at the pointed end of the thin filaments. Mutations in leiomodin-3 are associated with lethal nemaline myopathy in humans, and leiomodin-2–knockout mice present with dilated cardiomyopathy. The arrangement of the N-terminal actin- and tropomyosin-binding sites in leiomodin is contradictory and functionally not well understood. Using one-dimensional nuclear magnetic resonance and the pointed-end actin polymerization assay, we find that leiomodin-2, a major cardiac isoform, has an N-terminal actin-binding site located within residues 43–90. Moreover, for the first time, we obtain evidence that there are additional interactions with actin within residues 124–201. Here we establish that leiomodin interacts with only one tropomyosin molecule, and this is the only site of interaction between leiomodin and tropomyosin. Introduction of mutations in both actin- and tropomyosin-binding sites of leiomodin affected its localization at the pointed ends of the thin filaments in cardiomyocytes. On the basis of our new findings, we propose a model in which leiomodin regulates actin poly­merization dynamics in myocytes by acting as a leaky cap at thin filament pointed ends.

scholarly journals Rickettsia Sca2 Recruits Two Actin Subunits for Nucleation but Lacks WH2 Domains

2018 ◽  
Author(s):  
SS Alqassim ◽  
IG Lee ◽  
R Dominguez
Keyword(s):  
Amino Acid ◽  
Actin Filament ◽  
Actin Polymerization ◽  
Intramolecular Interactions ◽  
Actin Binding ◽  
Globular Domain ◽  
Comet Tail ◽  
Capping Protein ◽  
High Affinity ◽  
Actin Nucleator

AbstractThe Rickettsia ~1,800 amino acid autotransporter protein Sca2 promotes actin polymerization on the surface of the bacterium to drive its movement using an actin comet tail mechanism. Sca2 mimics eukaryotic formins in that it promotes both actin filament nucleation and elongation and competes with capping protein to generate filaments that are long and unbranched. However, despite these functional similarities, Sca2 is structurally unrelated to eukaryotic formins and achieves these functions through an entirely different mechanism. Thus, while formins are dimeric, Sca2 functions as a monomer. However, Sca2 displays intramolecular interactions and functional cooperativity between its N- and C-terminal domains that are crucial for actin nucleation and elongation. Here, we map the interaction of N- and C-terminal fragments of Sca2 and their contributions to actin binding and nucleation. We find that both the N- and C-terminal regions of Sca2 interact with actin monomers, but only weakly, whereas the full-length protein binds two actin monomers with high affinity. Moreover, deletions at both ends of the N- and C-terminal regions disrupt their ability to interact with each other, suggesting that they form a contiguous ring-like structure that wraps around two actin subunits, analogous to the formin homology-2 (FH2) domain. The discovery of Sca2 as an actin nucleator followed the identification of what appeared to be a repeat of three WH2 domains in the middle of the molecule, consistent with the presence of WH2 domains in most actin nucleators. However, we show here that contrary to previous assumptions Sca2 does not contain WH2 domains, and that the corresponding region is folded as a globular domain that cooperates with other parts of the Sca2 molecule for actin binding and nucleation.

scholarly journals Tropomodulin caps the pointed ends of actin filaments.

1994 ◽  
Vol 127 (6) ◽  
pp. 1627-1635 ◽  
Author(s):  
A Weber ◽  
C R Pennise ◽  
G G Babcock ◽  
V M Fowler
Keyword(s):  
Red Blood Cells ◽  
Blood Cells ◽  
Actin Filaments ◽  
Striated Muscle ◽  
Critical Concentration ◽  
Thin Filaments ◽  
Capping Protein ◽  
End Capping ◽  
Pointed End ◽  
A Site

Many proteins have been shown to cap the fast growing (barbed) ends of actin filaments, but none have been shown to block elongation and depolymerization at the slow growing (pointed) filament ends. Tropomodulin is a tropomyosin-binding protein originally isolated from red blood cells that has been localized by immunofluorescence staining to a site at or near the pointed ends of skeletal muscle thin filaments (Fowler, V. M., M. A., Sussman, P. G. Miller, B. E. Flucher, and M. P. Daniels. 1993. J. Cell Biol. 120: 411-420). Our experiments demonstrate that tropomodulin in conjunction with tropomyosin is a pointed end capping protein: it completely blocks both elongation and depolymerization at the pointed ends of tropomyosin-containing actin filaments in concentrations stoichiometric to the concentration of filament ends (Kd < or = 1 nM). In the absence of tropomyosin, tropomodulin acts as a "leaky" cap, partially inhibiting elongation and depolymerization at the pointed filament ends (Kd for inhibition of elongation = 0.1-0.4 microM). Thus, tropomodulin can bind directly to actin at the pointed filament end. Tropomodulin also doubles the critical concentration at the pointed ends of pure actin filaments without affecting either the rate of extent of polymerization at the barbed filament ends, indicating that tropomodulin does not sequester actin monomers. Our experiments provide direct biochemical evidence that tropomodulin binds to both the terminal tropomyosin and actin molecules at the pointed filament end, and is the long sought-after pointed end capping protein. We propose that tropomodulin plays a role in maintaining the narrow length distributions of the stable, tropomyosin-containing actin filaments in striated muscle and in red blood cells.

scholarly journals Regulation of the formation of osteoclastic actin rings by proline-rich tyrosine kinase 2 interacting with gelsolin

2003 ◽  
Vol 160 (4) ◽  
pp. 565-575 ◽  
Author(s):  
Qiang Wang ◽  
Yi Xie ◽  
Quan-Sheng Du ◽  
Xiao-Jun Wu ◽  
Xu Feng ◽  
...  
Keyword(s):  
Cell Adhesion ◽  
Tyrosine Kinase ◽  
Actin Polymerization ◽  
Actin Binding ◽  
Cell Periphery ◽  
Cytoskeletal Organization ◽  
Capping Protein ◽  
Tyrosine Kinase 2 ◽  
Multiple Cells ◽  
Actin Rings

Osteoclast activation is important for bone remodeling and is altered in multiple bone disorders. This process requires cell adhesion and extensive actin cytoskeletal reorganization. Proline-rich tyrosine kinase 2 (PYK2), a major cell adhesion–activated tyrosine kinase in osteoclasts, plays an important role in regulating this event. The mechanisms by which PYK2 regulates actin cytoskeletal organization and osteoclastic activation remain largely unknown. In this paper, we provide evidence that PYK2 directly interacts with gelsolin, an actin binding, severing, and capping protein essential for osteoclastic actin cytoskeletal organization. The interaction is mediated via the focal adhesion–targeting domain of PYK2 and an LD motif in gelsolin's COOH terminus. PYK2 phosphorylates gelsolin at tyrosine residues and regulates gelsolin bioactivity, including decreasing gelsolin binding to actin monomer and increasing gelsolin binding to phosphatidylinositol lipids. In addition, PYK2 increases actin polymerization at the fibroblastic cell periphery. Finally, PYK2 interacts with gelsolin in osteoclasts, where PYK2 activation is required for the formation of actin rings. Together, our results suggest that PYK2 is a regulator of gelsolin, revealing a novel PYK2–gelsolin pathway in regulating actin cytoskeletal organization in multiple cells, including osteoclasts.

scholarly journals The nebulin repeat protein Lasp regulates I-band architecture and filament spacing in myofibrils

2014 ◽  
Vol 206 (4) ◽  
pp. 559-572 ◽  
Author(s):  
Isabelle Fernandes ◽  
Frieder Schöck
Keyword(s):  
Thin Filament ◽  
Muscle Protein ◽  
Nemaline Myopathy ◽  
Single Amino Acid ◽  
Actin Binding ◽  
Filament Length ◽  
Thin Filaments ◽  
Single Amino Acid Change ◽  
Filament Spacing

Mutations in nebulin, a giant muscle protein with 185 actin-binding nebulin repeats, are the major cause of nemaline myopathy in humans. Nebulin sets actin thin filament length in sarcomeres, potentially by stabilizing thin filaments in the I-band, where nebulin and thin filaments coalign. However, the precise role of nebulin in setting thin filament length and its other functions in regulating power output are unknown. Here, we show that Lasp, the only member of the nebulin family in Drosophila melanogaster, acts at two distinct sites in the sarcomere and controls thin filament length with just two nebulin repeats. We found that Lasp localizes to the Z-disc edges to control I-band architecture and also localizes at the A-band, where it interacts with both actin and myosin to set proper filament spacing. Furthermore, introducing a single amino acid change into the two nebulin repeats of Lasp demonstrated different roles for each domain and established Lasp as a suitable system for studying nebulin repeat function.

scholarly journals Mapping the binding site of thymosin β4 on actin by competition with G-actin binding proteins indicates negative co-operativity between binding sites located on opposite subdomains of actin

1997 ◽  
Vol 327 (3) ◽  
pp. 787-793 ◽  
Author(s):  
Edda BALLWEBER ◽  
Ewald HANNAPPEL ◽  
Thomas HUFF ◽  
Hans Georg MANNHERZ
Keyword(s):  
Binding Site ◽  
Binding Sites ◽  
Ternary Complex ◽  
Actin Polymerization ◽  
Binding Proteins ◽  
Critical Concentration ◽  
Dnase I ◽  
Actin Binding ◽  
Thymosin Β4 ◽  
Actin Binding Proteins

The β-thymosins are small monomeric (G-)actin-binding proteins of 5 kDa that are supposed to act intracellularly as actin-sequestering factors stabilizing the cytoplasmic monomeric pool of actin. The binding region of thymosin β4 was determined by analysing the binding of thymosin β4 to actin complexed with DNase I, gelsolin or gelsolin segment 1. Binding was analysed by determining the increase in the critical concentration of actin polymerization by native gel electrophoresis or chemical cross-linking. The formation of a ternary complex including thymosin β4 should indicate that the actin-binding proteins attach to different sites on actin. Competition would be indicative of binding to identical or overlapping sites on actin or of a negative co-operative linkage between the two binding sites. Competition of thymosin β4 for actin binding was observed in the presence of intact gelsolin or the N-terminal gelsolin fragment, segment 1, indicating that thymosin β4 binds to a site close to or identical with the gelsolin segment 1-binding site. The ternary complex of actin-DNase I-thymosin β4 was obtained only when using the chemically cross-linked actin-thymosin β4 complex, indicating that thymosin β4 is dissociated by the binding of DNase I to actin. It is suggested that the dissociation of thymosin β4 by DNase I binding to actin is caused by negative co-operativity between their spatially separated binding sites on actin. A similar negative co-operativity was observed between DNase I and gelsolin segment 1 binding to actin. The results therefore indicate that the respective binding sites for DNase I and segment 1 on subdomains 1 and 2 of actin are linked in a negative co-operative manner.

scholarly journals Sequences, structural models, and cellular localization of the actin-related proteins Arp2 and Arp3 from Acanthamoeba.

1995 ◽  
Vol 131 (2) ◽  
pp. 385-397 ◽  
Author(s):  
J F Kelleher ◽  
S J Atkinson ◽  
T D Pollard
Keyword(s):  
Phylogenetic Analysis ◽  
Binding Site ◽  
Divalent Cation ◽  
Cellular Localization ◽  
Actin Polymerization ◽  
Binding Proteins ◽  
Structural Models ◽  
Actin Binding ◽  
Pointed End ◽  
Related Proteins

We cloned and sequenced the two actin-related proteins (Arps) present in the profilin-binding complex of Acanthamoeba (Machesky, L. M., S. J. Atkinson, C. Ampe, J. Vandekerckhove, and T. D. Pollard. 1994, J. Cell Biol. 127:107-115). The sequence of Arp2 is more similar to other Arp2s than to actin, while the sequence of Arp3 is more similar to other Arp3s than to actin. Phylogenetic analysis of all known Arps demonstrates that most group into three major families, which are likely to be shared across all eukaryotic phyla. Together with conventional actins, the Arps form a larger family distinct from structurally related ATPases such as Hsp70's and sugar kinases. Atomic models of the Arps based on their sequences and the structure of actin provide some clues about function. Both Arps have atoms appropriately placed to bind ATP and divalent cation. Arp2, but not Arp3, has a conserved profilin-binding site. Neither Arp has the residues required to copolymerize with actin, but an Arp heterodimer present in the profilin-binding complex might serve as a pointed end nucleus for actin polymerization. Both Acanthamoeba Arps are soluble in cell homogenates, and both are concentrated in the cortex of Acanthamoeba. The cellular concentrations are 1.9 microM Arp2 and 5.1 microM Arp3, substoichiometric to actin (200 microM) but comparable to many actin-binding proteins.

scholarly journals Dynamics of capping protein and actin assembly in vitro: uncapping barbed ends by polyphosphoinositides.

1996 ◽  
Vol 135 (1) ◽  
pp. 169-179 ◽  
Author(s):  
D A Schafer ◽  
P B Jennings ◽  
J A Cooper
Keyword(s):  
Protein Binding ◽  
Rate Constant ◽  
Half Life ◽  
Actin Polymerization ◽  
Beta Subunit ◽  
Actin Binding ◽  
Capping Protein ◽  
Subunit Isoforms

Bursts of actin polymerization in vivo involve the transient appearance of free barbed ends. To determine how rapidly barbed ends might appear and how long they might remain free in vivo, we studied the kinetics of capping protein, the major barbed end capper, binding to barbed ends in vitro. First, the off-rate constant for capping protein leaving a barbed end is slow, predicting a half-life for a capped barbed end of approximately 30 min. This half-life implies that cells cannot wait for capping protein to spontaneously dissociate from capped barbed ends in order to create free barbed ends. However, we find that phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 4-mono-phosphate (PIP) cause rapid and efficient dissociation of capping protein from capped filaments. PIP2 is a strong candidate for a second messenger regulating actin polymerization; therefore, the ability of PIP2 to remove capping protein from barbed ends is a potential mechanism for stimulating actin polymerization in vivo. Second, the on-rate constant for capping protein binding to free barbed ends predicts that actin filaments could grow to the length of filaments observed in vivo during one lifetime. Third, capping protein beta-subunit isoforms did not differ in their actin binding properties, even in tests with different actin isoforms. A major hypothesis for why capping protein beta-subunit isoforms exist is thereby excluded. Fourth, the proposed capping protein regulators, Hsc70 and S100, had no effect on capping protein binding to actin in vitro.

scholarly journals Heterodimeric Capping Protein fromArabidopsisIs Regulated by Phosphatidic Acid

2006 ◽  
Vol 17 (4) ◽  
pp. 1946-1958 ◽  
Author(s):  
Shanjin Huang ◽  
Lisa Gao ◽  
Laurent Blanchoin ◽  
Christopher J. Staiger
Keyword(s):  
Pollen Tube ◽  
Phosphatidic Acid ◽  
Actin Polymerization ◽  
Tip Growth ◽  
Pollen Grains ◽  
Actin Binding ◽  
Filamentous Actin ◽  
Suspension Cells ◽  
Capping Protein ◽  
Extracellular Stimuli

The cytoskeleton is a key regulator of morphogenesis, sexual reproduction, and cellular responses to extracellular stimuli. Changes in the cellular architecture are often assumed to require actin-binding proteins as stimulus-response modulators, because many of these proteins are regulated directly by binding to intracellular second messengers or signaling phospholipids. Phosphatidic acid (PA) is gaining widespread acceptance as a major, abundant phospholipid in plants that is required for pollen tube tip growth and mediates responses to osmotic stress, wounding, and phytohormones; however, the number of identified effectors of PA is rather limited. Here we demonstrate that exogenous PA application leads to significant increases in filamentous actin levels in Arabidopsis suspension cells and poppy pollen grains. To investigate further these lipid-induced changes in polymer levels, we analyzed the properties of a key regulator of actin filament polymerization, the heterodimeric capping protein from Arabidopsis thaliana (AtCP). AtCP binds to PA with a Kdvalue of 17 μM and stoichiometry of ∼1:2. It also binds well to PtdIns(4,5)P2, but not to several other phosphoinositide or acidic phospholipids. The interaction with PA inhibited the actin-binding activity of CP. In the presence of PA, CP is unable to block the barbed or rapidly growing and shrinking end of actin filaments. Precapped filament barbed ends can also be uncapped by addition of PA, allowing rapid filament assembly from an actin monomer pool that is buffered with profilin. The findings support a model in which the inhibition of CP activity in cells by elevated PA results in the stimulation of actin polymerization from a large pool of profilin-actin. Such regulation may be important for the response of plant cells to extracellular stimuli as well as for the normal process of pollen tube tip growth.

scholarly journals Myosin motors that cannot bind actin leave their folded OFF state on activation of skeletal muscle

2021 ◽  
Vol 153 (11) ◽  
Author(s):  
Massimo Reconditi ◽  
Elisabetta Brunello ◽  
Luca Fusi ◽  
Marco Linari ◽  
Vincenzo Lombardi ◽  
...  
Keyword(s):  
Skeletal Muscle ◽  
Binding Sites ◽  
Muscle Activation ◽  
Sarcomere Length ◽  
Rapid Change ◽  
Thick Filament ◽  
Actin Binding ◽  
Thin Filaments ◽  
Thick Filaments ◽  
Myosin Motors

The myosin motors in resting skeletal muscle are folded back against their tails in the thick filament in a conformation that makes them unavailable for binding to actin. When muscles are activated, calcium binding to troponin leads to a rapid change in the structure of the actin-containing thin filaments that uncovers the myosin binding sites on actin. Almost as quickly, myosin motors leave the folded state and move away from the surface of the thick filament. To test whether motor unfolding is triggered by the availability of nearby actin binding sites, we measured changes in the x-ray reflections that report motor conformation when muscles are activated at longer sarcomere length, so that part of the thick filaments no longer overlaps with thin filaments. We found that the intensity of the M3 reflection from the axial repeat of the motors along the thick filaments declines almost linearly with increasing sarcomere length up to 2.8 µm, as expected if motors in the nonoverlap zone had left the folded state and become relatively disordered. In a recent article in JGP, Squire and Knupp challenged this interpretation of the data. We show here that their analysis is based on an incorrect assumption about how the interference subpeaks of the M3 reflection were reported in our previous paper. We extend previous models of mass distribution along the filaments to show that the sarcomere length dependence of the M3 reflection is consistent with <10% of no-overlap motors remaining in the folded conformation during active contraction, confirming our previous conclusion that unfolding of myosin motors on muscle activation is not due to the availability of local actin binding sites.

scholarly journals Twinfilin, a molecular mailman for actin monomers

2002 ◽  
Vol 115 (5) ◽  
pp. 881-886 ◽  
Author(s):  
Sandra Palmgren ◽  
Maria Vartiainen ◽  
Pekka Lappalainen
Keyword(s):  
Drosophila Melanogaster ◽  
Actin Filament ◽  
Binding Sites ◽  
Actin Filaments ◽  
Actin Binding ◽  
Actin Monomer ◽  
Nucleotide Exchange ◽  
Capping Protein ◽  
Filament Assembly

Twinfilin is a ubiquitous actin-monomer-binding protein that is composed of two ADF-homology domains. It forms a 1:1 complex with ADP-actin-monomers,inhibits nucleotide exchange on actin monomers and prevents assembly of the monomer into filaments. The two ADF-H domains in twinfilin probably have 3D structures similar to those of the ADF/cofilin proteins and overlapping actin-binding sites. Twinfilin also interacts with PtdIns(4,5)P2, which inhibits its actin-monomer-sequestering activity in vitro. Mutations in the twinfilin gene result in defects in the bipolar budding pattern in S. cerevisiae and in a rough eye phenotype and aberrant bristle morphology in Drosophila melanogaster. These phenotypes are caused by the uncontrolled polymerization of actin filaments in the absence of twinfilin. Studies on budding yeast suggest that twinfilin contributes to actin filament turnover by localizing actin monomers, in their `inactive'ADP-form, to the sites of rapid filament assembly. This is mediated through direct interactions between twinfilin and capping protein. Therefore,twinfilin might serve as a link between rapid actin filament depolymerization and assembly in cells.

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