Domain structure of tropomodulin: distinct properties of the N-terminal and C-terminal halves

A nemaline myopathy-linked mutation inhibits the actin-regulatory functions of tropomodulin and leiomodin

Actin is a highly expressed protein in eukaryotic cells and is essential for numerous cellular processes. In particular, efficient striated muscle contraction is dependent upon the precise regulation of actin-based thin filament structure and function. Alterations in the lengths of actin-thin filaments can lead to the development of myopathies. Leiomodins and tropomodulins are members of an actin-binding protein family that fine-tune thin filament lengths, and their dysfunction is implicated in muscle diseases. An Lmod3 mutation [G326R] was previously identified in patients with nemaline myopathy (NM), a severe skeletal muscle disorder; this residue is conserved among Lmod and Tmod isoforms and resides within their homologous leucine-rich repeat (LRR) domain. We mutated this glycine to arginine in Lmod and Tmod to determine the physiological function of this residue and domain. This G-to-R substitution disrupts Lmod and Tmod's LRR domain structure, altering their binding interface with actin and destroying their abilities to regulate thin filament lengths. Additionally, this mutation renders Lmod3 nonfunctional in vivo. We found that one single amino acid is essential for folding of Lmod and Tmod LRR domains, and thus is essential for the opposing actin-regulatory functions of Lmod (filament elongation) and Tmod (filament shortening), revealing a mechanism underlying the development of NM.

Myofilament-associated proteins with intrinsic disorder (MAPIDs) and their resolution by computational modeling

The cardiac sarcomere is a cellular structure in the heart that enables muscle cells to contract. Dozens of proteins belong to the cardiac sarcomere, which work in tandem to generate force and adapt to demands on cardiac output. Intriguingly, the majority of these proteins have significant intrinsic disorder that contributes to their functions, yet the biophysics of these intrinsically disordered regions (IDRs) have been characterized in limited detail. In this review, we first enumerate these myofilament-associated proteins with intrinsic disorder (MAPIDs) and recent biophysical studies to characterize their IDRs. We secondly summarize the biophysics governing IDR properties and the state-of-the-art in computational tools toward MAPID identification and characterization of their conformation ensembles. We conclude with an overview of future computational approaches toward broadening the understanding of intrinsic disorder in the cardiac sarcomere.

The role of leiomodin in actin dynamics: a new road or a secret gate

Leiomodin is an important emerging regulator of thin filaments. As novel molecular, cellular, animal model, and human data accumulate, the mechanisms of its action become clearer. Structural studies played a significant part in understanding the functional significance of leiomodin's interacting partners and functional domains. In this review, we present the current state of knowledge on the structural and cellular properties of leiomodin which has led to two proposed mechanisms of its function. Although it is known that leiomodin is essential for life, numerous domains within leiomodin remain unstudied and as such, we outline future directions for investigations that we predict will provide evidence that leiomodin is a multifunctional protein.

Leiomodin creates a leaky cap at the pointed end of actin-thin filaments

Improper lengths of actin-thin filaments are associated with altered contractile activity and lethal myopathies. Leiomodin, a member of the tropomodulin family of proteins, is critical in thin filament assembly and maintenance; however, its role is under dispute. Using nuclear magnetic resonance data and molecular dynamics simulations, we generated the first atomic structural model of the binding interface between the tropomyosin-binding site of cardiac leiomodin and the N-terminus of striated muscle tropomyosin. Our structural data indicate that the leiomodin/tropomyosin complex only forms at the pointed end of thin filaments, where the tropomyosin N-terminus is not blocked by an adjacent tropomyosin protomer. This discovery provides evidence supporting the debated mechanism where leiomodin and tropomodulin regulate thin filament lengths by competing for thin filament binding. Data from experiments performed in cardiomyocytes provide additional support for the competition model; specifically, expression of a leiomodin mutant that is unable to interact with tropomyosin fails to displace tropomodulin at thin filament pointed ends and fails to elongate thin filaments. Together with previous structural and biochemical data, we now propose a molecular mechanism of actin polymerization at the pointed end in the presence of bound leiomodin. In the proposed model, the N-terminal actin-binding site of leiomodin can act as a "swinging gate" allowing limited actin polymerization, thus making leiomodin a leaky pointed-end cap. Results presented in this work answer long-standing questions about the role of leiomodin in thin filament length regulation and maintenance.

Congenital myopathy-related mutations in tropomyosin disrupt regulatory function through altered actin affinity and tropomodulin binding

Tropomyosin (Tpm) binds along actin filaments and regulates myosin binding to control muscle contraction. Tropomodulin binds to the pointed end of a filament and regulates actin dynamics, which maintains the length of a thin filament. To define the structural determinants of these Tpm functions, we examined the effects of two congenital myopathy mutations, A4V and R91C, in the Tpm gene, TPM3, which encodes the Tpm3.12 isoform, specific for slow-twitch muscle fibers. Mutation A4V is located in the tropomodulin-binding, N-terminal region of Tpm3.12. R91C is located in the actin-binding period 3 and directly interacts with actin. The A4V and R91C mutations resulted in a 2.5-fold reduced affinity of Tpm3.12 homodimers for F-actin in the absence and presence of troponin, and a two-fold decrease in actomyosin ATPase activation in the presence of Ca2+ . Actomyosin ATPase inhibition in the absence of Ca2+ was not affected. The Ca2+ sensitivity of ATPase activity was decreased by R91C, but not by A4V. In vitro, R91C altered the ability of tropomodulin 1 (Tmod1) to inhibit actin polymerization at the pointed end of the filaments, which correlated with the reduced affinity of Tpm3.12-R91C for Tmod1. Molecular dynamics simulations of Tpm3.12 in complex with F-actin suggested that both mutations reduce the affinity of Tpm3.12 for F-actin binding by perturbing the van der Waals energy, which may be attributable to two different molecular mechanisms-a reduced flexibility of Tpm3.12-R91C and an increased flexibility of Tpm3.12-A4V.

Role of intrinsic disorder in muscle sarcomeres

The role and utility of intrinsically disordered regions (IDRs) is reviewed for two groups of sarcomeric proteins, such as members of tropomodulin/leiomodin (Tmod/Lmod) protein homology group and myosin binding protein C (MyBP-C). These two types of sarcomeric proteins represent very different but strongly interdependent functions, being responsible for maintaining structure and operation of the muscle sarcomere. The role of IDRs in the formation of complexes between thin filaments and Tmods/Lmods is discussed within the framework of current understanding of the thin filament length regulation. For MyBP-C, the function of IDRs is discussed in the context of MYBP-C-dependent sarcomere contraction and actomyosin activation.

Effects of cardiomyopathy-linked mutations K15N and R21H in tropomyosin on thin-filament regulation and pointed-end dynamics

Missense mutations K15N and R21H in striated muscle tropomyosin are linked to dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM), respectively. Tropomyosin, together with the troponin complex, regulates muscle contraction and, along with tropomodulin and leiomodin, controls the uniform thin-filament lengths crucial for normal sarcomere structure and function. We used Förster resonance energy transfer to study effects of the tropomyosin mutations on the structure and kinetics of the cardiac troponin core domain associated with the Ca²⁺-dependent regulation of cardiac thin filaments. We found that the K15N mutation desensitizes thin filaments to Ca²⁺ and slows the kinetics of structural changes in troponin induced by Ca²⁺ dissociation from troponin, while the R21H mutation has almost no effect on these parameters. Expression of the K15N mutant in cardiomyocytes decreases leiomodin’s thin-filament pointed-end assembly but does not affect tropomodulin’s assembly at the pointed end. Our in vitro assays show that the R21H mutation causes a twofold decrease in tropomyosin’s affinity for F-actin and affects leiomodin’s function. We suggest that the K15N mutation causes DCM by altering Ca²⁺-dependent thin-filament regulation and that one of the possible HCM-causing mechanisms by the R21H mutation is through alteration of leiomodin’s function.

Tropomodulin's Actin-Binding Abilities Are Required to Modulate Dendrite Development

There are many unanswered questions about the roles of the actin pointed end capping and actin nucleation by tropomodulins (Tmod) in regulating neural morphology. Previous studies indicate that Tmod1 and Tmod2 regulate morphology of the dendritic arbor and spines. Tmod3, which is expressed in the brain, had only a minor influence on morphology. Although these studies established a defined role of Tmod in regulating dendritic and synaptic morphology, the mechanisms by which Tmods exert these effects are unknown. Here, we overexpressed a series of mutated forms of Tmod1 and Tmod2 with disrupted actin-binding sites in hippocampal neurons and found that Tmod1 and Tmod2 require both of their actin-binding sites to regulate dendritic morphology and dendritic spine shape. Proximity ligation assays (PLAs) indicate that these mutations impact the interaction of Tmod1 and Tmod2 with tropomyosins Tpm3.1 and Tpm3.2. This impact on Tmod/Tpm interaction may contribute to the morphological changes observed. Finally, we use molecular dynamics simulations (MDS) to characterize the structural changes, caused by mutations in the C-terminal helix of the leucine-rich repeat (LRR) domain of Tmod1 and Tmod2 alone and when bound onto actin monomers. Our results expand our understanding of how neurons utilize the different Tmod isoforms in development.

Characterizing interaction forces between actin and proteins of the tropomodulin family reveals the presence of the N-terminal actin-binding site in leiomodin

Tropomodulin family of proteins includes several isoforms of tropomodulins (Tmod) and leiomodins (Lmod). These proteins can sequester actin monomers or nucleate actin polymerization. Although it is known that their actin-binding properties are isoform-dependent, knowledge on how they vary in strengths of interactions with G-actin is missing. While it is confirmed in many studies that Tmods have two actin-binding sites, information on number and location of actin-binding sites in Lmod2 is controversial. We used atomic force microscopy to study interactions between G-actin and proteins of the tropomodulin family. Unbinding forces between G-actin and Tmod1, Tmod2, Tmod3, or Lmod2 were quantified. Our results indicated that Tmod1 and Tmod3 had unimodal force distributions, Tmod2 had a bimodal distribution and Lmod2 had a trimodal distribution. The number of force distributions correlates with the proteins' abilities to sequester actin or to nucleate actin polymerization. We assigned specific unbinding forces to the individual actin-binding sites of Tmod2 and Lmod2 using mutations that destroy actin-binding sites of Tmod2 and truncated Lmod2. Our results confirm the existence of the N-terminal actin-binding site in Lmod2. Altogether, our data demonstrate how the differences between the number and the strength of actin-binding sites of Tmod or Lmod translate to their functional abilities.

The cardiomyopathy-associated K15N mutation in tropomyosin alters actin filament pointed end dynamics

Correct assembly of thin filaments composed of actin and actin-binding proteins is of crucial importance for properly functioning muscle cells. Tropomyosin (Tpm) mediates the binding of tropomodulin (Tmod) and leiomodin (Lmod) at the slow-growing, or pointed, ends of the thin filaments. Together these proteins regulate thin filament lengths and actin dynamics in cardiac muscle. The K15N mutation in the TPM1 gene is associated with familial dilated cardiomyopathy (DCM) but the effect of this mutation on Tpm's function is unknown. In this study, we introduced the K15N mutation in striated muscle α-Tpm (Tpm1.1) and investigated its interaction with actin, Tmod and Lmod. The mutation caused a ∼3-fold decrease in the affinity of Tpm1.1 for actin. The binding of Lmod and Tmod to Tpm1.1-covered actin filaments also decreased in the presence of the K15N mutation. Furthermore, the K15N mutation in Tpm1.1 disrupted the inhibition of actin polymerization and affected the competition between Tmod1 and Lmod2 for binding at the pointed ends. Our data demonstrate that the K15N mutation alters pointed end dynamics by affecting molecular interactions between Tpm1.1, Lmod2 and Tmod1.

Actin regulation by tropomodulin and tropomyosin in neuronal morphogenesis and function

Actin is a profoundly influential protein; it impacts, among other processes, membrane morphology, cellular motility, and vesicle transport. Actin can polymerize into long filaments that push on membranes and provide support for intracellular transport. Actin filaments have polar ends: the fast-growing (barbed) end and the slow-growing (pointed) end. Depolymerization from the pointed end supplies monomers for further polymerization at the barbed end. Tropomodulins (Tmods) cap pointed ends by binding onto actin and tropomyosins (Tpms). Tmods and Tpms have been shown to regulate many cellular processes; however, very few studies have investigated their joint role in the nervous system. Recent data directly indicate that they can modulate neuronal morphology. Additional studies suggest that Tmod and Tpm impact molecular processes influential in synaptic signaling. To facilitate future research regarding their joint role in actin regulation in the nervous system, we will comprehensively discuss Tpm and Tmod and their known functions within molecular systems that influence neuronal development.

Tropomodulin isoforms utilize specific binding functions to modulate dendrite development

Tropomodulins (Tmods) cap F-actin pointed ends and have altered expression in the brain in neurological diseases. The function of Tmods in neurons has been poorly studied and their role in neurological diseases is entirely unknown. In this article, we show that Tmod1 and Tmod2, but not Tmod3, are positive regulators of dendritic complexity and dendritic spine morphology. Tmod1 increases dendritic branching distal from the cell body and the number of filopodia/thin spines. Tmod2 increases dendritic branching proximal to the cell body and the number of mature dendritic spines. Tmods utilize two actin-binding sites and two tropomyosin (Tpm)-binding sites to cap F-actin. Overexpression of Tmods with disrupted Tpm-binding sites indicates that Tmod1 and Tmod2 differentially utilize their Tpm- and actin-binding sites to affect morphology. Disruption of Tmod1's Tpm-binding sites abolished the overexpression phenotype. In contrast, overexpression of the mutated Tmod2 caused the same phenotype as wild type overexpression. Proximity ligation assays indicate that the mutated Tmods are shuttled similarly to wild type Tmods. Our data begins to uncover the roles of Tmods in neural development and the mechanism by which Tmods alter neural morphology. These observations in combination with altered Tmod expression found in several neurological diseases also suggest that dysregulation of Tmod expression may be involved in the pathology of these diseases. © 2016 Wiley Periodicals, Inc.

Tropomyosin-binding properties modulate competition between tropomodulin isoforms

The formation and fine-tuning of cytoskeleton in cells are governed by proteins that influence actin filament dynamics. Tropomodulin (Tmod) regulates the length of actin filaments by capping the pointed ends in a tropomyosin (TM)-dependent manner. Tmod1, Tmod2 and Tmod3 are associated with the cytoskeleton of non-muscle cells and their expression has distinct consequences on cell morphology. To understand the molecular basis of differences in the function and localization of Tmod isoforms in a cell, we compared the actin filament-binding abilities of Tmod1, Tmod2 and Tmod3 in the presence of Tpm3.1, a non-muscle TM isoform. Tmod3 displayed preferential binding to actin filaments when competing with other isoforms. Mutating the second or both TM-binding sites of Tmod3 destroyed its preferential binding. Our findings clarify how Tmod1, Tmod2 and Tmod3 compete for binding actin filaments. Different binding mechanisms and strengths of Tmod isoforms for Tpm3.1 contribute to their divergent functional capabilities.

Overexpression of TMOD1 is associated with enhanced regional lymph node metastasis in human oral cancer

Tropomodulin1 (TMOD1), which regulates the length and depolymerization of actin filaments by binding to the pointed end of the actin filament, has been reported to be a powerful diagnostic marker for ALK-negative anaplastic large-cell lymphoma; however, little is known about the relevance of TMOD1 in the behavior of oral squamous cell carcinoma (OSCC). We evaluated TMOD1 expression in OSCC-derived cell lines and primary OSCC samples (n=200) using quantitative reverse transcriptase-polymerase chain reaction, immunoblotting and semi-quantitative immunohistochemistry. We also analyzed the clinical correlation between TMOD1 expression status and clinical parameters in patients with OSCC and performed a prospective study using 40 primary OSCC samples. TMOD1 expression was upregulated significantly (p<0.05) in OSCC in vitro and in vivo compared with normal counterparts. TMOD1 expression also was correlated significantly (p=0.0199 and p=0.0064, respectively) with regional lymph node metastasis (RLNM) and 5-year survival rates. This prospective study also showed that high TMOD1 expression was seen in 12 (75%) of 16 cases in RLNM-positive patients and 9 (37.5%) of 24 cases in RLNM-negative patients. The current data provide the first evidence that TMOD1 expression is a critical biomarker for RLNM and prognosis of patients with OSCC.

Phosphorylation of tropomodulin1 contributes to the regulation of actin filament architecture in cardiac muscle

Tropomodulin1 (Tmod1) is an actin-capping protein that plays an important role in actin filament pointed-end dynamics and length in striated muscle. No mechanisms have been identified to explain how Tmod1's functional properties are regulated. The purpose of this investigation was to explore the functional significance of the phosphorylation of Tmod1 at previously identified Thr54. Rat cardiomyocytes were assessed for phosphorylation of Tmod1 using Pro-Q Diamond staining and (32)P labeling. Green fluorescent protein-tagged phosphorylation-mimic (T54E) and phosphorylation-deficient (T54A) versions of Tmod1 were expressed in cultured cardiomyocytes, and the ability of these mutants to assemble and restrict actin lengths was observed. We report for the first time that Tmod1 is phosphorylated endogenously in cardiomyocytes, and phosphorylation at Thr54 causes a significant reduction in the ability of Tmod1 to assemble to the pointed end compared with that of the wild type (WT; 48 vs. 78%, respectively). In addition, overexpression of Tmod1-T54E restricts actin filament lengths by only ∼3%, whereas Tmod1-WT restricts the lengths significantly by ∼8%. Finally, Tmod1-T54E altered the actin filament-capping activity in polymerization assays. Taken together, our data suggest that pointed-end assembly and Tmod1's thin filament length regulatory function are regulated by its phosphorylation state.

Role of tropomodulin's leucine rich repeat domain in the formation of neurite-like processes

Actin dynamics is fundamental for neurite development; monomer depolymerization from pointed ends is rate-limiting in actin treadmilling. Tropomodulins (Tmod) make up a family of actin pointed end-capping proteins. Of the four known isoforms, Tmod1-Tmod3 are expressed in brain cells. We investigated the role of Tmod's C-terminal (LRR) domain in the formation of neurite-like processes by overexpressing Tmod1 and Tmod2 with deleted or mutated LRR domains in PC12 cells, a model system used to study neuritogenesis. Tmod1 overexpression results in a normal quantity and a normal length of processes, while Tmod2 overexpression reduces both measures. The Tmod2 overexpression phenotype is mimicked by overexpression of Tmod1 with the LRR domain removed or with three point mutations in the LRR domain that disrupt exposed clusters of conserved residues. Removal of Tmod2's LRR domain does not significantly alter the outgrowth of neurite-like processes compared to that of Tmod2. Overexpression of chimeras with the N-terminal and C-terminal domains switched between Tmod1 and Tmod2 reinforces the idea that Tmod1's LRR domain counteracts the reductive effect of the Tmod N-terminal domain upon formation of processes while Tmod2's LRR domain does not. We suggest that the TM-dependent actin capping ability of both Tmods inhibits the formation of processes, but in Tmod1, this inhibition can be controlled via its LRR domain. Circular dichroism, limited proteolysis, and molecular dynamics demonstrate structural differences in the C-terminal region of the LRR domains of Tmod1, Tmod2, and the Tmod1 mutant.

Differential actin-regulatory activities of Tropomodulin1 and Tropomodulin3 with diverse tropomyosin and actin isoforms

Tropomodulins (Tmods) are F-actin pointed end capping proteins that interact with tropomyosins (TMs) and cap TM-coated filaments with higher affinity than TM-free filaments. Here, we tested whether differences in recognition of TM or actin isoforms by Tmod1 and Tmod3 contribute to the distinct cellular functions of these Tmods. We found that Tmod3 bound ~5-fold more weakly than Tmod1 to α/βTM, TM5b, and TM5NM1. However, surprisingly, Tmod3 was as effective as Tmod1 at capping pointed ends of skeletal muscle α-actin (αsk-actin) filaments coated with α/βTM, TM5b, or TM5NM1. Tmod3 only capped TM-coated αsk-actin filaments more weakly than Tmod1 in the presence of recombinant αTM2, which is unacetylated at its NH2 terminus, binds F-actin weakly, and has a disabled Tmod-binding site. Moreover, both Tmod1 and Tmod3 were similarly effective at capping pointed ends of platelet β/cytoplasmic γ (γcyto)-actin filaments coated with TM5NM1. In the absence of TMs, both Tmod1 and Tmod3 had similarly weak abilities to nucleate β/γcyto-actin filament assembly, but only Tmod3 could sequester cytoplasmic β- and γcyto-actin (but not αsk-actin) monomers and prevent polymerization under physiological conditions. Thus, differences in TM binding by Tmod1 and Tmod3 do not appear to regulate the abilities of these Tmods to cap TM-αsk-actin or TM-β/γcyto-actin pointed ends and, thus, are unlikely to determine selective co-assembly of Tmod, TM, and actin isoforms in different cell types and cytoskeletal structures. The ability of Tmod3 to sequester β- and γcyto-actin (but not αsk-actin) monomers in the absence of TMs suggests a novel function for Tmod3 in regulating actin remodeling or turnover in cells.

Tropomodulins and tropomyosins: working as a team

Actin filaments are major components of the cytoskeleton in eukaryotic cells and are involved in vital cellular functions such as cell motility and muscle contraction. Tmod and TM are crucial constituents of the actin filament network, making their presence indispensable in living cells. Tropomyosin (TM) is an alpha-helical, coiled coil protein that covers the grooves of actin filaments and stabilizes them. Actin filament length is optimized by tropomodulin (Tmod), which caps the slow growing (pointed end) of thin filaments to inhibit polymerization or depolymerization. Tmod consists of two structurally distinct regions: the N-terminal and the C-terminal domains. The N-terminal domain contains two TM-binding sites and one TM-dependent actin-binding site, whereas the C-terminal domain contains a TM-independent actin-binding site. Tmod binds to two TM molecules and at least one actin molecule during capping. The interaction of Tmod with TM is a key regulatory factor for actin filament organization. The binding efficacy of Tmod to TM is isoform-dependent. The affinities of Tmod/TM binding influence the proper localization and capping efficiency of Tmod at the pointed end of actin filaments in cells. Here we describe how a small difference in the sequence of the TM-binding sites of Tmod may result in dramatic change in localization of Tmod in muscle cells or morphology of non-muscle cells. We also suggest most promising directions to study and elucidate the role of Tmod-TM interaction in formation and maintenance of sarcomeric and cytoskeletal structure.

Mutations changing tropomodulin affinity for tropomyosin alter neurite formation and extension

Assembly of the actin cytoskeleton is an important part of formation of neurites in developing neurons. Tropomodulin, a tropomyosin-dependent capping protein for the pointed end of the actin filament, is one of the key players in this process. Tropomodulin binds tropomyosin in two binding sites. Tmod1 and Tmod2, tropomodulin isoforms found in neurons, were overexpressed in PC12 cells, a model system for neuronal differentiation. Tmod1 did not affect neuronal differentiation; while cells expressing Tmod2 showed a significant reduction in the number and the length of neurites. Both tropomodulins bind short α-, γ- and δ-tropomyosin isoforms. Mutations in one of the tropomyosin-binding sites of Tmod1, which increased its affinity to short γ- and δ-tropomyosin isoforms, caused a decrease in binding short α-tropomyosin isoforms along with a 2-fold decrease in the length of neurites. Our data demonstrate that Tmod1 is involved in neuronal differentiation for proper neurite formation and outgrowth, and that Tmod2 inhibits these processes. The mutations in the tropomyosin-binding site of Tmod1 impair neurite outgrowth, suggesting that the integrity of this binding site is critical for the proper function of Tmod1 during neuronal differentiation.

Alteration of tropomyosin-binding properties of tropomodulin-1 affects its capping ability and localization in skeletal myocytes

Tropomodulin (Tmod) is an actin-capping protein that binds to the two tropomyosins (TM) at the pointed end of the actin filament to prevent further actin polymerization and depolymerization. Therefore, understanding the role of Tmod is very important when studying actin filament dependent processes such as muscle contraction and intracellular transport. The capping ability of Tmod is highly influenced by TM and is 1000-fold greater in the presence of TM. There are four Tmod isoforms (Tmod1-4), three of which, Tmod1, Tmod3, and Tmod4, are expressed in skeletal muscles. The affinity of Tmod1 to skeletal striated TM (stTM) is higher than that of Tmod3 and Tmod4 to stTM. In this study, we tested mutations in the TM-binding sites of Tmod1, using circular dichroism (CD) and prediction analysis (PONDR). The mutations R11K, D12N, and Q144K were chosen because they decreased the affinity of Tmod1 to stTM, making it similar to that of affinity of Tmod3 and Tmod4 to stTM. Significant reduction of inhibition of actin pointed-end polymerization in the presence of stTM was shown for Tmod1 (R11K/D12N/Q144K) as compared with WT Tmod1. When GFP-Tmod1 and mutants were expressed in primary chicken skeletal myocytes, decreased assembly of Tmod1 mutants was revealed. This indicates a direct correlation between TM-binding and the actin-capping abilities of Tmod. Our data confirmed the hypothesis that assembly of Tmod at the pointed-end of the actin filament depends on its TM-binding affinity.

Myopodin is an F-actin bundling protein with multiple independent actin-binding regions

The assembly of striated muscle myofibrils is a multistep process in which a variety of proteins is involved. One of the first and most important steps in myofibrillogenesis is the arrangement of thin myofilaments into ordered I-Z-I brushes, requiring the coordinated activity of numerous actin binding proteins. The early expression of myopodin prior to sarcomeric α-actinin, as well as its binding to actin, α-actinin and filamin indicate an important role for this protein in actin cytoskeleton remodelling with the precise function of myopodin in this process yet remaining to be resolved. While myopodin was previously described as a protein capable of cross-linking actin filaments into thick bundles upon transient transfections, it has remained unclear whether myopodin alone is capable of bundling actin, or if additional proteins are involved. We have therefore investigated the in vitro actin binding properties of myopodin. High speed cosedimentation assays with skeletal muscle actin confirmed direct binding of myopodin to F-actin and showed that this interaction is mediated by at least two independent actin binding sites, found in all myopodin isoforms identified to date. Furthermore, low-speed cosedimentation assays revealed that not only full length myopodin, but also the fragment containing only the second binding site, bundles microfilaments in the absence of accessory proteins. Ultrastructural analysis demonstrated that this bundling activity resembled that of α-actinin. Biochemical experiments revealed that bundling was not achieved by myopodin's ability to dimerize, indicating the presence of two individual F-actin binding sites within the second binding segment. Thus full length myopodin contains at least three F-actin binding sites. These data provide further understanding of the mechanisms by which myopodin contributes to actin reorganization during myofibril assembly.

Tropomodulins: pointed-end capping proteins that regulate actin filament architecture in diverse cell types

Tropomodulins are a family of four proteins (Tmods 1-4) that cap the pointed ends of actin filaments in actin cytoskeletal structures in a developmentally regulated and tissue-specific manner. Unique among capping proteins, Tmods also bind tropomyosins (TMs), which greatly enhance the actin filament pointed-end capping activity of Tmods. Tmods are defined by a TM-regulated/Pointed-End Actin Capping (TM-Cap) domain in their unstructured N-terminal portion, followed by a compact, folded Leucine-Rich Repeat/Pointed-End Actin Capping (LRR-Cap) domain. By inhibiting actin monomer association and dissociation from pointed ends, Tmods regulate actin dynamics and turnover, stabilizing actin filament lengths and cytoskeletal architecture. In this review, we summarize the genes, structural features, molecular and biochemical properties, actin regulatory mechanisms, expression patterns, and cell and tissue functions of Tmods. By understanding Tmods' functions in the context of their molecular structure, actin regulation, binding partners, and related variants (leiomodins 1-3), we can draw broad conclusions that can explain the diverse morphological and functional phenotypes that arise from Tmod perturbation experiments in vitro and in vivo. Tmod-based stabilization and organization of intracellular actin filament networks provide key insights into how the emergent properties of the actin cytoskeleton drive tissue morphogenesis and physiology.

Tropomodulin capping of actin filaments in striated muscle development and physiology

Efficient striated muscle contraction requires precise assembly and regulation of diverse actin filament systems, most notably the sarcomeric thin filaments of the contractile apparatus. By capping the pointed ends of actin filaments, tropomodulins (Tmods) regulate actin filament assembly, lengths, and stability. Here, we explore the current understanding of the expression patterns, localizations, and functions of Tmods in both cardiac and skeletal muscle. We first describe the mechanisms by which Tmods regulate myofibril assembly and thin filament lengths, as well as the roles of closely related Tmod family variants, the leiomodins (Lmods), in these processes. We also discuss emerging functions for Tmods in the sarcoplasmic reticulum. This paper provides abundant evidence that Tmods are key structural regulators of striated muscle cytoarchitecture and physiology.

Identification of residues within tropomodulin-1 responsible for its localization at the pointed ends of the actin filaments in cardiac myocytes

Tropomodulin is a tropomyosin-dependent actin filament capping protein involved in the structural formation of thin filaments and in the regulation of their lengths through its localization at the pointed ends of actin filaments. The disordered N-terminal domain of tropomodulin contains three functional sites: two tropomyosin-binding and one tropomyosin-dependent actin-capping sites. The C-terminal half of tropomodulin consists of one compact domain containing a tropomyosin-independent actin-capping site. Here we determined the structural properties of tropomodulin-1 that affect its roles in cardiomyocytes. To explore the significance of individual tropomyosin-binding sites, GFP-tropomodulin-1 with single mutations that destroy each tropomyosin-binding site was expressed in cardiomyocytes. We demonstrated that both sites are necessary for the optimal localization of tropomodulin-1 at thin filament pointed ends, with site 2 acting as the major determinant. To investigate the functional properties of the tropomodulin C-terminal domain, truncated versions of GFP-tropomodulin-1 were expressed in cardiomyocytes. We discovered that the leucine-rich repeat (LRR) fold and the C-terminal helix are required for its proper targeting to the pointed ends. To investigate the structural significance of the LRR fold, we generated three mutations within the C-terminal domain (V232D, F263D, and L313D). Our results show that these mutations affect both tropomyosin-independent actin-capping activity and pointed end localization, most likely by changing local conformations of either loops or side chains of the surfaces involved in the interactions of the LRR domain. Studying the influence of these mutations individually, we concluded that, in addition to the tropomyosin-independent actin-capping site, there appears to be another regulatory site within the tropomodulin C-terminal domain.

Leiomodin-2 is an antagonist of tropomodulin-1 at the pointed end of the thin filaments in cardiac muscle

Regulation of actin filament assembly is essential for efficient contractile activity in striated muscle. Leiomodin is an actin-binding protein and homolog of the pointed-end capping protein, tropomodulin. These proteins are structurally similar, sharing a common domain organization that includes two actin-binding sites. Leiomodin also contains a unique C-terminal extension that has a third actin-binding WH2 domain. Recently, the striated-muscle-specific isoform of leiomodin (Lmod2) was reported to be an actin nucleator in cardiomyocytes. Here, we have identified a function of Lmod2 in the regulation of thin filament lengths. We show that Lmod2 localizes to the pointed ends of thin filaments, where it competes for binding with tropomodulin-1 (Tmod1). Overexpression of Lmod2 results in loss of Tmod1 assembly and elongation of the thin filaments from their pointed ends. The Lmod2 WH2 domain is required for lengthening because its removal results in a molecule that caps the pointed ends similarly to Tmod1. Furthermore, Lmod2 transcripts are first detected in the heart after it has begun to beat, suggesting that the primary function of Lmod2 is to maintain thin filament lengths in the mature heart. Thus, Lmod2 antagonizes the function of Tmod1, and together, these molecules might fine-tune thin filament lengths.

Capping complex formation at the slow-growing end of the actin filament

Actin filaments are polar; their barbed (fast-growing) and pointed (slow-growing) ends differ in structure and dynamic properties. The slow-growing end is regulated by tropomodulins, a family of capping proteins that require tropomyosins for optimal function. There are four tropomodulin isoforms; their distributions vary depending on tissue type and change during development. The C-terminal half of tropomodulin contains one compact domain represented by alternating alpha-helices and beta-structures. The tropomyosin-independent actin-capping site is located at the C-terminus. The N-terminal half has no regular structure; however, it contains a tropomyosin-dependent actin-capping site and two tropomyosin-binding sites. One tropomodulin molecule can bind two tropomyosin molecules. Effectiveness of tropomodulin binding to tropomyosin depends on the tropomyosin isoform. Regulation of tropomodulin binding at the pointed end as well as capping effectiveness in the presence of specific tropomyosins may affect formation of local cytoskeleton and dynamics of actin filaments in cells.

Leiomodin/tropomyosin interactions are isoform specific

Leiomodins are larger homologs of tropomodulin, a tropomyosin-binding, actin-capping protein. There are several leiomodin isoforms, one of them found in smooth muscles (Lmod1) and another one found in cardiac and skeletal muscles (Lmod2). In this work, the tropomyosin-binding abilities of these two isoforms were studied. The tropomyosin-binding sites were localized in the N-terminal regions of Lmod1 and Lmod2. The affinities of the leiomodin fragments containing the tropomyosin-binding sites for tropomyosin peptides containing N-termini of different tropomyosin isoforms, alpha, gamma and delta, were determined and compared using non-denaturing gel-electrophoresis and circular dichroism. It was shown that leiomodin/tropomyosin binding is isoform-specific and differs almost 100-fold for different tropomyosin isoforms.

Molecular basis of tropomyosin binding to tropomodulin, an actin-capping protein

The tropomodulin (Tmod) family of proteins that cap the pointed, slow-growing end of actin filaments require tropomyosin (TM) for optimal function. Earlier studies identified two regions in Tmod1 that bind the N terminus of TM, though the ability of different isoforms to bind the two sites is controversial. We used model peptides to determine the affinity and define the specificity of the highly conserved N termini of three short, non-muscle TMs (alpha, gamma, delta-TM) for the two Tmod1 binding sites using circular dichroism spectroscopy, native gel electrophoresis, and chemical crosslinking. All TM peptides have high affinity for the second Tmod1 binding site (within residues 109-144; alpha-TM, 2.5 nM; gamma-TM, delta-TM, 40-90 nM), but differ >100-fold for the first site (residues 1-38; alpha-TM, 90 nM; undetectable at 10 microM, gamma-TM, delta-TM). Residue 14 (R in alpha; Q in gamma and delta) and, to a lesser extent, residue 4 (S in alpha; T in gamma and delta) are primarily responsible for the differences. The functional consequence of the sequence differences is reflected in more effective inhibition of actin filament elongation by full-length alpha-TMs than gamma-TM in the presence of Tmod1. The binding sites of the two Tmod1 peptides on a model TM peptide differ, as defined by comparing (15)N,(1)H HSQC spectra of a (15)N-labeled model TM peptide in both the absence and presence of Tmod1 peptide. The NMR and CD studies show that there is an increase in alpha-helix upon Tmod1-TM complex formation, indicating that intrinsically disordered regions of the two proteins become ordered upon binding. A model proposed for the binding of Tmod to actin and TM at the pointed end of the filament shows how the Tmod-TM accentuates the asymmetry of the pointed end and suggests how subtle differences among TM isoforms may modulate actin filament dynamics.

Tropomodulin binds two tropomyosins: a novel model for actin filament capping

Tropomodulin, a tropomyosin-binding protein, caps the slow-growing (pointed) end of the actin filament regulating its dynamics. Tropomodulin, therefore, is important for determining cell morphology, cell movement, and muscle contraction. For the first time we show that one tropomodulin molecule simultaneously binds two tropomyosin molecules in a cooperative manner. On the basis of the tropomodulin solution structure and predicted secondary structure, we introduced a series of point mutations in regions important for tropomyosin binding and actin capping. Capping activity of these mutants was assayed by measuring actin polymerization using pyrene fluorescence. Using direct methods (circular dichroism and native gel electrophoresis) for detecting tropomodulin/tropomyosin binding, we localized the second tropomyosin-binding site to residues 109-144. Despite previous reports that the second binding site is for erythrocyte tropomyosin only, we found that both short nonmuscle and long muscle alpha-tropomyosins bind there as well, though with different affinities. We propose a model for actin capping where one tropomodulin molecule can bind to two tropomyosin molecules at the pointed end.

Tropomodulin 3 binds to actin monomers

Regulation of the actin cytoskeleton by filament capping proteins is critical to myriad dynamic cellular functions. The ability of these proteins to bind both filaments as well as monomers is often central to their cellular functions. The ubiquitous pointed end capping protein Tmod3 (tropomodulin 3) acts as a negative regulator of cell migration, yet mechanisms behind its cellular functions are not understood. Analysis of Tmod3 effects on kinetics of actin polymerization and steady state monomer levels revealed that Tmod3, unlike previously characterized tropomodulins, sequesters actin monomers with an affinity similar to its affinity for capping pointed ends. Furthermore, Tmod3 is found bound to actin in high speed supernatant cytosolic extracts, suggesting that Tmod3 can bind to monomers in the context of other cytosolic monomer binding proteins. The Tmod3-actin complex can be efficiently cross-linked with 1-ethyl-3-(dimethylaminopropyl)carbodiimide/N-hydroxylsulfosuccinimide in a 1:1 complex. Subsequent tryptic digestion and liquid chromatography/tandem mass spectrometry revealed two binding interfaces on actin, one distinct from other actin monomer binding proteins, and two potential binding sites in Tmod3, which are independent of the previously characterized leucine-rich repeat structure involved in pointed end capping. These data suggest that the Tmod3 isoform may regulate actin dynamics differently in cells than the previously described tropomodulin isoforms.

Structure and tropomyosin binding properties of the N-terminal capping domain of tropomodulin 1

Two families of actin regulatory proteins are the tropomodulins and tropomyosins. Tropomodulin binds to tropomyosin (TM) and to the pointed end of actin filaments and "caps" the pointed end (i.e., inhibits its polymerization and depolymerization). Tropomodulin 1 has two distinct actin-capping regions: a folded C-terminal domain (residues 160-359), which does not bind to TM, and a conserved, N-terminal region, within residues 1-92 that binds TM and requires TM for capping activity. NMR and circular dichroism were used to determine the structure of a peptide containing residues 1-92 of tropomodulin (Tmod1(1-92)) and to define its TM binding site. Tmod1(1-92) is mainly disordered with only one helical region, residues 24-35. This helix forms part of the TM binding domain, residues 1-35, which become more ordered upon binding a peptide containing the N-terminus of an alpha-TM. Mutation of L27 to E or G in the Tmod helix reduces TM affinity. Residues 49-92 are required for capping but do not bind TM. Of these, residues 67-75 have the sequence of an amphipathic helix, but are not helical. Residues 55-62 and 76-92 display negative 1H-15N heteronuclear Overhauser enhancements showing they are flexible. The conformational dynamics of these residues may be important for actin capping activity.

Xin repeats define a novel actin-binding motif

Xin is a protein that is expressed during early developmental stages of cardiac and skeletal muscles. Immunolocalization studies indicated a peripheral localization in embryonic mouse heart, where Xin localizes with beta-catenin and N-cadherin. In adult tissues, Xin is found primarily in the intercalated discs of cardiomyocytes and the myotendinous junctions of skeletal muscle cells, both specialized attachment sites of the myofibrillar ends to the sarcolemma. A large part of the Xin protein consists of unique 16 amino acid repeats with unknown function. We have investigated the characteristics of the Xin repeats by transfection experiments and actin-binding assays and ascertained that, upon expression in cultured cells, these repeats bind to and stabilize the actin-based cytoskeleton. In vitro co-sedimentation assays with skeletal muscle actin indicated that they not only directly bind actin filaments, but also have the capability of arranging microfilaments into networks that sediment upon low-speed centrifugation. Very similar repeats were also found in 'Xin-repeat protein 2' (XIRP2), a novel protein that seems to be expressed mainly in striated muscles. Human XIRP2 contains 28 Xin repeats with properties identical to those of Xin. We conclude that the Xin repeats define a novel, repetitive actin-binding motif present in at least two different muscle proteins. These Xin-repeat proteins therefore constitute the first two members of a novel family of actin-binding proteins.

Effect of the Structure of the N Terminus of Tropomyosin on Tropomodulin Function

Tropomodulins (Tmod) bind to the N terminus of tropomyosin and cap the pointed end of actin filaments. Tropomyosin alone also inhibits the rate of actin depolymerization at the pointed end of filaments. Here we have defined 1) the structural requirements of the N terminus of tropomyosin important for regulating the pointed end alone and with erythrocyte Tmod (Tmod1), and 2) the Tmod1 subdomains required for binding to tropomyosin and for regulating the pointed end. Changes in pyrene-actin fluorescence during polymerization and depolymerization were measured with actin filaments blocked at the barbed end with gelsolin. Three tropomyosin isoforms differently influence pointed end dynamics. Recombinant TM5a, a short non-muscle alpha-tropomyosin, inhibited depolymerization. Recombinant (unacetylated) TM2 and N-acetylated striated muscle TM (stTM), long alpha-tropomyosin isoforms with the same N-terminal sequence, different from TM5a, also inhibited depolymerization but were less effective than TM5a. All blocked the pointed end with Tmod1 in the order of effectiveness TM5a >stTM >TM2, showing the importance of the N-terminal sequence and modification. Tmod1-(1-344), lacking the C-terminal 15 residues, did not nucleate polymerization but blocked the pointed end with all three tropomyosin isoforms as does a shorter fragment, Tmod1-(1-92), lacking the C-terminal "capping" domain though higher concentrations were required. An even shorter fragment, Tmod1-(1-48), bound tropomyosin but did not influence actin filament elongation. Tropomyosin-Tmod may function to locally regulate cytoskeletal dynamics in cells by stabilizing actin filaments.

cDNA Microarray Gene Expression Analysis and Toxicological Phenotype for Anticancer Drug

Toxicogenomics, the subdiscipline that merges genomics with toxicology, hold the promise to contributing toward the goal of elucidating mechanism by studying genomic profiling related with various drugs. The application of gene expression profiling technology to examine multiple genes and signaling pathways promises a significant advance in understanding the toxic mechanisms of various drugs and prediction of new drug candidate. Toxicogenomics is emerging field combining genomics and bioinformatics to identify and characterize mechanisms of toxicity of drug and various compounds. The principal hypothesis underlying on this field is that chemical-specific pattern of altered gene expression is related with each chemicals properties, especially toxicological property, and it will be revealed using high-density microarray analysis of sample from exposed organisms. So, in this study we compare the gene expression pattern of two anticancer drugs paclitaxel and orally absorbable paclitaxel, using the cDNA microarray. And from the result of this study, it is possible to provide the new possibility for genome-wide insight into mechanism of their anticancer activity and toxicological phenotype.

The interaction between E-tropomodulin and thymosin β-10 rescues tumor cells from thymosin β-10 mediated apoptosis by restoring actin architecture

Thymosin beta-10 (TB10) is a small G-actin binding protein that induces depolymerization of intracellular F-actin pools by sequestering actin monomers. Previously, we demonstrated that overexpression of TB10 in ovarian tumor cells increased the rate of cell death. As an initial step to define molecular mechanism of TB10-dependent apoptotic process in ovarian tumor cells, we searched a human ovary cDNA library for a novel TB10 binding protein using a yeast two-hybrid system. The selected protein was human E-tropomodulin (E-Tmod), another component of the actin binding proteins. Subsequently, two interacting protein components were determined quantitatively. Results showed that the full-length TB10 is required to bind with E-Tmod, and the TB10 binding site on E-Tmod partially overlaps with the actin binding site on E-Tmod. Moreover, introduction of E-Tmod cDNA into a tumor cell line reversed TB10 mediated apoptosis and restored actin architectures. These results may suggest that TB10 regulates apoptotic homeostasis by not only just binding to actin but also competing or blocking the protein complex formation of E-Tmod with actin.

Tropomodulin contains two actin filament pointed end-capping domains

Tropomodulin 1 (Tmod1) is a approximately 40-kDa tropomyosin binding and actin filament pointed end-capping protein that regulates pointed end dynamics and controls thin filament length in striated muscle. In vitro, the capping affinity of Tmod1 for tropomyosin-actin filaments (Kd approximately 50 pm) is several thousand-fold greater than for capping of pure actin filaments (Kd approximately 0.1 microM). The tropomyosin-binding region of Tmod1 has been localized to the amino-terminal portion between residues 1 and 130, but the location of the actin-capping domain is not known. We have now identified two distinct actin-capping regions on Tmod1 by testing a series of recombinant Tmod1 fragments for their ability to inhibit actin elongation from gelsolin-actin seeds using pyrene-actin polymerization assays. The carboxyl-terminal portion of Tmod1 (residues 160-359) contains the principal actin-capping activity (Kd approximately 0.4 microM), requiring residues between 323 and 359 for full activity, whereas the amino-terminal portion of Tmod1 (residues 1-130) contains a second, weaker actin-capping activity (Kd approximately 1.8 microM). Interestingly, 160-359 but not 1-130 enhances spontaneous actin nucleation, suggesting that the carboxyl-terminal domain may bind to two actin subunits across the actin helix at the pointed end, whereas the amino-terminal domain may bind to only one actin subunit. On the other hand, the actin-capping activity of the amino-terminal but not the carboxyl-terminal portion of Tmod1 is enhanced several thousand-fold in the presence of skeletal muscle tropomyosin. We conclude that the carboxyl-terminal capping domain of Tmod1 contains a TM-independent actin pointed end-capping activity, whereas the amino-terminal domain contains a TM-regulated pointed end actin-capping activity.

The interaction of tropomodulin with tropomyosin stabilizes thin filaments in cardiac myocytes

Actin (thin) filament length regulation and stability are essential for striated muscle function. To determine the role of the actin filament pointed end capping protein, tropomodulin1 (Tmod1), with tropomyosin, we generated monoclonal antibodies (mAb17 and mAb8) against Tmod1 that specifically disrupted its interaction with tropomyosin in vitro. Microinjection of mAb17 or mAb8 into chick cardiac myocytes caused a dramatic loss of the thin filaments, as revealed by immunofluorescence deconvolution microscopy. Real-time imaging of live myocytes expressing green fluorescent protein-alpha-tropomyosin and microinjected with mAb17 revealed that the thin filaments depolymerized from their pointed ends. In a thin filament reconstitution assay, stabilization of the filaments before the addition of mAb17 prevented the loss of thin filaments. These studies indicate that the interaction of Tmod1 with tropomyosin is critical for thin filament stability. These data, together with previous studies, indicate that Tmod1 is a multifunctional protein: its actin filament capping activity prevents thin filament elongation, whereas its interaction with tropomyosin prevents thin filament depolymerization.

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

Identification of the putrescine biosynthetic genes in Pseudomonas aeruginosa and characterization of agmatine deiminase and N-carbamoylputrescine amidohydrolase of the arginine decarboxylase pathway

Putrescine can be synthesized either directly from ornithine by ornithine decarboxylase (ODC; the speC product) or indirectly from arginine via arginine decarboxylase (ADC; the speA product). The authors identified the speA and speC genes in Pseudomonas aeruginosa PAO1. The activities of the two decarboxylases were similar and each enzyme alone appeared to direct sufficient formation of the polyamine for normal growth. A mutant defective in both speA and speC was a putrescine auxotroph. In this strain, agmatine deiminase (the aguA product) and N-carbamoylputrescine amidohydrolase (the aguB product), which were initially identified as the catabolic enzymes of agmatine, biosynthetically convert agmatine to putrescine in the ADC pathway: a double mutant of aguAB and speC was a putrescine auxotroph. AguA was purified as a homodimer of 43 kDa subunits and AguB as a homohexamer of 33 kDa subunits. AguA specifically deiminated agmatine with K(m) and K(cat) values of 0.6 mM and 4.2 s(-1), respectively. AguB was specific to N-carbamoylputrescine and the K(m) and K(cat) values of the enzyme for the substrate were 0.5 mM and 3.3 s(-1), respectively. Whereas AguA has no structural relationship to any known C-N hydrolases, AguB is a protein of the nitrilase family that performs thiol-assisted catalysis. Inhibition by SH reagents and the conserved cysteine residue in AguA and its homologues suggested that this enzyme is also involved in thiol-mediated catalysis.

Crystal structure of the C-terminal half of tropomodulin and structural basis of actin filament pointed-end capping

Tropomodulin is the unique pointed-end capping protein of the actin-tropomyosin filament. By blocking elongation and depolymerization, tropomodulin regulates the architecture and the dynamics of the filament. Here we report the crystal structure at 1.45-A resolution of the C-terminal half of tropomodulin (C20), the actin-binding moiety of tropomodulin. C20 is a leucine-rich repeat domain, and this is the first actin-associated protein with a leucine-rich repeat. Binding assays suggested that C20 also interacts with the N-terminal fragment, M1-M2-M3, of nebulin. Based on the crystal structure, we propose a model for C20 docking to the actin subunit at the pointed end. Although speculative, the model is consistent with the idea that a tropomodulin molecule competes with an actin subunit for a pointed end. The model also suggests that interactions with tropomyosin, actin, and nebulin are all possible sources of influences on the dynamic properties of pointed-end capping by tropomodulin.

Tropomyosin requires an intact N-terminal coiled coil to interact with tropomodulin

Tropomodulins (Tmods) are tropomyosin (TM) binding proteins that bind to the pointed end of actin filaments and modulate thin filament dynamics. They bind to the N termini of both "long" TMs (with the N terminus encoded by exon 1a of the alpha-TM gene) and "short" nonmuscle TMs (with the N terminus encoded by exon 1b). In this present study, circular dichroism was used to study the interaction of two designed chimeric proteins, AcTM1aZip and AcTM1bZip, containing the N terminus of a long or a short TM, respectively, with protein fragments containing residues 1 to 130 of erythrocyte or skeletal muscle Tmod. The binding of either TMZip causes similar conformational changes in both Tmod fragments promoting increases in both alpha-helix and beta-structure, although they differ in binding affinity. The circular dichroism changes in the Tmod upon binding and modeling of the Tmod sequences suggest that the interface between TM and Tmod includes a three- or four-stranded coiled coil. An intact coiled coil at the N terminus of the TMs is essential for Tmod binding, as modifications that disrupt the N-terminal helix, such as removal of the N-terminal acetyl group from AcTM1aZip or striated muscle alpha-TM, or introduction of a mutation that causes nemaline myopathy, Met-8-Arg, into AcTM1aZip destroyed Tmod binding.

Natively unfolded proteins: a point where biology waits for physics

The experimental material accumulated in the literature on the conformational behavior of intrinsically unstructured (natively unfolded) proteins was analyzed. Results of this analysis showed that these proteins do not possess uniform structural properties, as expected for members of a single thermodynamic entity. Rather, these proteins may be divided into two structurally different groups: intrinsic coils, and premolten globules. Proteins from the first group have hydrodynamic dimensions typical of random coils in poor solvent and do not possess any (or almost any) ordered secondary structure. Proteins from the second group are essentially more compact, exhibiting some amount of residual secondary structure, although they are still less dense than native or molten globule proteins. An important feature of the intrinsically unstructured proteins is that they undergo disorder-order transition during or prior to their biological function. In this respect, the Protein Quartet model, with function arising from four specific conformations (ordered forms, molten globules, premolten globules, and random coils) and transitions between any two of the states, is discussed.

Folding properties of functional domains of tropomodulin

Tropomodulin (Tmod) stabilizes the actin-tropomyosin filament by capping the slow-growing end (P-end). The N- and C-terminal halves play distinct roles; the N-terminal half interacts with the N-terminal region of tropomyosin, whereas the C-terminal half interacts with actin. Our previous study (A. Kostyukova, K. Maeda, E. Yamauchi, I. Krieger, and Y. Maéda Y., 2000, Eur. J. Biochem. 267:6470-6475) suggested that the two halves are also structurally distinct from each other. We have now studied the folding properties of the two halves, by circular dichroism spectroscopy and by differential scanning calorimetry of the expressed chicken E-type tropomodulin and its large fragments. The results showed that the C-terminal half represents a single, independently folded unit that melts cooperatively through a two-state transition. In contrast, the N-terminal half lacks a definite tertiary structure in solution. The binding of N11, a fragment that corresponds to the first 91 amino acids of the tropomodulin, to tropomyosin substantially stabilized the tropomyosin. This may indicate that the flexible structure of the N-terminal half of tropomodulin in solution is required for binding to tropomyosin and that the N-terminal half acquires its tertiary structure upon binding to tropomyosin.

The shapes and sizes of two domains of tropomodulin, the P-end-capping protein of actin-tropomyosin

Tropomodulin, the P-end (slow-growing end)-capping protein of the actin-tropomyosin filament, and its fragment (C20) of the C-terminal half were studied by synchrotron small-angle X-ray scattering, restoring low-resolution shapes using an ab initio shape-determining procedure. Tropomodulin is elongated (115 A long) and consists of two domains, one of 65 A in length and the other being similar to C20 in shape and size if the long axes of the two are tilted by about 40 degrees relative to each other. We propose a model for tropomodulin in association with tropomyosin and actin: the N-terminal half of tropomodulin, a rod, binds to the N-terminus of tropomyosin and the C-terminal triangle domain protrudes from the P-end being slightly bent towards the actin subunit at the end, thereby blocking the P-end.