Cap Z(36/32), a barbed end actin-capping protein, is a component of the Z-line of skeletal muscle

Actin-capping protein regulates actomyosin contractility to maintain germline architecture in C. elegans

Actin dynamics play an important role in tissue morphogenesis, yet the control of actin filament growth takes place at the molecular level. A challenge in the field is to link the molecular function of actin regulators with their physiological function. Here, we report an in vivo role of the actin-capping protein CAP-1 in the Caenorhabditis elegans germline. We show that CAP-1 is associated with actomyosin structures in the cortex and rachis, and its depletion or overexpression led to severe structural defects in the syncytial germline and oocytes. A 60% reduction in the level of CAP-1 caused a twofold increase in F-actin and non-muscle myosin II activity, and laser incision experiments revealed an increase in rachis contractility. Cytosim simulations pointed to increased myosin as the main driver of increased contractility following loss of actin-capping protein. Double depletion of CAP-1 and myosin or Rho kinase demonstrated that the rachis architecture defects associated with CAP-1 depletion require contractility of the rachis actomyosin corset. Thus, we uncovered a physiological role for actin-capping protein in regulating actomyosin contractility to maintain reproductive tissue architecture.

Functional requirements for a Samd14-capping protein complex in stress erythropoiesis

Acute anemia induces rapid expansion of erythroid precursors and accelerated differentiation to replenish erythrocytes. Paracrine signals—involving cooperation between stem cell factor (SCF)/Kit signaling and other signaling inputs—are required for the increased erythroid precursor activity in anemia. Our prior work revealed that the sterile alpha motif (SAM) domain 14 (Samd14) gene increases the regenerative capacity of the erythroid system in a mouse genetic model and promotes stress-dependent Kit signaling. However, the mechanism underlying Samd14’s role in stress erythropoiesis is unknown. We identified a protein-protein interaction between Samd14 and the α- and β-heterodimers of the F-actin capping protein (CP) complex. Knockdown of the CP β subunit increased erythroid maturation in murine ex vivo cultures and decreased colony forming potential of stress erythroid precursors. In a genetic complementation assay for Samd14 activity, our results revealed that the Samd14-CP interaction is a determinant of erythroid precursor cell levels and function. Samd14-CP promotes SCF/Kit signaling in CD71^med spleen erythroid precursors. Given the roles of Kit signaling in hematopoiesis and Samd14 in Kit pathway activation, this mechanism may have pathological implications in acute/chronic anemia.

Anemia is a condition in which the body has a shortage of healthy red blood cells to carry enough oxygen to support its organs. A range of factors are known to cause anemia, including traumatic blood loss, toxins or nutritional deficiency. An estimated one-third of all women of reproductive age are anemic, which can cause tiredness, weakness and shortness of breath. Severe anemia drives the release of hormones and growth factors, leading to a rapid regeneration of precursor red blood cells to replenish the supply in the blood.

To understand how red blood cell regeneration is controlled, Ray et al. studied proteins involved in regenerating blood using mice in which anemia had been induced with chemicals. Previous research had shown that the protein Samd14 is produced at higher quantities in individuals with anemia, and is involved with the recovery of lost red blood cells. However, it is not known how the Samd14 protein plays a role in regenerating blood cells, or whether Samd14 interacts with other proteins required for red blood cell production.

To shed light on these questions, mouse cells exposed to anemia conditions were used to see what proteins Samd14 binds to. Purifying Samd14 revealed that it interacts with the actin capping protein. This interaction relies on a specific region of Samd14 that is similar to regions in other proteins that bind capping proteins. Ray et al. found that the interaction between Samd14 and the actin capping protein increased the signals needed for the development and survival of new red blood cells.

These results identify a signaling mechanism that, if disrupted, could cause anemia to develop. They lead to a better understanding of how our bodies recover from anemia, and potential avenues to treat this condition.

Thick filament-associated myosin undergoes frequent replacement at the tip of the thick filament

Myosin plays a fundamental role in muscle contraction. Approximately 300 myosins form a bipolar thick filament, in which myosin is continuously replaced by protein turnover. However, it is unclear how rapidly this process occurs and whether the myosin exchange rate differs depending on the region of the thick filament. To answer this question, we first measured myosin release and insertion rates over a short period and monitored myotubes expressing a photoconvertible fluorescence protein-tagged myosin, which enabled us to monitor myosin release and insertion simultaneously. About 20% of myosins were replaced within 10 min, while 70% of myosins were exchanged over 10 h with symmetrical and biphasic alteration of myosin release and insertion rates. Next, a fluorescence pulse-chase assay was conducted to investigate whether myosin is incorporated into specific regions in the thick filament. Newly synthesized myosin was located at the tip of the thick filament rather than the center in the first 7 min of pulse-chase labeling and was observed in the remainder of the thick filament by 30 min. These results suggest that the myosin replacement rate differs depending on the regions of the thick filament. We concluded that myosin release and insertion occur concurrently and that myosin is more frequently exchanged at the tip of the thick filament.

Cofilin Loss in Drosophila Muscles Contributes to Muscle Weakness through Defective Sarcomerogenesis during Muscle Growth

Sarcomeres, the fundamental contractile units of muscles, are conserved structures composed of actin thin filaments and myosin thick filaments. How sarcomeres are formed and maintained is not well understood. Here, we show that knockdown of Drosophila cofilin (DmCFL), an actin depolymerizing factor, disrupts both sarcomere structure and muscle function. The loss of DmCFL also results in the formation of sarcomeric protein aggregates and impairs sarcomere addition during growth. The activation of the proteasome delays muscle deterioration in our model. Furthermore, we investigate how a point mutation in CFL2 that causes nemaline myopathy (NM) in humans affects CFL function and leads to the muscle phenotypes observed in vivo. Our data provide significant insights to the role of CFLs during sarcomere formation, as well as mechanistic implications for disease progression in NM patients.

How sarcomeres are added and maintained in a growing muscle cell is unclear. Balakrishnan et al. observed that DmCFL loss in growing muscles affects sarcomere size and addition through unregulated actin polymerization. This results in a collapse of sarcomere and muscle structure, formation of large protein aggregates, and muscle weakness.

Towards a structural understanding of the remodeling of the actin cytoskeleton

Actin filaments (F-actin) are a key component of eukaryotic cells. Whether serving as a scaffold for myosin or using their polymerization to push onto cellular components, their function is always related to force generation. To control and fine-tune force production, cells have a large array of actin-binding proteins (ABPs) dedicated to control every aspect of actin polymerization, filament localization, and their overall mechanical properties. Although great advances have been made in our biochemical understanding of the remodeling of the actin cytoskeleton, the structural basis of this process is still being deciphered. In this review, we summarize our current understanding of this process. We outline how ABPs control the nucleation and disassembly, and how these processes are affected by the nucleotide state of the filaments. In addition, we highlight recent advances in the understanding of actomyosin force generation, and describe recent advances brought forward by the developments of electron cryomicroscopy.

Cofilin-1 phosphorylation catalyzed by ERK1/2 alters cardiac actin dynamics in dilated cardiomyopathy caused by lamin A/C gene mutation

Hyper-activation of extracellular signal-regulated kinase (ERK) 1/2 contributes to heart dysfunction in cardiomyopathy caused by mutations in the lamin A/C gene (LMNA cardiomyopathy). The mechanism of how this affects cardiac function is unknown. We show that active phosphorylated ERK1/2 directly binds to and catalyzes the phosphorylation of the actin depolymerizing factor cofilin-1 on Thr25. Cofilin-1 becomes active and disassembles actin filaments in a large array of cellular and animal models of LMNA cardiomyopathy. In vivo expression of cofilin-1, phosphorylated on Thr25 by endogenous ERK1/2 signaling, leads to alterations in left ventricular function and cardiac actin. These results demonstrate a novel role for cofilin-1 on actin dynamics in cardiac muscle and provide a rationale on how increased ERK1/2 signaling leads to LMNA cardiomyopathy.

Actin-associated proteins and cardiomyopathy-the 'unknown' beyond troponin and tropomyosin

It has been known for several decades that mutations in genes that encode for proteins involved in the control of actomyosin interactions such as the troponin complex, tropomyosin and MYBP-C and thus regulate contraction can lead to hereditary hypertrophic cardiomyopathy. In recent years, it has become apparent that actin-binding proteins not directly involved in the regulation of contraction also can exhibit changed expression levels, show altered subcellular localisation or bear mutations that might lead to hereditary cardiomyopathies. The aim of this review is to look beyond the troponin/tropomyosin mechanism and to give an overview of the different types of actin-associated proteins and their potential roles in cardiomyocytes. It will then discuss recent findings relevant to their involvement in heart disease.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

MoCAP proteins regulated by MoArk1-mediated phosphorylation coordinate endocytosis and actin dynamics to govern development and virulence of Magnaporthe oryzae

Actin organization is a conserved cellular process that regulates the growth and development of eukaryotic cells. It also governs the virulence process of pathogenic fungi, such as the rice blast fungus Magnaporthe oryzae, with mechanisms not yet fully understood. In a previous study, we found that actin-regulating kinase MoArk1 displays conserved functions important in endocytosis and actin organization, and MoArk1 is required for maintaining the growth and full virulence of M. oryzae. To understand how MoArk1 might function, we identified capping protein homologs from M. oryzae (MoCAP) that interact with MoArk1 in vivo. MoCAP is heterodimer consisting of α and β subunits MoCapA and MoCapB. Single and double deletions of MoCAP subunits resulted in abnormal mycelial growth and conidia formation. The ΔMocap mutants also exhibited reduced appressorium penetration and invasive hyphal growth within host cells. Furthermore, the ΔMocap mutants exhibited delayed endocytosis and abnormal cytoskeleton assembly. Consistent with above findings, MoCAP proteins interacted with MoAct1, co-localized with actin during mycelial development, and participated in appressorial actin ring formation. Further analysis revealed that the S85 residue of MoCapA and the S285 residue of MoCapB were subject to phosphorylation by MoArk1 that negatively regulates MoCAP functions. Finally, the addition of exogenous phosphatidylinositol 4,5-bisphosphate (PIP₂) failed to modulate actin ring formation in ΔMocap mutants, in contrast to the wild-type strain, suggesting that MoCAP may also mediate phospholipid signaling in the regulation of the actin organization. These results together demonstrate that MoCAP proteins whose functions are regulated by MoArk1 and PIP₂ are important for endocytosis and actin dynamics that are directly linked to growth, conidiation and pathogenicity of M. oryzae.

The actin-regulating kinase MoArk1 plays a conserved function in endocytosis and actin organization and is also essential for growth and full virulence of the rice blast fungus Magnaporthe oryzae. To understand how MoArk1 functions, we identified the F-actin capping protein α (MoCapA) and β (MoCapB) subunits that interact with MoArk1. We showed that single and double deletions of MoCAPA and MoCAPB result in slowed growth, reduced conidia production, abnormal morphogenesis, and attenuated virulence. We found that ΔMocap mutants are defective in endocytosis and actin organization and that MoCAP proteins are subject to regulation by MoArk1 through protein phosphorylation. Finally, we provided evidence demonstrating that MoCAP proteins modulate actin dynamics in response to phosphatidylinositol 4,5-biphosphate (PIP₂). These combined results suggest that MoCAP proteins play an important role in endocytosis, actin organization, and virulence. Further studies of MoCAP proteins could lead to a better understanding of the connections between actin organization and host infection by M. oryzae.

Myofibril growth during cardiac hypertrophy is regulated through dual phosphorylation and acetylation of the actin capping protein CapZ

The mechanotransduction signaling pathways initiated in heart muscle by increased mechanical loading are known to lead to long-term transcriptional changes and hypertrophy, but the rapid events for adaptation at the sarcomeric level are not fully understood. The goal of this study was to test the hypothesis that actin filament assembly during cardiomyocyte growth is regulated by post-translational modifications (PTMs) of CapZβ1. In rapidly hypertrophying neonatal rat ventricular myocytes (NRVMs) stimulated by phenylephrine (PE), two-dimensional gel electrophoresis (2DGE) of CapZβ1 revealed a shift toward more negative charge. Consistent with this, mass spectrometry identified CapZβ1 phosphorylation on serine-204 and acetylation on lysine-199, two residues which are near the actin binding surface of CapZβ1. Ectopic expression of dominant negative PKCɛ (dnPKCɛ) in NRVMs blunted the PE-induced increase in CapZ dynamics, as evidenced by the kinetic constant (Kfrap) of fluorescence recovery after photobleaching (FRAP), and concomitantly reduced phosphorylation and acetylation of CapZβ1. Furthermore, inhibition of class I histone deacetylases (HDACs) increased lysine-199 acetylation on CapZβ1, which increased Kfrap of CapZ and stimulated actin dynamics. Finally, we show that PE treatment of NRVMs results in decreased binding of HDAC3 to myofibrils, suggesting a signal-dependent mechanism for the regulation of sarcomere-associated CapZβ1 acetylation. Taken together, this dual regulation through phosphorylation and acetylation of CapZβ1 provides a novel model for the regulation of myofibril growth during cardiac hypertrophy.

Pivotal and distinct role for Plasmodium actin capping protein alpha during blood infection of the malaria parasite

Accurate regulation of microfilament dynamics is central to cell growth, motility and response to environmental stimuli. Stabilizing and depolymerizing proteins control the steady-state levels of filamentous (F-) actin. Capping protein (CP) binds to free barbed ends, thereby arresting microfilament growth and restraining elongation to remaining free barbed ends. In all CPs characterized to date, alpha and beta subunits form the active heterodimer. Here, we show in a eukaryotic parasitic cell that the two CP subunits can be functionally separated. Unlike the beta subunit, the CP alpha subunit of the apicomplexan parasite Plasmodium is refractory to targeted gene deletion during blood infection in the mammalian host. Combinatorial complementation of Plasmodium berghei CP genes with the orthologs from Plasmodium falciparum verified distinct activities of CP alpha and CP alpha/beta during parasite life cycle progression. Recombinant Plasmodium CP alpha could be produced in Escherichia coli in the absence of the beta subunit and the protein displayed F-actin capping activity. Thus, the functional separation of two CP subunits in a parasitic eukaryotic cell and the F-actin capping activity of CP alpha expand the repertoire of microfilament regulatory mechanisms assigned to CPs.

Capping protein regulators fine-tune actin assembly dynamics

Capping protein (CP) binds the fast growing barbed end of the actin filament and regulates actin assembly by blocking the addition and loss of actin subunits. Recent studies provide new insights into how CP and barbed-end capping are regulated. Filament elongation factors, such as formins and ENA/VASP (enabled/vasodilator-stimulated phosphoprotein), indirectly regulate CP by competing with CP for binding to the barbed end, whereas other molecules, including V-1 and phospholipids, directly bind to CP and sterically block its interaction with the filament. In addition, a diverse and unrelated group of proteins interact with CP through a conserved 'capping protein interaction' (CPI) motif. These proteins, including CARMIL (capping protein, ARP2/3 and myosin I linker), CD2AP (CD2-associated protein) and the WASH (WASP and SCAR homologue) complex subunit FAM21, recruit CP to specific subcellular locations and modulate its actin-capping activity via allosteric effects.

A two-segment model for thin filament architecture in skeletal muscle

Correct specification of myofilament length is essential for efficient skeletal muscle contraction. The length of thin actin filaments can be explained by a novel 'two-segment' model, wherein the thin filaments consist of two concatenated segments, which are of either constant or variable length. This is in contrast to the classic 'nebulin ruler' model, which postulates that thin filaments are uniform structures, the lengths of which are dictated by nebulin. The two-segment model implicates position-specific microregulation of actin dynamics as a general principle underlying actin filament length and stability.

Conservation and divergence between cytoplasmic and muscle-specific actin capping proteins: insights from the crystal structure of cytoplasmic Cap32/34 from Dictyostelium discoideum

BACKGROUND

Capping protein (CP), also known as CapZ in muscle cells and Cap32/34 in Dictyostelium discoideum, plays a major role in regulating actin filament dynamics. CP is a ubiquitously expressed heterodimer comprising an α- and β-subunit. It tightly binds to the fast growing end of actin filaments, thereby functioning as a "cap" by blocking the addition and loss of actin subunits. Vertebrates contain two somatic variants of CP, one being primarily found at the cell periphery of non-muscle tissues while the other is mainly localized at the Z-discs of skeletal muscles.

RESULTS

To elucidate structural and functional differences between cytoplasmic and sarcomercic CP variants, we have solved the atomic structure of Cap32/34 (32=β- and 34=α-subunit) from the cellular slime mold Dictyostelium at 2.2 Å resolution and compared it to that of chicken muscle CapZ. The two homologs display a similar overall arrangement including the attached α-subunit C-terminus (α-tentacle) and the flexible β-tentacle. Nevertheless, the structures exhibit marked differences suggesting considerable structural flexibility within the α-subunit. In the α-subunit we observed a bending motion of the β-sheet region located opposite to the position of the C-terminal β-tentacle towards the antiparallel helices that interconnect the heterodimer. Recently, a two domain twisting attributed mainly to the β-subunit has been reported. At the hinge of these two domains Cap32/34 contains an elongated and highly flexible loop, which has been reported to be important for the interaction of cytoplasmic CP with actin and might contribute to the more dynamic actin-binding of cytoplasmic compared to sarcomeric CP (CapZ).

CONCLUSIONS

The structure of Cap32/34 from Dictyostelium discoideum allowed a detailed analysis and comparison between the cytoplasmic and sarcomeric variants of CP. Significant structural flexibility could particularly be found within the α-subunit, a loop region in the β-subunit, and the surface of the α-globule where the amino acid differences between the cytoplasmic and sarcomeric mammalian CP are located. Hence, the crystal structure of Cap32/34 raises the possibility of different binding behaviours of the CP variants toward the barbed end of actin filaments, a feature, which might have arisen from adaptation to different environments.

Electron microscopic visualization of the filament binding mode of actin-binding proteins

A large number of actin-binding proteins (ABPs) regulate various kinds of cellular events in which the superstructure of the actin cytoskeleton is dynamically changed. Thus, to understand the actin dynamics in the cell, the mechanisms of actin regulation by ABPs must be elucidated. Moreover, it is particularly important to identify the side, barbed-end or pointed-end ABP binding sites on the actin filament. However, a simple, reliable method to determine the ABP binding sites on the actin filament is missing. Here, a novel electron microscopic method for determining the ABP binding sites is presented. This approach uses a gold nanoparticle that recognizes a histidine tag on an ABP and an image analysis procedure that can determine the polarity of the actin filament. This method will facilitate future study of ABPs.

Dynamic regulation of sarcomeric actin filaments in striated muscle

In striated muscle, the actin cytoskeleton is differentiated into myofibrils. Actin and myosin filaments are organized in sarcomeres and specialized for producing contractile forces. Regular arrangement of actin filaments with uniform length and polarity is critical for the contractile function. However, the mechanisms of assembly and maintenance of sarcomeric actin filaments in striated muscle are not completely understood. Live imaging of actin in striated muscle has revealed that actin subunits within sarcomeric actin filaments are dynamically exchanged without altering overall sarcomeric structures. A number of regulators for actin dynamics have been identified, and malfunction of these regulators often result in disorganization of myofibril structures or muscle diseases. Therefore, proper regulation of actin dynamics in striated muscle is critical for assembly and maintenance of functional myofibrils. Recent studies have suggested that both enhancers of actin dynamics and stabilizers of actin filaments are important for sarcomeric actin organization. Further investigation of the regulatory mechanism of actin dynamics in striated muscle should be a key to understanding how myofibrils develop and operate.

BAG3 and Hsc70 interact with actin capping protein CapZ to maintain myofibrillar integrity under mechanical stress

RATIONALE

A homozygous disruption or genetic mutation of the bag3 gene, a member of the Bcl-2-associated athanogene (BAG) family proteins, causes cardiomyopathy and myofibrillar myopathy that is characterized by myofibril and Z-disc disruption. However, the detailed disease mechanism is not yet fully understood.

OBJECTIVE

bag3(-/-) mice exhibit differences in the extent of muscle degeneration between muscle groups with muscles experiencing the most usage degenerating at an accelerated rate. Usage-dependent muscle degeneration suggests a role for BAG3 in supporting cytoskeletal connections between the Z-disc and myofibrils under mechanical stress. The mechanism by which myofibrillar structure is maintained under mechanical stress remains unclear. The purpose of the study is to clarify the detailed molecular mechanism of BAG3-mediated muscle maintenance under mechanical stress.

METHODS AND RESULTS

To address the question of whether bag3 gene knockdown induces myofibrillar disorganization caused by mechanical stress, in vitro mechanical stretch experiments using rat neonatal cardiomyocytes and a short hairpin RNA-mediated gene knockdown system of the bag3 gene were performed. As expected, mechanical stretch rapidly disrupts myofibril structures in bag3 knockdown cardiomyocytes. BAG3 regulates the structural stability of F-actin through the actin capping protein, CapZβ1, by promoting association between Hsc70 and CapZβ1. BAG3 facilitates the distribution of CapZβ1 to the proper location, and dysfunction of BAG3 induces CapZ ubiquitin-proteasome-mediated degradation. Inhibition of CapZβ1 function by overexpressing CapZβ2 increased myofibril vulnerability and fragmentation under mechanical stress. On the other hand, overexpression of CapZβ1 inhibits myofibrillar disruption in bag3 knockdown cells under mechanical stress. As a result, heart muscle isolated from bag3(-/-) mice exhibited myofibrillar degeneration and lost contractile activity after caffeine contraction.

CONCLUSIONS

These results suggest novel roles for BAG3 and Hsc70 in stabilizing myofibril structure and inhibiting myofibrillar degeneration in response to mechanical stress. These proteins are possible targets for further research to identify therapies for myofibrillar myopathy or other degenerative diseases.

Temporal proteomic analysis of IGF-1R signalling in MCF-7 breast adenocarcinoma cells

Dysregulation of the insulin-like growth factor 1 receptor signalling network is implicated in tumour growth and resistance to chemotherapy. We explored proteomic changes resulting from insulin-like growth factor 1 stimulation of MCF-7 adenocarcinoma cells as a function of time. Quantitative analysis using iTRAQ reagents and 2-D LC-MS/MS analysis of three biological replicates resulted in the identification of 899 proteins (p<or=0.05) with an estimated mean false-positive rate of 2.6%. Quantitative protein expression was obtained from 681 proteins. Further analysis by supervised k-means clustering identified five temporal clusters, which were submitted to the FuncAssociate server to assign overrepresented gene ontology terms. Proteins associated with vesicle transport were significantly overrepresented. We further analyzed our data set for proteins showing temporal significance using the software, extraction and analysis of differential gene expression, resulting in 20 significantly and temporally changing proteins (p<or=0.1). These significant proteins play roles in, among others, altered glucose metabolism (lactate dehydrogenase A and pyruvate kinase M1/M2) and cellular stress (nascent polypeptide-associated complex subunit alpha and heat shock (HSC70) proteins). We used multiple reaction monitoring to validate these interesting proteins and have revealed several differences in relative peptide expression corresponding to protein isoforms and variants.

Activity-dependent localization in spines of the F-actin capping protein CapZ screened in a rat model of dementia

Actin reorganization in dendritic spines is hypothesized to underlie neuronal plasticity. Actin-related proteins, therefore, might serve as useful markers of plastic changes in dendritic spines. Here, we utilized memory deficits induced by fimbria-fornix transection (FFT) in rats as a dementia model to screen candidate memory-associated molecules by using a two-dimensional gel method. Comparison of protein profiles between the transected and control sides of hippocampi after unilateral FFT revealed a reduction in the F-actin capping protein (CapZ) signal on the FFT side. Subsequent immunostaining of brain sections and cultured hippocampal neurons revealed that CapZ localized in dendritic spines and the signal intensity in each spine varied widely. The CapZ content decreased after suppression of neuronal firing by tetrodotoxin treatment in cultured neurons, indicating rapid and activity-dependent regulation of CapZ accumulation in spines. To test input specificity of CapZ accumulation in vivo, we delivered high-frequency stimuli to the medial perforant path unilaterally in awake rats. This path selectively inputs to the middle molecular layer of the dentate gyrus, where CapZ immunoreactivity increased. We conclude that activity-dependent, synapse-specific regulation of CapZ redistribution might be important in both maintenance and remodeling of synaptic connections in neurons receiving specific spatial and temporal patterns of inputs.

Nebulin plays a direct role in promoting strong actin-myosin interactions

The role of the actin filament-associated protein nebulin on mechanical and kinetic properties of the actomyosin motor was investigated in skeletal muscle of wild-type (wt) and nebulin-deficient (nebulin(-)(/)(-)) mice that were 1 d old, an age at which sarcomeric structure is still well preserved. In Ca2+-activated skinned fibers from psoas muscle, we determined the Ca2+ dependence of isometric force and stiffness, the rate of force redevelopment after unloaded shortening (k(TR)), the power during isotonic shortening, and the unloaded shortening velocity (V(0)). Our results show a 65% reduction in isometric force in nebulin(-)(/)(-) fibers at saturating [Ca2+], whereas neither thin-filament length nor the Ca2+ sensitivity of the contractile system is affected. Stiffness measurements indicate that the reduction in isometric force is due to a reduction in the number of actin-attached myosin motors, whereas the force of the motor is unchanged. Furthermore, in nebulin(-)(/)(-) fibers, k(TR) is decreased by 57%, V(0) is increased by 63%, and the maximum power is decreased by 80%. These results indicate that, in the absence of nebulin, the attachment probability of the myosin motors to actin is decreased, revealing a direct role for nebulin in promoting strong actomyosin interactions responsible for force and power production.

Contractility-dependent actin dynamics in cardiomyocyte sarcomeres

In contrast to the highly dynamic actin cytoskeleton in non-muscle cells, actin filaments in muscle sarcomeres are thought to be relatively stable and undergo dynamics only at their ends. However, many proteins that promote rapid actin dynamics are also expressed in striated muscles. We show that a subset of actin filaments in cardiomyocyte sarcomeres displays rapid turnover. Importantly, we found that turnover of these filaments depends on contractility of the cardiomyocytes. Studies using an actin-polymerization inhibitor suggest that the pool of dynamic actin filaments is composed of filaments that do not contribute to contractility. Furthermore, we provide evidence that ADF/cofilins, together with myosin-induced contractility, are required to disassemble non-productive filaments in developing cardiomyocytes. These data indicate that an excess of actin filaments is produced during sarcomere assembly, and that contractility is applied to recognize non-productive filaments that are subsequently destined for depolymerization. Consequently, contractility-induced actin dynamics plays an important role in sarcomere maturation.

Characterizing mouse male germ cell-specific actin capping protein alpha3 (CPalpha3): dynamic patterns of expression in testicular and epididymal sperm

AIM

To characterize mouse capping protein alpha3 (CPalpha3) during spermatogenesis and sperm maturation.

METHODS

We produced rat anti-CPalpha3 antiserum and examined the expression of CPalpha3 in various mouse tissues using Western blot analysis and the localization of CPalpha3 in testicular and epididymal sperm using immunohistochemical analyses. We also examined how the localization of CPalpha3 and beta-actin (ACTB) in sperm changed after the acrosomal reaction by performing immunohistochemical analyses using anti-CPalpha3 antiserum and anti-actin antibody.

RESULTS

Western blot analysis using specific antiserum revealed that CPalpha3 was expressed specifically in testes. Interestingly, the molecular weight of CPalpha3 changed during sperm maturation in the epididymis. Furthermore, the subcellular localization of CPalpha3 in sperm changed dynamically from the flagellum to the post-acrosomal region of the head during epididymal maturation. The distribution of ACTB was in the post-acrosomal region of the head and the flagellum. After inducing the acrosomal reaction, the CPalpha3 and ACTB localization was virtually identical to the localization before the acrosomal reaction.

CONCLUSION

CPalpha3 might play an important role in sperm morphogenesis and/or sperm function.

Caenorhabditis elegans gelsolin-like protein 1 is a novel actin filament-severing protein with four gelsolin-like repeats

The gelsolin family of proteins is a major class of actin regulatory proteins that sever, cap, and nucleate actin filaments in a calcium-dependent manner and are involved in various cellular processes. Typically, gelsolin-related proteins have three or six repeats of gelsolin-like (G) domain, and each domain plays a distinct role in severing, capping, and nucleation. The Caenorhabditis elegans gelsolin-like protein-1 (gsnl-1) gene encodes an unconventional gelsolin-related protein with four G domains. Sequence alignment suggests that GSNL-1 lacks two G domains that are equivalent to fourth and fifth G domains of gelsolin. In vitro, GSNL-1 severed actin filaments and capped the barbed end in a calcium-dependent manner. However, unlike gelsolin, GSNL-1 remained bound to the side of F-actin with a submicromolar affinity and did not nucleate actin polymerization, although it bound to G-actin with high affinity. These results indicate that GSNL-1 is a novel member of the gelsolin family of actin regulatory proteins and provide new insight into functional diversity and evolution of gelsolin-related proteins.

Differential protein expression following low temperature culture of suspension CHO-K1 cells

Background

To ensure maximal productivity of recombinant proteins (rP) during production culture it is typical to encourage an initial phase of rapid cell proliferation to achieve high biomass followed by a stationary phase where cellular energies are directed towards production of rP. During many such biphasic cultures, the initial phase of rapid cell growth at 37°C is followed by a growth arrest phase induced through reduction of the culture temperature. Low temperature induced growth arrest is associated with many positive phenotypes including increased productivity, sustained viability and an extended production phase, although the mechanisms regulating these phenotypes during mild hypothermia are poorly understood.

Results

In this study differential protein expression in suspension CHO-K1 cells was investigated following a reduction of the culture temperature from 37°C to 31°C in comparison to standard batch culture maintained at 37°C using 2D-DIGE (Fluorescence 2-D Difference Gel Electrophoresis) and mass spectrometry (MS). There is only limited proteomic analysis of suspension-grown CHO cells describing a direct comparison of temperature shifted versus non-temperature shifted cultures using 2D-DIGE. This investigation has enabled the identification of temperature-dependent as well as temperature-independent proteomic changes. 201 proteins were observed as differentially expressed following temperature shift, of which 118 were up regulated. Of the 53 proteins identified by MALDI-ToF MS, 23 were specifically differentially expressed upon reduction of the culture temperature and were found related to a variety of cellular functions such as regulation of growth (HNRPC), cap-independent translation (EIF4A), apoptosis (importin-α), the cytoskeleton (vimentin) and glycoprotein quality control (alpha glucosidase 2).

Conclusion

These results indicate the extent of the temperature response in CHO-K1 cells and suggest a number of key regulatory proteins and pathways that are involved in modulating the response of cells to mild hypothermia. Regulation of these identified proteins and pathways could be useful for future approaches to engineer CHO cells for improved recombinant protein production.

New insights into mechanism and regulation of actin capping protein

The heterodimeric actin capping protein, referred to here as "CP," is an essential element of the actin cytoskeleton, binding to the barbed ends of actin filaments and regulating their polymerization. In vitro, CP has a critical role in the dendritic nucleation process of actin assembly mediated by Arp2/3 complex, and in vivo, CP is important for actin assembly and actin-based process of morphogenesis and differentiation. Recent studies have provided new insight into the mechanism of CP binding the barbed end, which raises new possibilities for the dynamics of CP and actin in cells. In addition, a number of molecules that bind and regulate CP have been discovered, suggesting new ideas for how CP may integrate into diverse processes of cell physiology.

Structural basis of actin filament capping at the barbed-end: a cryo-electron microscopy study

The intracellular distribution and migration of many protein complexes and organelles is regulated by the dynamics of the actin filament. Many actin filament end-binding proteins play crucial roles in actin dynamics, since polymerization and depolymerization of actin protomers occur only at the filament ends. We present here an EM structure of the complex of the actin filament and hetero-dimeric capping protein (CP) bound to the barbed-end at 23 A resolution, by applying a newly developed methods of image analysis to cryo-electron micrographs. This structure was fitted by the crystal structure of CP and the proposed actin filament structure, allowing us to construct a model that depicts two major binding regions between CP and the barbed-end. This binding scheme accounted for the results of newly performed and previously published mutation experiments, and led us to propose a two-step binding model. This is the first determination of an actin filament end structure.

Protein-lipid interactions: correlation of a predictive algorithm for lipid-binding sites with three-dimensional structural data

Background

Over the past decade our laboratory has focused on understanding how soluble cytoskeleton-associated proteins interact with membranes and other lipid aggregates. Many protein domains mediating specific cell membrane interactions appear by fluorescence microscopy and other precision techniques to be partially inserted into the lipid bilayer. It is unclear whether these protein-lipid-interactions are dependent on shared protein motifs or unique regional physiochemistry, or are due to more global characteristics of the protein.

Results

We have developed a novel computational program that predicts a protein's lipid-binding site(s) from primary sequence data. Hydrophobic labeling, Fourier transform infrared spectroscopy (FTIR), film balance, T-jump, CD spectroscopy and calorimetry experiments confirm that the interfaces predicted for several key cytoskeletal proteins (alpha-actinin, Arp2, CapZ, talin and vinculin) partially insert into lipid aggregates. The validity of these predictions is supported by an analysis of the available three-dimensional structural data. The lipid interfaces predicted by our algorithm generally contain energetically favorable secondary structures (e.g., an amphipathic alpha-helix flanked by a flexible hinge or loop region), are solvent-exposed in the intact protein, and possess favorable local or global electrostatic properties.

Conclusion

At present, there are few reliable methods to determine the region of a protein that mediates biologically important interactions with lipids or lipid aggregates. Our matrix-based algorithm predicts lipid interaction sites that are consistent with the available biochemical and structural data. To determine whether these sites are indeed correctly identified, and whether use of the algorithm can be safely extended to other classes of proteins, will require further mapping of these sites, including genetic manipulation and/or targeted crystallography.

Differential effects of Latrunculin-A on myofibrils in cultures of skeletal muscle cells: insights into mechanisms of myofibrillogenesis

To test different models of myofibrillogenesis, we followed live cells expressing Green Fluorescent Proteins ligated to either actin or alpha-actinin and analyzed stress fibers, premyofibrils, and myofibrils in quail myotube cultures. Actin filaments in the three types of fibers were compared by analyzing the effects of Latrunculin-A (Lat-A), a monomeric actin binding macrolide drug (M.W. = 422 Daltons), on stress fibers in fibroblasts and on myofibrils in skeletal myotubes in the same culture. Lat-A, at low concentrations (0.2 microM), induced the loss of stress fibers in fibroblasts within a few hours and within 10 min when Lat-A was increased to 1.0 microM. The effect was reversible with reformation of the stress fibers when the drug was removed. In contrast to the Lat-A induced disassembly of stress fibers in fibroblasts, assembling myofibrils in the skeletal muscle cells were not affected by 1.0-microM concentrations of Lat-A. With increasing concentrations of Lat-A (up to 5 microM), and increasing incubation times, however, the drug induced premyofibrils, the precursors of mature myofibrils, to disassemble and the accumulation of mature myofibrils to be halted. Removal of the drug led to the reformation of premyofibrils and the resumption of myofibrillogenesis in the spreading edges of the myotubes. In contrast, the mature myofibrils in the central shaft of the myotubes were stable in doses of Lat-A as high as 50 microM. The newly assembled mature myofibrils located adjacent to the premyofibrils at the ends and sides of the myotube were intermediate in sensitivity to Lat-A, disassembling when exposed to 10 microM Lat-A for one hour. To determine how a change in the actin filaments during myofibrillogenesis might confer greater resistance to depolymerization by Lat-A, we stained the myotubes with an antibody directed against CapZ, a protein that blocks the release of monomer actin from the barbed ends of actin filaments. CapZ was absent from premyofibrils. It was distributed uniformly along nascent myofibrils where F-actin was unstriated, and was localized in a clearly striated Z-band pattern in the mature myofibrils where F-actin patterns were fully striated. These Lat-A and CapZ results are discussed in the context of various models of myofibrillogenesis.

Three-dimensional reconstruction of the dynactin complex by single-particle image analysis

Dynactin is a large complex of at least nine distinct proteins that co-complexes with cytoplasmic dynein within cells, where it plays a major role as a regulator of the motor's function. Owing to its large size and complexity, relatively little is known about dynactin's 3D structure or the structural basis of its function. Use of single-particle image analysis techniques has enabled us to produce the first 3D reconstruction of the dynactin complex, to a resolution of 3 nm. The actin-related protein (Arp) backbone of the filament has been clearly visualized. Fitting of models of the Arp backbone showed that it consists of 10 subunits. Additional mass, not part of the Arp backbone, was also seen. A preliminary fitting of the capping protein CapZ structure into our 3D reconstruction of the dynactin complex suggests that it is optimally placed to perform its proposed function as a stabilizer of the Arp1 backbone and gives clues as to likely interaction points between the capping protein and Arp subunits. The results provide the first detailed visualization of the dynactin complex and shed light on the mode of interaction between several of its constituent proteins and their possible functions.

The erythrocyte skeletons of β-adducin deficient mice have altered levels of tropomyosin, tropomodulin and EcapZ

The erythrocyte membrane cytoskeleton is organized as a polygonal spectrin network linked to short actin filaments that are capped by adducin at the barbed ends. We have constructed a mouse strain deficient in beta-adducin having abnormal erythrocytes. We show here that the levels of several skeletal proteins from beta-adducin mutant erythrocytes are altered. In fact, CapZ, the main muscle actin-capping protein of the barbed ends that in the erythrocytes is cytoplasmic, is 9-fold upregulated in mutant skeletons of erythrocytes suggesting a compensatory mechanism. We also detected upregulation of tropomodulin and downregulation of alpha-tropomyosin and actin. In addition, purified adducin can be re-incorporated into adducin-deficient ghosts.

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.

Crystal structure of CapZ: structural basis for actin filament barbed end capping

Capping protein, a heterodimeric protein composed of alpha and beta subunits, is a key cellular component regulating actin filament assembly and organization. It binds to the barbed ends of the filaments and works as a 'cap' by preventing the addition and loss of actin monomers at the end. Here we describe the crystal structure of the chicken sarcomeric capping protein CapZ at 2.1 A resolution. The structure shows a striking resemblance between the alpha and beta subunits, so that the entire molecule has a pseudo 2-fold rotational symmetry. CapZ has a pair of mobile extensions for actin binding, one of which also provides concomitant binding to another protein for the actin filament targeting. The mobile extensions probably form flexible links to the end of the actin filament with a pseudo 2(1) helical symmetry, enabling the docking of the two in a symmetry mismatch.

Actin binding proteins: regulation of cytoskeletal microfilaments

The actin cytoskeleton is a complex structure that performs a wide range of cellular functions. In 2001, significant advances were made to our understanding of the structure and function of actin monomers. Many of these are likely to help us understand and distinguish between the structural models of actin microfilaments. In particular, 1) the structure of actin was resolved from crystals in the absence of cocrystallized actin binding proteins (ABPs), 2) the prokaryotic ancestral gene of actin was crystallized and its function as a bacterial cytoskeleton was revealed, and 3) the structure of the Arp2/3 complex was described for the first time. In this review we selected several ABPs (ADF/cofilin, profilin, gelsolin, thymosin beta4, DNase I, CapZ, tropomodulin, and Arp2/3) that regulate actin-driven assembly, i.e., movement that is independent of motor proteins. They were chosen because 1) they represent a family of related proteins, 2) they are widely distributed in nature, 3) an atomic structure (or at least a plausible model) is available for each of them, and 4) each is expressed in significant quantities in cells. These ABPs perform the following cellular functions: 1) they maintain the population of unassembled but assembly-ready actin monomers (profilin), 2) they regulate the state of polymerization of filaments (ADF/cofilin, profilin), 3) they bind to and block the growing ends of actin filaments (gelsolin), 4) they nucleate actin assembly (gelsolin, Arp2/3, cofilin), 5) they sever actin filaments (gelsolin, ADF/cofilin), 6) they bind to the sides of actin filaments (gelsolin, Arp2/3), and 7) they cross-link actin filaments (Arp2/3). Some of these ABPs are essential, whereas others may form regulatory ternary complexes. Some play crucial roles in human disorders, and for all of them, there are good reasons why investigations into their structures and functions should continue.

Aberrant expression of centractin and capping proteins, integral constituents of the dynactin complex, in fetal down syndrome brain

Down syndrome (DS, trisomy 21) is the most frequent genetic cause of mental retardation. Although known for more than a hundred years the underlying pathomechanisms for the phenotype and impaired brain functions remain elusive. Performing protein hunting in fetal DS brain, we detected a series of cytoskeleton proteins with aberrant expression in fetal DS cortex. Fetal brain cortex samples of controls and DS of the early second trimenon of gestation were used for the experiments. We applied two-dimensional electrophoresis with in-gel digestion of protein spots, subsequent mass spectroscopical (MALDI) identification, and quantification of spots using specific software. Centractin alpha, F-actin capping protein alpha-1, alpha-2 and beta subunits were significantly reduced in fetal DS cortex, whereas dynein intermediate clear 2, dynein intermediate chain 2, and kinesin light chain protein levels were unchanged. Centractins and F-actin capping proteins are major determinants of the cytoskeleton and are involved in pivotal functions including cellular, organelle, and nuclear motility. Deranged centractins and F-actin capping proteins may represent or induce deficient axonal transport and may well contribute to deterioration of the cytoskeleton's mitotic functions in trisomy 21.

Molecular dissection of the interaction of desmin with the C-terminal region of nebulin

In vertebrate skeletal muscle, ultrastructural studies have suggested that the Z-line and extracellular intermediate filaments are linked, although a structural basis for this has remained elusive. We searched for potential novel ligands of the Z-line portion of nebulin by a yeast two-hybrid (Y2H) approach. This identified that the nebulin modules M160 to M170 interact with desmin. In desmin, deletion series experiments assigned a 19-kDa central coiled-coil domain as the nebulin-binding site. The specific interactions of nebulin and desmin were confirmed in vitro by GST pull-down experiments. In situ, the nebulin modules M176 to M181 colocalize with desmin in a Z-line-associated, striated pattern as shown by immunofluorescence studies. Our data are consistent with a model that desmin attaches directly to the Z-line through its interaction with the nebulin repeats M163-M170. This interaction may link myofibrillar Z-discs to the intermediate filament system, thereby forming a lateral linkage system which contributes to maintain adjacent Z-lines in register.

S100: a multigenic family of calcium-modulated proteins of the EF-hand type with intracellular and extracellular functional roles

S100 is a multigenic family of non-ubiquitous Ca(2+)-modulated proteins of the EF-hand type expressed in vertebrates exclusively and implicated in intracellular and extracellular regulatory activities. Within cells, most of S100 members exist in the form of antiparallelly packed homodimers (in some cases heterodimers), capable of functionally crossbridging two homologous or heterologous target proteins in a Ca(2+)-dependent (and, in some instances, Ca(2+)-independent) manner. S100 oligomers can also form, under the non-reducing conditions found in the extracellular space and/or within cells upon changes in the cell redox status. Within cells, S100 proteins have been implicated in the regulation of protein phosphorylation, some enzyme activities, the dynamics of cytoskeleton components, transcription factors, Ca(2+) homeostasis, and cell proliferation and differentiation. Certain S100 members are released into the extracellular space by an unknown mechanism. Extracellular S100 proteins stimulate neuronal survival and/or differentiation and astrocyte proliferation, cause neuronal death via apoptosis, and stimulate (in some cases) or inhibit (in other cases) the activity of inflammatory cells. A cell surface receptor, RAGE, has been identified on inflammatory cells and neurons for S100A12 and S100B, which transduces S100A12 and S100B effects. It is not known whether RAGE is a universal S100 receptor, S100 members interact with other cell surface receptors, or S100 protein interaction with other extracellular factors specifies the biological effects of a given S100 protein on a target cell. The variety of intracellular target proteins of S100 proteins and, in some cases, of a single S100 protein, and the cell specificity of expression of certain S100 members suggest that these proteins might have a role in the fine regulation of effector proteins and/or specific steps of signaling pathways/cellular functions. Future analyses should discriminate between functionally relevant S100 interactions with target proteins and in vitro observations devoid of physiological importance.

Actin dynamics at pointed ends regulates thin filament length in striated muscle

Regulation of actin dynamics at filament ends determines the organization and turnover of actin cytoskeletal structures. In striated muscle, it is believed that tight capping of the fast-growing (barbed) ends by CapZ and of the slow-growing (pointed) ends by tropomodulin (Tmod) stabilizes the uniform lengths of actin (thin) filaments in myofibrils. Here we demonstrate for the first time that both CapZ and Tmod are dynamic on the basis of the rapid incorporation of microinjected rhodamine-labelled actin (rho-actin) at both barbed and pointed ends and from the photobleaching of green fluorescent protein (GFP)-labelled Tmod. Unexpectedly, the inhibition of actin dynamics at pointed ends by GFP-Tmod overexpression results in shorter thin filaments, whereas the inhibition of actin dynamics at barbed ends by cytochalasin D has no effect on length. These data demonstrate that the actin filaments in myofibrils are relatively dynamic despite the presence of capping proteins, and that regulated actin assembly at pointed ends determines the length of thin filaments.

Interaction of actin with the capping protein, CapZ from sea bass (Dicentrarchus labrax) white skeletal muscle

We have compared the functional properties of CapZ from fish white skeletal muscle with those of CapZ from chicken muscle. CapZ is a heterodimer, which enhances actin nucleation and inhibits the depolymerization process by binding to the barbed ends of microfilaments. Here, we report the interaction of CapZ not only with F-actin, but also with monomeric actin. The affinity of sea bass CapZ for G-actin estimated by enzyme-linked immunosorbent assay (ELISA) was in the microM range. This association was PIP2 dependent. Binding contacts with the barbed end of actin were delimited by both ELISA and fluorescence approaches. One site (actin sequence 338-348) was located in a helical region of the subdomain 1, region already implicated in the interaction with other actin binding proteins such as gelsolin. Another site implicates the C-terminal region (sequence 360-372) of actin. Finally, the partial competition of antibodies directed against CapZ alpha or beta-subunits towards CapZ interaction with actin filaments suggests both subunits participate in the complex with actin.

Effect of capping protein, CapZ, on the length of actin filaments and mechanical properties of actin filament networks

We report on how physiological concentrations of capping protein shorten actin filaments and on the remarkably fluid nature of solutions of such short filaments even at the high concentrations that exist in cells. We measured the lengths of actin filaments formed by spontaneous polymerization of highly purified actin monomers by fluorescence microscopy after labeling with rhodamine-phalloidin. The length distributions are exponential with a mean of about 7 microm (2600 subunits). As observed previously with less quantitative assays, copolymerization with the actin capping protein, CapZ, reduces the length of the filaments. At cellular concentrations of capping protein, one filament forms for each molecule of capping protein and the population of filaments is uniformly short. Using CapZ to vary the length of actin filaments, we measured how their mechanical properties depend on length. The stiffness (elastic modulus) of actin filament networks depends steeply on the length, with long filaments contributing far out of proportion to their numbers to the stiffness. Even at physiological concentrations (300 microM), networks of filaments limited to lengths observed in cells with a 1 to 500 molar ratio of CapZ are more fluid and much less elastic than lower concentrations of longer actin filaments. Thus the high concentration of short actin filaments in cells must be crosslinked to produce the observed stiffness of the cortex.

In vitro refolding of heterodimeric CapZ expressed in E. coli as inclusion body protein

CapZ is a heterodimeric Ca(2+)-independent actin binding protein which plays an important role in organizing the actin filament lattice of cross-striated muscle cells. It caps the barbed end of actin filaments and promotes nucleation of actin polymerization, thereby regulating actin filament length. Here we report the expression of the two muscle-specific isoforms alpha2 and beta1, from chicken in Escherichia coli as individual subunits using the pQE60 expression vector and the subsequent renaturation of the functional CapZ heterodimer from inclusion bodies. Optimal renaturation conditions were obtained both by simultaneous refolding of urea-solubilized subunits and by rapid dilution into a buffer containing 20% glycerol, 5 mM EGTA, 2 mM DTT, 1 mM PMSF, and 100 mM Tris, pH 7.4. The refolding mixture was incubated for 24 h at 15 degrees C and the protein was concentrated by ultrafiltration. Biochemical characterization of the recombinant heterodimer revealed actin binding activities indistinguishable from those of native CapZ as purified from chicken skeletal muscle. Using the same protocol, we were able to refold the beta1, but not the alpha2 isoform as a single polypeptide, indicating a role for beta1 as a molecular template for the folding of alpha2. The reported recombinant approach leads to high yields of active heterodimer and allows the renaturation and characterization of the beta subunit.

Protein interactions at the tight junction. Actin has multiple binding partners, and ZO-1 forms independent complexes with ZO-2 and ZO-3

Defining how the molecular constituents of the tight junction interact is a prerequisite to understanding tight junction physiology. We utilized in vitro binding assays with purified recombinant proteins and immunoprecipitation analyses to define interactions between ZO-1, ZO-2, ZO-3, occludin, and the actin cytoskeleton. Actin cosedimentation studies showed that ZO-2, ZO-3, and occludin all interact directly with F-actin in vitro, indicating that actin is engaged in multiple interactions at the tight junction. Low speed sedimentation analyses demonstrated that neither ZO-2, ZO-3, nor occludin act as F-actin cross-linking proteins, and further evidence indicates that these proteins do not bind to actin filament ends. The binding interactions of ZO-2, ZO-3, and occludin were corroborated in vivo by immunofluorescence colocalization experiments which showed that all three proteins colocalized with actin aggregates at cell borders in cytochalasin D-treated Madin-Darby canine kidney cells. Exploration of other tight junction protein interactions demonstrated that ZO-2 binds directly to both ZO-1 and occludin. Contrary to previous beliefs, our immunoprecipitation results indicate that ZO-1, ZO-2, and ZO-3 exist in situ primarily as independent ZO-1.ZO-2 and ZO-1.ZO-3 complexes rather than a trimeric ZO-1.ZO-2.ZO-3 grouping. These studies elucidate direct binding interactions among tight junction-associated proteins, giving insight into their organization as a multimolecular structure.

Use of a chaotropic anion iodide in the purification of Z-line proteins: isolation of CapZ from fish white muscle

In the present study, we have described an improved method allowing the isolation of proteins which form tightly associated complexes in organized structures such as Z line in skeletal muscle. This procedure is based on both extraction and chromatography in the presence of a chaotropic agent. KI at medium concentration (0.6 M) was selected, taking into account its dissociating activity and mild effect on the native state of proteins. This procedure was applied to purify and to characterize for the first time a CapZ from fish white muscle, a protein involved in the stabilization of the filaments in Z line. The alpha and beta CapZ subunits were identified using anti-synthetic peptide antibodies directed against conserved sequences derived from chicken CapZ. The protocol can be also used for the isolation of other muscular proteins such as alpha-actinin and actin. Finally this technique may be utilized to obtain a good amount of capping protein which could be employed in experiments of microfilament dynamics.

Exchange of alpha-actinin in isolated rigor myofibrils

In the current study, the process of alpha-actinin binding to the myofibrillar Z-line was investigated to determine its mechanism. Pretreatment of rigor myofibrils with unlabeled alpha-actinin did not prevent or slow the incorporation of fluorescein skeletal alpha-actinin into myofibrils suggesting that incorporation was not the filling of empty binding sites but rather an exchange reaction. Further support for this was obtained using quantitative measures of labeled alpha-actinin incorporation and measures of total myofibrillar alpha-actinin. These results showed that there was no change in myofibrillar alpha-actinin content when up to 15% of the total alpha-actinin was the labeled protein. Measurement of the time-course of fluorescein alpha-actinin incorporation by quantitative fluorescence microscopy showed that the increase in Z-line fluorescence was well described by a rapid (unresolved) incorporation of fluorescence followed by a much slower phase. The slower phase was independent of fluorescein alpha-actinin concentration (2.5-160 nM) and had an apparent rate of 0.008-0.016 min(-1). Pretreatment of myofibrils with fluorescein alpha-actinin followed by incubation with unlabeled alpha-actinin resulted in a decrease in Z-line fluorescence with an apparent rate of 0.021 min(-1). The slow phase was interpreted as representing the dissociation rate of intrinsic Z-line alpha-actinin. Thus, the dissociation rate for the in situ interaction of alpha-actinin with actin appears to be three orders of magnitude slower than that determined from solution studies.