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Current Biology : CB Sep 2001The actin cytoskeleton has long been believed to play a role in endocytosis, but its actual function in this process has been unclear. Now, three proteins that promote... (Review)
Review
The actin cytoskeleton has long been believed to play a role in endocytosis, but its actual function in this process has been unclear. Now, three proteins that promote actin nucleation have been found to provide a link between the actin cytoskeleton and the endocytic machinery.
Topics: Actin-Related Protein 2; Actin-Related Protein 3; Actins; Animals; Cortactin; Cytoskeletal Proteins; Cytoskeleton; Endocytosis; Fungal Proteins; Microfilament Proteins; Proteins; Saccharomyces cerevisiae Proteins; Wiskott-Aldrich Syndrome Protein; Wiskott-Aldrich Syndrome Protein Family
PubMed: 11553341
DOI: 10.1016/s0960-9822(01)00410-9 -
JCI Insight Jun 2019TRIOBP remodels the cytoskeleton by forming unusually dense F-actin bundles and is implicated in human cancer, schizophrenia, and deafness. Mutations ablating human and...
TRIOBP remodels the cytoskeleton by forming unusually dense F-actin bundles and is implicated in human cancer, schizophrenia, and deafness. Mutations ablating human and mouse TRIOBP-4 and TRIOBP-5 isoforms are associated with profound deafness, as inner ear mechanosensory hair cells degenerate after stereocilia rootlets fail to develop. However, the mechanisms regulating formation of stereocilia rootlets by each TRIOBP isoform remain unknown. Using 3 new Triobp mouse models, we report that TRIOBP-5 is essential for thickening bundles of F-actin in rootlets, establishing their mature dimensions and for stiffening supporting cells of the auditory sensory epithelium. The coiled-coil domains of this isoform are required for reinforcement and maintenance of stereocilia rootlets. A loss of TRIOBP-5 in mouse results in dysmorphic rootlets that are abnormally thin in the cuticular plate but have increased widths and lengths within stereocilia cores, and causes progressive deafness recapitulating the human phenotype. Our study extends the current understanding of TRIOBP isoform-specific functions necessary for life-long hearing, with implications for insight into other TRIOBPopathies.
Topics: Actins; Animals; Deafness; Hearing; Mice; Mice, Knockout; Microfilament Proteins; Protein Isoforms; Stereocilia
PubMed: 31217345
DOI: 10.1172/jci.insight.128561 -
Journal of Muscle Research and Cell... Aug 2013Our thesis is that thin filament function can only be fully understood and muscle regulation then elucidated if atomic structures of the thin filament are available to... (Review)
Review
Our thesis is that thin filament function can only be fully understood and muscle regulation then elucidated if atomic structures of the thin filament are available to reveal the positions of tropomyosin on actin in all physiological states. After all, it is tropomyosin influenced by troponin that regulates myosin-crossbridge cycling on actin and therefore controls contraction in all muscles. In addition, we maintain that a complete appreciation of thin filament activation also requires that the mechanical properties of tropomyosin itself are recognized and then related to the effect of myosin-association on actin. Taking the Gestalt-binding of tropomyosin into account, coupled with our electron microscopy structures and computational chemistry, we propose a comprehensive mechanism for tropomyosin regulatory movement over the actin filament surface that explains the cooperative muscle activation process. In fact, well-known point mutations of critical amino acids on the actin-tropomyosin binding interface disrupt Gestalt-binding and are associated with a number of inherited myopathies. Moreover, dysregulation of tropomyosin may also be a factor that interferes with the gatekeeping operation of non-muscle tropomyosin in the controlling interactions of a wide variety of cellular actin-binding proteins. The clinical relevance of Gestalt-binding is discussed in articles by the Marston and the Gunning groups in this special journal issue devoted to the impact of tropomyosin on biological systems.
Topics: Actin Cytoskeleton; Actins; Humans; Microfilament Proteins; Muscles; Tropomyosin; Troponin
PubMed: 23666668
DOI: 10.1007/s10974-013-9342-0 -
Biophysical Journal Feb 2015The functional relevance of regulating proteins is often limited to specific binding sites such as the ends of microtubules or actin-filaments. A localization of...
The functional relevance of regulating proteins is often limited to specific binding sites such as the ends of microtubules or actin-filaments. A localization of proteins on these functional sites is of great importance. We present a quantitative theory for a diffusion and capture process, where proteins diffuse on a filament and stop diffusing when reaching the filament's end. It is found that end-association after one-dimensional diffusion is the main source for tip-localization of such proteins. As a consequence, diffusion and capture is highly efficient in enhancing the reaction velocity of enzymatic reactions, where proteins and filament ends are to each other as enzyme and substrate. We show that the reaction velocity can effectively be described within a Michaelis-Menten framework. Together, one-dimensional diffusion and capture beats the (three-dimensional) Smoluchowski diffusion limit for the rate of protein association to filament ends.
Topics: Diffusion; Microfilament Proteins; Models, Biological
PubMed: 25692582
DOI: 10.1016/j.bpj.2014.12.053 -
ELife May 2021Despite extensive studies on the actin regulators that direct microfilament dynamics, how these regulators are combinatorially utilized in organismal tissues to generate...
Despite extensive studies on the actin regulators that direct microfilament dynamics, how these regulators are combinatorially utilized in organismal tissues to generate 3D structures is an unresolved question. Here, we present an in-depth characterization of cortical actin cap dynamics and their regulation in vivo. We identify rapid phases of initiation, expansion, duplication, and disassembly and examine the functions of seven different actin and/or nucleator regulators (ANRPs) in guiding these behaviors. We find ANRPs provide distinct activities in building actin cap morphologies - specifically, while DPod1 is a major regulator of actin intensities, Cortactin is required for continued cortical growth, while Coronin functions in both growth and intensity and is required for Cortactin localization to the cap periphery. Unexpectedly, cortical actin populations recover after regulator disruption, suggestive of a deep competition for limited G-actin pools, and we measure in vivo Arp2/3 recruitment efficiencies through an ectopic relocalization strategy. Our results illustrate how the coordination of multiple actin regulators can orchestrate organized and dynamic actin structures in a developmental system.
Topics: 4-Butyrolactone; Actins; Animals; Cell Line; Clustered Regularly Interspaced Short Palindromic Repeats; Cortactin; Drosophila; Female; Gene Expression Regulation; Microfilament Proteins
PubMed: 33949307
DOI: 10.7554/eLife.63046 -
The Biochemical Journal Jun 2004The polymerization of actin is catalysed by the Arp (actin-related protein) 2/3 complex, which acts downstream of a variety of receptors and signalling cascades.... (Review)
Review
The polymerization of actin is catalysed by the Arp (actin-related protein) 2/3 complex, which acts downstream of a variety of receptors and signalling cascades. Intermediary molecules such as cortactin bind to the Arp2/3 complex and stimulate its activity, thus promoting actin polymerization and actin filament stabilization. New data in this issue of the Biochemical Journal by the Kapus group suggest that cortactin is reciprocally regulated by filamentous (F) actin and tyrosine kinases. This suggests a new paradigm for considering the cellular processes that regulate the dynamic organization of the actin cytoskeleton.
Topics: Actins; Animals; Cortactin; Humans; Microfilament Proteins; Protein-Tyrosine Kinases
PubMed: 15154835
DOI: 10.1042/BJ20040559 -
PloS One 2011Twinfilins are evolutionarily conserved regulators of cytoskeletal dynamics. They inhibit actin polymerization by binding both actin monomers and filament barbed ends....
Twinfilins are evolutionarily conserved regulators of cytoskeletal dynamics. They inhibit actin polymerization by binding both actin monomers and filament barbed ends. Inactivation of the single twinfilin gene from budding yeast and fruit fly results in defects in endocytosis, cell migration, and organization of the cortical actin filament structures. Mammals express three twinfilin isoforms, of which twinfilin-1 and twinfilin-2a display largely overlapping expression patterns in non-muscle tissues of developing and adult mice. The expression of twinfilin-2b, which is generated through alternative promoter usage of the twinfilin-2 gene, is restricted to heart and skeletal muscles. However, the physiological functions of mammalian twinfilins have not been reported. As a first step towards understanding the function of twinfilin in vertebrates, we generated twinfilin-2a deficient mice by deleting exon 1 of the twinfilin-2 gene. Twinfilin-2a knockout mice developed normally to adulthood, were fertile, and did not display obvious morphological or behavioural abnormalities. Tissue anatomy and morphology in twinfilin-2a deficient mice was similar to that of wild-type littermates. These data suggest that twinfilin-2a plays a redundant role in cytoskeletal dynamics with the biochemically similar twinfilin-1, which is typically co-expressed in same tissues with twinfilin-2a.
Topics: Animals; Blotting, Northern; Blotting, Western; Brain; Gene Expression Regulation, Developmental; Growth and Development; Mice; Mice, Knockout; Microfilament Proteins; Organ Specificity; Reverse Transcriptase Polymerase Chain Reaction; Tissue Extracts
PubMed: 21876732
DOI: 10.1371/journal.pone.0022894 -
Cytoskeleton (Hoboken, N.J.) Jun 2012Formins are a conserved family of actin assembly-promoting factors with essential and diverse biological roles. Most of our biochemical understanding of formin effects...
Formins are a conserved family of actin assembly-promoting factors with essential and diverse biological roles. Most of our biochemical understanding of formin effects on actin dynamics is derived from studies using formin fragments. In addition, all structural information on formins has been limited to fragments. This has left open key questions about the structure, activity and regulation of intact formin proteins. Here, we isolated full-length mouse mDia1 (mDia1-FL) and found that it forms tightly autoinhibited dimers that can only be partially activated by RhoA. We solved the structure of autoinhibited mDia1-FL using electron microscopy and single particle analysis. Docking of crystal structures into the three dimensional reconstruction revealed that the fork-shaped N-terminal diaphanous inhibitory domain-coiled coil domain region hangs over the ring-shaped formin homology (FH)2 domain, suggesting that autoinhibition results from steric obstruction of actin binding. Deletion of the C-terminal diaphanous autoregulatory domain extended mDia1 structure and activated it for actin assembly. Using total internal reflection fluorescence microscopy, we observed that RhoA-activated mDia1-FL persistently accelerated filament elongation in the presence of profilin similar to mDia1 FH1-FH2 fragment. These observations validate the known activities of FH1-FH2 fragments as reflecting those of the intact molecule. Our results further suggest that mDia1-FL does not readily snap back into the autoinhibited conformation and dissociate from growing filament ends, and thus additional factors may be required to displace formins and restrict filament length.
Topics: Actins; Animals; Carrier Proteins; Enzyme Activation; Formins; Mice; Microfilament Proteins; Microscopy, Fluorescence; Models, Biological; Protein Multimerization; Protein Structure, Tertiary; Sequence Deletion; Structure-Activity Relationship; rhoA GTP-Binding Protein
PubMed: 22605659
DOI: 10.1002/cm.21033 -
Molecular Biology of the Cell Feb 2022Actin nucleation is achieved by collaborative teamwork of actin nucleator factors (NFs) and nucleation-promoting factors (NPFs) into functional protein complexes....
Actin nucleation is achieved by collaborative teamwork of actin nucleator factors (NFs) and nucleation-promoting factors (NPFs) into functional protein complexes. Selective inter- and intramolecular interactions between the nucleation complex constituents enable diverse modes of complex assembly in initiating actin polymerization on demand. Budding yeast has two formins, Bni1 and Bnr1, which are teamed up with different NPFs. However, the selective pairing between formin NFs and NPFs into the nucleation core for actin polymerization is not completely understood. By examining the functions and interactions of NPFs and NFs via biochemistry, genetics, and mathematical modeling approaches, we found that two NPFs, Aip5 and Bud6, showed joint teamwork effort with Bni1 and Bnr1, respectively, by interacting with the C-terminal intrinsically disordered region (IDR) of formin, in which two NPFs work together to promote formin-mediated actin nucleation. Although the C-terminal IDRs of Bni1 and Bnr1 are distinct in length, each formin IDR orchestrates the recruitment of Bud6 and Aip5 cooperatively by different positioning strategies to form a functional complex. Our study demonstrated the dynamic assembly of the actin nucleation complex by recruiting multiple partners in budding yeast, which may be a general feature for effective actin nucleation by formins.
Topics: Actins; Amino Acid Sequence; Cytoskeletal Proteins; Formins; Microfilament Proteins; Saccharomyces cerevisiae; Saccharomyces cerevisiae Proteins
PubMed: 34818061
DOI: 10.1091/mbc.E21-06-0285 -
The Journal of Cell Biology Aug 2022Actin filament dynamics must be precisely controlled in cells to execute behaviors such as vesicular trafficking, cytokinesis, and migration. Coronins are conserved...
Actin filament dynamics must be precisely controlled in cells to execute behaviors such as vesicular trafficking, cytokinesis, and migration. Coronins are conserved actin-binding proteins that regulate several actin-dependent subcellular processes. Here, we describe a new conditional knockout cell line for two ubiquitous coronins, Coro1B and Coro1C. These coronins, which strongly co-localize with Arp2/3-branched actin, require Arp2/3 activity for proper subcellular localization. Coronin null cells have altered lamellipodial protrusion dynamics due to increased branched actin density and reduced actin turnover within lamellipodia, leading to defective haptotaxis. Surprisingly, excessive cofilin accumulates in coronin null lamellipodia, a result that is inconsistent with the current models of coronin-cofilin functional interaction. However, consistent with coronins playing a pro-cofilin role, coronin null cells have increased F-actin levels. Lastly, we demonstrate that the loss of coronins increases accompanied by an increase in cellular contractility. Together, our observations reveal that coronins are critical for proper turnover of branched actin networks and that decreased actin turnover leads to increased cellular contractility.
Topics: Actin Cytoskeleton; Actin Depolymerizing Factors; Actins; Animals; Cell Movement; Mice; Microfilament Proteins; Pseudopodia
PubMed: 35657370
DOI: 10.1083/jcb.202111126