Interaction of actin with divalent cations. 1. The effect of various cations on the physical state of actin

Zinc is a master-regulator of sperm function associated with binding, motility, and metabolic modulation during porcine sperm capacitation

Sperm capacitation is a post-testicular maturation step endowing spermatozoa with fertilizing capacity within the female reproductive tract, significant for fertility, reproductive health, and contraception. Recently discovered mammalian sperm zinc signatures and their changes during sperm in vitro capacitation (IVC) warranted a more in-depth study of zinc interacting proteins (further zincoproteins). Here, we identified 1752 zincoproteins, with 102 changing significantly in abundance (P < 0.05) after IVC. These are distributed across 8 molecular functions, 16 biological processes, and 22 protein classes representing 130 pathways. Two key, paradigm-shifting observations were made: i) during sperm capacitation, molecular functions of zincoproteins are both upregulated and downregulated within several molecular function categories; and ii) Huntington's and Parkinson's disease pathways were the two most represented, making spermatozoon a candidate model for studying neurodegenerative diseases. These findings highlight the importance of Zn2+ homeostasis in reproduction, offering new avenues in semen processing for human-assisted reproductive therapy, identification of somatic-reproductive comorbidities, and livestock breeding.

Human deafness-associated variants alter the dynamics of key molecules in hair cell stereocilia F-actin cores

Stereocilia protrude up to 100 µm from the apical surface of vertebrate inner ear hair cells and are packed with cross-linked filamentous actin (F-actin). They function as mechanical switches to convert sound vibration into electrochemical neuronal signals transmitted to the brain. Several genes encode molecular components of stereocilia including actin monomers, actin regulatory and bundling proteins, motor proteins and the proteins of the mechanotransduction complex. A stereocilium F-actin core is a dynamic system, which is continuously being remodeled while maintaining an outwardly stable architecture under the regulation of F-actin barbed-end cappers, severing proteins and crosslinkers. The F-actin cores of stereocilia also provide a pathway for motor proteins to transport cargos including components of tip-link densities, scaffolding proteins and actin regulatory proteins. Deficiencies and mutations of stereocilia components that disturb this "dynamic equilibrium" in stereocilia can induce morphological changes and disrupt mechanotransduction causing sensorineural hearing loss, best studied in mouse and zebrafish models. Currently, at least 23 genes, associated with human syndromic and nonsyndromic hearing loss, encode proteins involved in the development and maintenance of stereocilia F-actin cores. However, it is challenging to predict how variants associated with sensorineural hearing loss segregating in families affect protein function. Here, we review the functions of several molecular components of stereocilia F-actin cores and provide new data from our experimental approach to directly evaluate the pathogenicity and functional impact of reported and novel variants of DIAPH1 in autosomal-dominant DFNA1 hearing loss using single-molecule fluorescence microscopy.

Low doses of zeolitic imidazolate framework-8 nanoparticles alter the actin organization and contractility of vascular smooth muscle cells

Zeolitic imidazolate framework-8 (ZIF-8) nanoparticles have emerged as a promising platform for drug delivery and controlled release. Considering most ZIF-8 nanoparticle drug carriers are designed to be administered intravenously, and thus would directly contact vascular smooth muscle cells (VSMCs) in many circumstances, the potential interactions of ZIF-8 nanoparticles with VSMCs require investigation. Here, the effects of low doses of ZIF-8 nanoparticles on VSMC morphology, actin organization, and contractility are investigated. Two nanoscale imaging tools, atomic force microscopy, and direct stochastic optical reconstruction microscopy, show that even at the concentrations (12.5 and 25 µg/ml) that were deemed "safe" by conventional biochemical cell assays (MTT and LDH assays), ZIF-8 nanoparticles can still cause changes in cell morphology and actin cytoskeleton organization at the cell apical and basal surfaces. These cytoskeletal structural changes impair the contractility function of VSMCs in response to Angiotensin II, a classic vasoconstrictor. Based on intracellular zinc and actin polymerization assays, we conclude that the increased intracellular Zn²⁺ concentration due to the uptake and dissociation of ZIF-8 nanoparticles could cause the actin cytoskeleton dis-organization, as the elevated Zn²⁺ directly disrupts the actin assembly process, leading to altered actin organization such as branches and networks. Since the VSMC phenotype change and loss of contractility are fundamental to the development of atherosclerosis and related cardiovascular diseases, it is worth noting that these low doses of ZIF-8 nanoparticles administered intravenously could still be a safety concern in terms of cardiovascular risks. Moving forward, it is imperative to re-consider the "safe" nanoparticle dosages determined by biochemical cell assays alone, and take into account the impact of these nanoparticles on the biophysical characteristics of VSMCs, including changes in the actin cytoskeleton and cell morphology.

In Vitro Cytotoxicity Effects of Zinc Oxide Nanoparticles on Spermatogonia Cells

Zinc Oxide Nanoparticles (ZnO NPs) are a type of metal oxide nanoparticle with an extensive use in biomedicine. Several studies have focused on the biosafety of ZnO NPs, since their size and surface area favor entrance and accumulation in the body, which can induce toxic effects. In previous studies, ZnO NPs have been identified as a dose- and time-dependent cytotoxic inducer in testis and male germ cells. However, the consequences for the first cell stage of spermatogenesis, spermatogonia, have never been evaluated. Therefore, the aim of the present work is to evaluate in vitro the cytotoxic effects of ZnO NPs in spermatogonia cells, focusing on changes in cytoskeleton and nucleoskeleton. For that purpose, GC-1 cell line derived from mouse testes was selected as a model of spermatogenesis. These cells were treated with different doses of ZnO NPs for 6 h and 12 h. The impact of GC-1 cells exposure to ZnO NPs on cell viability, cell damage, and cytoskeleton and nucleoskeleton dynamics was assessed. Our results clearly indicate that higher concentrations of ZnO NPs have a cytotoxic effect in GC-1 cells, leading to an increase of intracellular Reactive Oxygen Species (ROS) levels, DNA damage, cytoskeleton and nucleoskeleton dynamics alterations, and consequently cell death. In conclusion, it is here reported for the first time that ZnO NPs induce cytotoxic effects, including changes in cytoskeleton and nucleoskeleton in mouse spermatogonia cells, which may compromise the progression of spermatogenesis in a time- and dose-dependent manner.

Structural evidence for the roles of divalent cations in actin polymerization and activation of ATP hydrolysis

Actin polymerization is a divalent cation-dependent process. Here we identify a cation binding site on the surface of actin in a 2.0-Å resolution X-ray structure of actin and find evidence of three additional sites in published high-resolution structures. These cations are stable in molecular dynamics (MD) simulations of the filament, suggesting a functional role in polymerization or filament rigidity. Polymerization activates the ATPase activity of the incorporating actin protomers. Careful analysis of water molecules that approach the ATP in the MD simulations revealed Gln137-activated water to be in a suitable position in F-actin, to initiate attack for ATP hydrolysis, and its occupancy was dependent on bound cations.

The structure of the actin filament is known at a resolution that has allowed the architecture of protein components to be unambiguously assigned. However, fully understanding the chemistry of the system requires higher resolution to identify the ions and water molecules involved in polymerization and ATP hydrolysis. Here, we find experimental evidence for the association of cations with the surfaces of G-actin in a 2.0-Å resolution X-ray structure of actin bound to a Cordon-Bleu WH2 motif and in previously determined high-resolution X-ray structures. Three of four reoccurring divalent cation sites were stable during molecular dynamics (MD) simulations of the filament, suggesting that these sites may play a functional role in stabilizing the filament. We modeled the water coordination at the ATP-bound Mg²⁺, which also proved to be stable during the MD simulations. Using this model of the filament with a hydrated ATP-bound Mg²⁺, we compared the cumulative probability of an activated hydrolytic water molecule approaching the γ-phosphorous of ATP, in comparison with G-actin, in the MD simulations. The cumulative probability increased in F-actin in line with the activation of actin’s ATPase activity on polymerization. However, inclusion of the cations in the filament lowered cumulative probability, suggesting the rate of hydrolysis may be linked to filament flexibility. Together, these data extend the possible roles of Mg²⁺ in polymerization and the mechanism of polymerization-induced activation of actin’s ATPase activity.

Cations Stiffen Actin Filaments by Adhering a Key Structural Element to Adjacent Subunits

Ions regulate the assembly and mechanical properties of actin filaments. Recent work using structural bioinformatics and site-specific mutagenesis favors the existence of two discrete and specific divalent cation binding sites on actin filaments, positioned in the long axis between actin subunits. Cation binding at one site drives polymerization, while the other modulates filament stiffness and plays a role in filament severing by the regulatory protein, cofilin. Existing structural methods have not been able to resolve filament-associated cations, and so in this work we turn to molecular dynamics simulations to suggest a candidate binding pocket geometry for each site and to elucidate the mechanism by which occupancy of the “stiffness site” affects filament mechanical properties. Incorporating a magnesium ion in the “polymerization site” does not seem to require any large-scale change to an actin subunit’s conformation. Binding of a magnesium ion in the “stiffness site” adheres the actin DNase-binding loop (D-loop) to its long-axis neighbor, which increases the filament torsional stiffness and bending persistence length. Our analysis shows that bound D-loops occupy a smaller region of accessible conformational space. Cation occupancy buries key conserved residues of the D-loop, restricting accessibility to regulatory proteins and enzymes that target these amino acids.

Regulation of actin by ion-linked equilibria

Actin assembly, filament mechanical properties, and interactions with regulatory proteins depend on the types and concentrations of salts in solution. Salts modulate actin through both nonspecific electrostatic effects and specific binding to discrete sites. Multiple cation-binding site classes spanning a broad range of affinities (nanomolar to millimolar) have been identified on actin monomers and filaments. This review focuses on discrete, low-affinity cation-binding interactions that drive polymerization, regulate filament-bending mechanics, and modulate interactions with regulatory proteins. Cation binding may be perturbed by actin post-translational modifications and linked equilibria. Partial cation occupancy under physiological and commonly used in vitro solution conditions likely contribute to filament mechanical heterogeneity and structural polymorphism. Site-specific cation-binding residues are conserved in Arp2 and Arp3, and may play a role in Arp2/3 complex activation and actin-filament branching activity. Actin-salt interactions demonstrate the relevance of ion-linked equilibria in the operation and regulation of complex biological systems.

Flagellar display of bone-protein-derived peptides for studying peptide-mediated biomineralization

A bacterial flagellum is self-assembled primarily from thousands of flagellin (FliC), a protein subunit. A foreign peptide can be fully displayed on the surface of the flagellum through inserting it into every constituent protein subunit. To shed light on the role of bone proteins during the nucleation of hydroxyapatite (HAP), representative domains from type I collagen, including part of the N-,C-terminal, N-,C-zone around the hole zone and an eight repeat unit Gly-Pro-Pro (GPP8) sequence similar to the central sequence of type I collagen, were separately displayed on the surface of the flagella. Moreover, eight negatively charged, contiguous glutamic acid residues (E8) and two other characteristic sequences derived from a representative noncollagenous protein called bone sialoprotein (BSP) were also displayed on flagella. After being incubated in an HAP supersaturated precursor solution, flagella displaying E8 or GPP8 sequences were found to be coated with a layer of HAP nanocrystals. Very weak or no nucleation was observed on flagella displaying other peptides being tested. We also found that calcium ions can induce the assembly of the negatively charged E8 flagella into bundles mimicking collagen fibers, followed by the formation of HAP nanocrystals with the crystallographic c axis preferentially aligned with long axis of flagella, which is similar to that along the collagen fibrils in bone. This work demonstrates that because of the ease of the peptide display on flagella and the self-assembly of flagella, flagella can serve as a platform for studying biomineralization and as a building block to generate bonelike biomaterials.

Identification of cation-binding sites on actin that drive polymerization and modulate bending stiffness

The assembly of actin monomers into filaments and networks plays vital roles throughout eukaryotic biology, including intracellular transport, cell motility, cell division, determining cellular shape, and providing cells with mechanical strength. The regulation of actin assembly and modulation of filament mechanical properties are critical for proper actin function. It is well established that physiological salt concentrations promote actin assembly and alter the overall bending mechanics of assembled filaments and networks. However, the molecular origins of these salt-dependent effects, particularly if they involve nonspecific ionic strength effects or specific ion-binding interactions, are unknown. Here, we demonstrate that specific cation binding at two discrete sites situated between adjacent subunits along the long-pitch helix drive actin polymerization and determine the filament bending rigidity. We classify the two sites as "polymerization" and "stiffness" sites based on the effects that mutations at the sites have on salt-dependent filament assembly and bending mechanics, respectively. These results establish the existence and location of the cation-binding sites that confer salt dependence to the assembly and mechanics of actin filaments.

Dystrophin and utrophin have distinct effects on the structural dynamics of actin

We have used time-resolved spectroscopy to investigate the structural dynamics of actin interaction with dystrophin and utrophin in relationship to the pathology of muscular dystrophy. Dystrophin and utrophin bind actin in vitro with similar affinities, but the molecular contacts of these two proteins with actin are different. It has been hypothesized that the presence of two low-affinity actin-binding sites in dystrophin allows more elastic response of the actin-dystrophin-sarcolemma linkage to muscle stretches, compared with utrophin, which binds via one contiguous actin-binding domain. We have directly tested this hypothesis by determining the effects of dystrophin and utrophin on the microsecond rotational dynamics of a phosphorescent dye attached to C374 on actin, as detected by transient phosphorescence anisotropy (TPA). Binding of dystrophin or utrophin to actin resulted in significant changes in the TPA decay, increasing the final anisotropy (restricting the rotational amplitude) and decreasing the rotational correlation times (increasing the rotational rates and the torsional flexibility). This paradoxical combination of effects on actin dynamics (decreased amplitude but increased rate) has not been observed for other actin-binding proteins. Thus, when dystrophin or utrophin binds, actin becomes less like cast iron (strong but brittle) and more like steel (stronger and more resilient). At low levels of saturation, the binding of dystrophin and utrophin has similar effects, but at higher levels, utrophin caused much greater restrictions in amplitude and increases in rate. The effects of dystrophin and utrophin on actin dynamics provide molecular insight into the pathology of muscular dystrophy.

Polyamine-induced bundling of F-actin

To better understand the mechanism of actin filament (F-actin) bundling by polyamines, we have measured the onset of bundling as a function of polyamine concentration. Samples were centrifuged at low speeds to separate bundles from unbundled actin, and the relative amounts of actin in the pellet and supernatant were determined via gel electrophoresis, yielding a description of the bundling transition as a function of actin and polyamine concentrations. These experiments were carried out for two different polyamines, spermine (tetravalent) and spermidine (trivalent). We found that the threshold concentration of polyamine needed to bundle actin is independent of both actin concentration and Mg2+ concentration over a wide range in Mg2+ concentration. We also find that spermine in F-actin bundles is essentially invisible in solution-phase proton NMR, suggesting that it is bound so tightly to F-actin that it is immobilized.

Molecular scale imaging of F-actin assemblies immobilized on a photopolymer surface

A photo-immobilization based process is presented for direct imaging of hierarchical assemblies of biopolymers using atomic force microscopy (AFM). The technique was used to investigate the phase behavior of F-actin aggregates as a function of concentration of the divalent cation Mg2+. The data provided direct experimental evidence of a coil-on-coil (braided) structure of F-actin bundles formed at high Mg2+ concentrations. At intermediate Mg2+ concentrations, the data showed the first images of the two-dimensional nematic rafts discovered by recent x-ray studies and theoretical treatments.

Changes in vacuolar and mitochondrial motility and tubularity in response to zinc in a Paxillus involutus isolate from a zinc-rich soil

Short-term effects of zinc on organelles were investigated in Paxillus involutus from a zinc-rich soil. Vacuoles were labelled with Oregon Green 488 carboxylic acid and mitochondria with DiOC(6)(3). Hyphae were treated with ZnSO(4) in the range 1-100 mM and examined by fluorescence microscopy. ZnSO(4) caused loss of tubularity and motility in both organelles depending on concentration and exposure time. Tubular vacuoles thickened after 15 min in 5 mM ZnSO(4) and became spherical at higher concentrations. Mitochondria fragmented after 30 min in 25 mM ZnSO(4). Vacuoles recovered their tubularity after transfer to reverse osmosis water depending on ZnSO(4) concentration and exposure time during treatment. Mitochondria recovered their tubularity with time, both with and without removal of the ZnSO(4) solution. K(2)SO(4) (as control) had no effect on vacuoles but disrupted mitochondria, the effect also depending on concentration and duration of exposure.

Direct stochastic simulation of Ca2+ motion in Xenopus eggs

The release of important intracellular ions has been widely modeled using two approaches, namely, (1) Fickian diffusion, in which sometimes tensorial diffusion coefficients are used to fit observed temporally varying concentrations of calcium, and (2) cellular automata, which produce a set of localized finite difference equations that result in complex global behavior. Here, we take a different approach, employing some assumed, a priori, distribution of ion-binding proteins in the cell, and some assumed biochemical capture and release characteristics to explain ionic motion, and ultimately, distribution. We study several scenarios for ion distribution, based on differences in binder action and distribution. The numbers and strengths of ion binders, spatial variation in inositol 1,4,5-triphosphate concentration, together with the escalating distribution of ionic diffusion speed, are found to be key factors leading to concavity in the Ca2+ wave shape. We also offer an explanation for geometrical effects on previously observed ion diffusion speeds in the cellular cortex of the Xenopus laevis egg during fertilization, based on an angle-of-view correction.

Connectivity, clusters, and transport: use of percolation concepts and atomistic simulation to track intracellular ion migration

The cytoskeleton is an intracellular highway system, teaming with signalling ions that zip from site to site along filaments. These tiny particles alternately embrace and slip free of protein receptors with wide-ranging affinities, as they propagate in a blur of motion along cytoskeletal corridors at transport rates far exceeding ordinary diffusive motion. Recent experimental breakthroughs have enabled optical tracking of these single ion-binding events in the physiological and diseased states. However, traditional continuum modelling methods have proven ineffective for modelling migration of biometals such as copper and zinc, whose cytosolic concentrations are putatively vanishingly small, or very tightly controlled. Rather, the key modelling problem that must be solved for biometals is determination of the optimal placement of biosensors that bind and detect the metal ions within the heterogeneous environment of the cell. We discuss herein how percolation concepts, in combination with atomistic simulation and sensor delivery models, have been used to gain insights on this problem, and a roadmap for future breakthroughs.

Chemical treatment of mica for atomic force microscopy can affect biological sample conformation

An important aspect in the preparation of substrate materials to use in atomic force microscopy lies in the question of interactions introduced by treatments designed to immobilize the sample over the substrate. Here we used a mica substrate that was chemically modified with cationic nickel to immobilize actin filaments (F-actin). Chemical modification could be followed quantitatively by measuring the interaction force between the scanning tip and the mica surface. This approach allowed us to observe polymeric F-actin in a structure that resembles an actin gel. It also improved sample throughput and conferred sample stability as well as repeatability from run to run.

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.

Receptors and channels regulating acrosome reactions

Prospective clinical studies informed by cloning and sequencing of sperm surface receptors and metal ion channels have elucidated critical early steps in the acrosome reaction that explain aspects of metal ion-related male infertility. Induction of the acrosome reaction is proposed to include non-nuclear progesterone receptor activation of Shaker-related sperm head voltage-gated potassium ion channels (VGKC). Men express VGKC isoforms with differing sensitivities to lead (Pb(2+)) inhibition, thus explaining interindividual variabilities in Pb(2+)-related male infertility. VGKC opening induces calcium (Ca(2+)) transients, and a signalling cascade induced by zona receptor aggregation requires an actin cytoskeleton created by the VGKC-induced Ca(2+) transients. Actin polymerization and stabilization, favoured by zinc (Zn(2+)) and depolymerized by cadmium (Cd(2+)), may mediate low Zn(2+) and high Cd(2+) infertile states. Zona receptor aggregation induces phosphotyrosine signals at sites, including sperm voltage-dependent Ca(2+) channels (VDCC), intermediate in electrophysiology between T- and L-type channels. Sperm surface VDCC localize at the sperm equatorial segment, the terminus of zona receptor translocation. Opening of VDCC admits a second Ca(2+) wave that activates phospholipase C phosphorylated in the zona receptor cascade. Phospholipase C induces fusogenic lipids and activates actin-severing proteins, depolymerizing the actin cytoskeleton and permitting apposition and fusion of acrosomal and plasma membranes.

Probing the structure of F-actin: cross-links constrain atomic models and modify actin dynamics

Cross-links between protomers in F-actin can be used as a very sensitive probe of both the dynamics and structure of F-actin. We have characterized filaments formed from a previously described yeast actin Q41C mutant, where disulfide bonds can be formed between the Cys41 that is introduced into subdomain-2 and Cys374 on an adjacent protomer. We find that the distribution of cross-linked n-mers shows no cooperativity and corresponds to a random probability cross-linking reaction. The random distribution suggests that disulfide formation does not cause a significant perturbation of the F-actin structure. Consistent with this lack of perturbation, three-dimensional reconstructions of extensively cross-linked filaments, using a new approach to helical image analysis, show very small structural changes with respect to uncross-linked filaments. This finding is in conflict with refined models but in agreement with the original Holmes et al. model for F-actin. Under conditions where 94 % of the protomers are linked by disulfide bonds, the distribution of filament twist becomes more heterogeneous with respect to control filaments. A molecular model suggests that strain, introduced by the disulfide, is relieved by increasing the twist of the long-pitch actin helices. Disulfide formation makes yeast actin filaments approximately three times less flexible in terms of bending and similar, in this respect, to vertebrate skeletal muscle F-actin. These observations support previous reports that the rigidity of F-actin can be controlled by the position of subdomain-2, and that this region is more flexible in yeast F-actin than in skeletal muscle F-actin.

The assembly of Ni2+-actin: some peculiarities

Nickel alters the organisation of highly dynamic cytoskeletal elements. In cultured cells Ni2+ causes microtubule aggregation and bundling as well as microfilament aggregation and redistribution. Here, we have analysed the effect(s) of Ni2+ on in vitro actin polymerisation. Using limited proteolysis by trypsin we have suggested that the regions around Arg-62 and Lys-68 change their conformation following Ni2+ binding to the single high-affinity site for divalent cations in the G-actin molecule. We have found that Ni2+ shortens the lag phase of actin polymerisation and increases the rate of assembly mainly because of an increased elongation rate. Ni2+ has no significant effect on the final plateau of actin polymerisation nor on the actin critical concentration. Electron microscopy revealed that actin filaments polymerised by 2 mM Ni2+ showed some tendency to lateral aggregation, being frequently formed by the cohesion of two or three filaments. Furthermore, they often appeared shorter than those of control as also confirmed by the larger amount of free filament ends as well as the faster depolymerisation rate than control.

Spectroscopic study of conformational changes in subdomain 1 of G-actin: influence of divalent cations

Temperature dependence of the fluorescence intensity and anisotropy decay of N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine attached to Cys374 of actin monomer was investigated to characterize conformational differences between Ca- and Mg-G-actin. The fluorescence lifetime is longer in Mg-G-actin than that in Ca-G-actin in the temperature range of 5-34 degrees C. The width of the lifetime distribution is smaller by 30% in Mg-saturated actin monomer at 5 degrees C, and the difference becomes negligible above 30 degrees C. The semiangle of the cone within which the fluorophore can rotate is larger in Ca-G-actin at all temperatures. Electron paramagnetic resonance measurements on maleimide spin-labeled (on Cys374) monomer actin gave evidence that exchange of Ca2+ for Mg2+ induced a rapid decrease in the mobility of the label immediately after the addition of Mg2+. These results suggest that the C-terminal region of the monomer becomes more rigid as a result of the replacement of Ca2+ by Mg2+. The change can be related to the difference between the polymerization abilities of the two forms of G-actin.

Intestinal paracellular permeability during malnutrition in guinea pigs: effect of high dietary zinc

BACKGROUND

Zinc has been shown to have beneficial effects in vitro on epithelial barrier function, and in vivo to reduce intestinal permeability in malnourished children with diarrhoea.

AIMS

To determine whether malnutrition alters intestinal paracellular permeability, and whether zinc prevents such alterations.

METHODS

Guinea pigs were fed a normal protein diet (NP group), a low protein diet (LP group), or a low protein diet enriched with 1800 ppm zinc (LPZn group) for three weeks. Intestinal permeability was measured on jejunal segments mounted in Ussing chambers by measuring ionic conductance and mucosal to serosal fluxes of 14C-mannitol, 22Na, and horseradish peroxidase. Tight junction morphology was assessed on cryofracture replicas.

RESULTS

Mannitol and Na fluxes and ionic conductance increased in the LP group compared with the NP group but remained normal in the LPZn group. Accordingly, jejunal epithelia from the LP group, but not from the LPZn group, showed a small decrease in number of tight junctional strands compared with epithelia from the NP group. Neither malnutrition nor zinc treatment modified horseradish peroxidase fluxes.

CONCLUSIONS

Malnutrition is associated with increased intestinal paracellular permeability to small molecules, and pharmacological doses of zinc prevent such functional abnormality.

Alpha B-crystallin in cardiac tissue. Association with actin and desmin filaments

alpha B-Crystallin is a 20-kd peptide highly homologous to the small heat-shock proteins. This protein forms soluble homomultimeric complexes (M(r), 300-700 kd) and is very abundant in cardiac muscle cells. In vitro experiments (affinity column chromatography and binding studies with isolated proteins) have shown that alpha B-crystallin interacts directly with actin and, in particular, with desmin filaments. The immunocytochemical localization of alpha B-crystallin within the cardiomyocytes showed that the protein is distributed exclusively in the central region of the I bands (Z lines), where desmin is localized. In vitro studies have further shown that the binding affinity of alpha B-crystallin to actin and desmin filaments increases considerably at slightly acidic pH (6.5) or after a heat treatment (45 degrees C). Moreover, alpha B-crystallin was found to prevent effectively the tendency of actin filaments to form aggregates (i.e., paracrystals) at acidic pH. These in vitro data suggest a protective role of alpha B-crystallin during stress conditions such as ischemia of the heart. Crystallin could prevent the aggregation of filaments, which might occur during the acidification of the cytosol and lead eventually to irreversible structural damage.

Formation of 2-D paracrystals of F-actin on phospholipid layers mixed with quaternary ammonium surfactants

Two-dimensional paracrystalline arrays of F-actin have been formed on positively charged lipid layers composed of phosphatidylcholine (PC) and quaternary ammonium surfactants. These quaternary ammonium surfactants were found to be better promoters of two-dimensional order than PC lipid layers mixed with stearylamine. In addition, the length of the hydrocarbon chain was found to influence the achievement of 2-D order. Lipid layers composed of dilauryl-PC and didodecyldimethylammonium bromide, which are saturated C12 lipids, promoted 2-D crystallization better than mixtures of dipalmitoyl-PC, a saturated C16 lipid, and dioctadecyldimethylammonium bromide, a saturated C18 lipid. Thus, the hydrocarbon chain length, which influences lipid layer fluidity, had a significant effect on paracrystal formation. We suggest that quaternary ammonium surfactants may have advantages in some cases for forming ordered arrays on lipid layers. In addition to investigating the effect of lipid layer composition on paracrystal formation, we found that the injection of G-actin rather than F-actin under a fluid lipid layer into a polymerizing solution produced better ordered paracrystals.

Osmotically induced electrical signals from actin filaments

Actin filaments, F-actin, a major component of the cortical cytoskeleton, play an important role in a variety of cell functions. In this report we have assessed the role of osmotic stress on the electrochemical properties of F-actin. The spontaneous Donnan potential of a polymerized actin solution (5 mg/ml) was -3.93 +/- 1.84 mV, which was linearly reduced by osmotic stress on the order of 1-20 mOsm (0.28 +/- 0.06 mV/mM). Calculated surface charge density was reduced and eventually reversed by increasing the osmotic stress as expected for a phase transition behavior. The electro-osmotic behavior of F-actin disappeared at pH 5.5 and was dependent on its filamentous nature. Furthermore, osmotically stressed F-actin displayed a nonlinear electric response upon application of electric fields on the order of 500-2,000 V/cm. These data indicate that F-actin in solution may display nonideal electro-osmotic properties consistent with ionic "cable" behavior which may be of biological significance in the processing and conduction of electrical signals within the cellular compartment.

pH dependence of actin self-assembly

Fluorescence enhancement and fluorescence photobleaching recovery have been utilized to examine actin self-assembly over the pH range 6.6-8.0. The kinetics of assembly are faster and the critical concentrations are lower at lower pH. Filament diffusion coefficients are not a function of pH, indicating that average filament lengths are not pH dependent. Although critical actin concentrations are a sensitive function of the concentrations of various cations in the medium, the relative pH dependences of critical concentrations are similar for all combinations of cations employed. The pH dependence of actin self-assembly is sufficiently great that it should be taken into account when comparing data from different reports and when relating in vitro measurements to cytoplasmic mechanisms.

Polymerization of actin by positively charged liposomes

By cosedimentation, spectrofluorimetry, and electron microscopy, we have established that actin is induced to polymerize at low salt concentrations by positively charged liposomes. This polymerization occurs only at the surface of the liposomes, and thus monomers not in direct contact with the liposome remain monomeric. The integrity of the liposome membrane is necessary to maintain actin in its polymerized state since disruption of the liposome depolymerizes actin. Actin polymerized at the surface of the liposome is organized into two filamentous structures: sheets of parallel filaments in register and a netlike organization. Spectrofluorimetric analysis with the probe N-pyrenyl-iodoacetamide shows that actin is in the F conformation, at least in the environment of the probe. However, actin assembly induced by the liposome is not accompanied by full ATP hydrolysis as observed in vitro upon addition of salts.

Representing and defining patterns by graphs: applications to sol-gel patterns and to cytoskeleton

The problems of interdisciplinary interests--applying methods of graph theory to sol-gel patterns and to cytoskeleton--are discussed. The importance of sol-gel transition phenomena in living cells and the possibility of periodic sol-gel transition phenomena are briefly reviewed. Representing patterns by graphs and using graph probabilistic representation for calculating structure-property relationships are discussed and applied to sol-gel transition patterns.

Isolation and characterization of actin from cultured BHK cells

Cytoplasmic actin from cultured fibroblasts has been purified to homogeneity and characterized with respect to its polymerization and structure. It was found to be qualitatively similar to muscle actin in all respects, but significant quantitative differences in its properties were demonstrated. Although BHK actin did not polymerize in unfractionated cytoplasmic extracts, the purified BHK actin polymerized into filaments both in magnesium and calcium. The critical concentration, measured by the DNase I inhibition assay and by fluorimetry, was the same as that of muscle actin both in magnesium and calcium. Polymerization of pyrene-labelled BHK and muscle actin was followed by fluorimetry. Significant differences in kinetics were found under both ionic conditions tested. In the absence of Mg2+ ions (0.2 mM CaCl2, 85 mM KCl), BHK actin polymerized at a much slower rate than muscle actin. In the presence of magnesium and EGTA, the nucleation phase for BHK actin polymerization was shorter than that for muscle actin and the kinetics of polymerization was different. The structure of BHK actin filaments in the electron micrographs was very similar to that of muscle actin. In high concentrations of magnesium, BHK actin formed paracrystals which had the same appearance as muscle actin paracrystals. However, calcium-induced formation of actin paracrystals required higher concentration of Ca2+ ions for BHK actin than for muscle actin (12 mM and 8 mM respectively). These results suggest differences in divalent cation binding to both high- and low-affinity sites of the two actins.

Effects of temperature on actin polymerized by Ca2+. Direct evidence of fragmentation

When the temperature is lowered from 20 to 4 degrees C, the specific viscosity of actin polymerized in the presence of either 4 mM-CaCl2 or 2 mM-MgCl2, but not of actin polymerized in the presence of 90 mM-KCl, is decreased by 50% in the absence of free ATP. Addition of ATP restores the viscosity of the actin polymerized by Mg2+, but not that of actin polymerized by Ca2+, to the original value. The effect of temperature on actin polymerized in the presence of Ca2+ is due to (a) polymer-into-monomer conversion, (b) latero-lateral aggregation of filaments, and (c) fragmentation of the filaments. Fragmentation, as demonstrated by fractional centrifugation and electron microscopy, was the most important of these.

The binding of Ca2+ to actin monomer is monitored by the fluorescence of actin-bound auramine O

The fluorescence of the cation auramine O was substantially enhanced by the presence of actin monomer. Titrations of this fluorescence enhancement indicated that actin monomer had two auramine O binding sites, each with a dissociation constant of approx. 20 microM. Calcium ions had no effect on the number of actin monomer-bound auramine O molecules or on the dissociation constant for that interaction. However, calcium ions increased the maximum change of fluorescence that occurs when actin monomer was fully saturated with auramine O. This effect of calcium was saturable and yielded a Ca2+ dissociation constant of 1.6 mM. It was concluded that auramine O bound to sites on actin monomer and independently monitored the binding of Ca2+ ion(s) to other site(s) on actin monomer. Further, the magnitude of the Ca2+ dissociation constant suggested that this Ca2+-binding site may be representative of the multiple bivalent cation-binding sites on actin monomer which are thought to be directly involved in actin polymerization. However, the exact relationship between these sites remains unclear.

Helical disorder and the filament structure of F-actin are elucidated by the angle-layered aggregate

Angle-layered aggregates of F-actin are net-like structures induced by Mg2+ concentrations below that used to form paracrystals. These aggregates incorporate the angular disorder of subunits, which has been described elsewhere for isolated actin filaments. Because this disorder is incorporated into the aggregates in solution at the time they are formed, the possibility of negative stain preparation being responsible for the disorder is excluded. The simple two-layered geometry of the angle-layered aggregate provides information about the shape of the component actin filaments free from the superposition of large numbers of layers. A model for the filament shape, derived from single filaments and confirmed by the angle-layered aggregate, is different from those that have previously emerged from paracrystal studies. An understanding of the interfilament bond in both the angle-layered aggregate and the paracrystal allows one to reconcile these different models. We have found a bipolar bonding rule, with staggered crossover points in the angle-layered aggregate, which we suggest is also responsible for Mg2+ paracrystals. This bonding rule can explain the apparent alignment of crossover points in adjacent filaments in paracrystals as a consequence of the superposition of staggered filaments.

The effect of ATP concentration on the rate of actin polymerization

It was found that the rate of polymerization of G-actin increased with the decrease of ATP concentration. When excess ATP was replaced by chloride through anion-exchange treatment, the extent of actin polymerization did not change provided that the ionic strength was raised immediately after the treatment. In the meantime, the rate of actin polymerization was greatly enhanced after the removal of excess ATP. The rate enhancement was much less when both excess Ca2+ and excess ATP were removed. G-actin with excess ATP replaced by chloride had larger light scattering and showed a "catalytic" effect on the polymerization of normal G-actin. The inhibition of actin polymerization by cytochalasin B in 100 mM KCl was much more obvious for G-actin with excess ATP removed than for normal G-actin. It is suggested that the reduction of excess ATP concentration in a G-actin solution increases the binding of weak-affinity Ca2+ and promotes the formation of oligomeric actin (actin nuclei).

Multiple supramolecular structures formed by interaction of actin with protamine

When protamine is added to actin, different supramolecular structures are formed depending on the molar ratio of the two proteins and of the ionic strength of the medium. At low ionic strength, and going from a molar ratio of protamine to G-actin of 4:1, 2:1 and 1:1, globular aggregates are first converted into extended structures and then to long threads in which the constituent ATP-G-actin is rapidly exchangeable with the actin of the medium. At high ionic strength {Tyrode [(1910) Arch. Int. Pharmacodyn. Ther.20, 205-212] solution}, starting from G-actin and protamine in the 1:1 molar ratio, long ropes are formed that can be resolved into intertwining filaments of 4-5nm diameter. The addition of protamine in a 1:1 molar ratio to a solution of F-actin in Tyrode solution causes the breakage of the actin filaments, which is also revealed by the decrease of the viscosity of the solution and the formation of ordered latero-lateral aggregates. The structures formed by reaction of protamine with G-actin can be separated from free G-actin and protamine by filtration through 0.45mum-pore-size Millipore filters. This technique has been exploited to study the exchange reaction between free actin and the actin-protamine complexes. For these studies the 1:1 actin-protamine complex formed at low ionic strength and the 2:1 actin-protamine complex formed in the presence of 23nm-free Mg(2+) have been selected. In the first case the exchange reaction is practically complete in the dead time of the experiment (20s). In the second case, where the complex operates like a true ATPase, the rate of the exchange is initially comparable with the rate of the ATP cleavage. Later on, however, the complex undergoes a change and the rate of the exchange between free actin and the actin bound to protamine becomes lower than the rate of the ATPase reaction. It is proposed that the ATP exchanges for ADP directly on the G-actin bound in the complex.

Polymorphism of actin paracrystals induced by polylysine

We describe a method for the induction of different polymorphic forms of actin filament paracrystals. This polymorphism is probably based on differences in the stagger and/or polarity of adjacent filaments in single-layered paracrystals and by superposition of different layers in multilayered paracrystals. The helical parameters defining the filament geometry are indistinguishable for the different polymorphic forms observed and for the four different actins used. Analysis of these paracrystals, some of which are ordered to better than 2.5 nm, should provide a reference structure suitable for alignment and orientation within the actin filament of high resolution models of the actin monomer obtained from crystal data.

Different polymeric forms of actin detected by the fluorescent probe terbium ion

The interaction of actin with Tb3+ was studied by following the fluorescence emitted at 545 nm when the protein was excited at 285 nm in the presence of Tb3+. It was shown that, at low ionic strength, each actin monomer binds, at saturation, six Tb3+ with an association constant of 0.8 microM-1. In the presence of 0.1 M KCl the association constant decreases to 0.15 and 0.24 microM-1 at subcritical and overcritical actin concentrations, respectively; the number of the binding sites remains six. When polymeric actin is formed by the addition of 2 mM MgCl2, the association constant drops to 0.008 micro M-1 and the number of the binding sites to four. The lower number of the Tb3+ binding sites (four) in the actin polymerized by MgCl2 as compared to the number of binding sites (six) of the actin polymerized by KCl is taken as evidence of a looser structure of this latter polymer. We have also shown that Tb3+ does not replace 45Ca2+ at the single, "high-affinity" site of G-actin. Removal of this Ca2+, in the presence of Tb3+, destroys the typical G- and F-actin structures.

Effects of formamide on the polymerization and depolymerization of muscle actin

Formamide was found to interfere with the polymerization of electrophoretically pure rabbit skeletal muscle G-actin to F-actin in vitro. It decreased the rate as well as the extent of polymerization. However, this influence was dependent on the way the polymerization reaction was initiated. If polymerization of G-actin was induced by 2 mM MgCl2, formamide inhibited the rate and extent of the polymerization much less than if the polymerization was induced by either 2 mM CaCl2 or 0.1 M KCl. The critical protein concentration was increased when formamide was present. This effect was small in the presence of MgCl2 but an approximately tenfold increase in the critical value was observed for the CaCl2-induced or KCl-induced polymerization. Depolymerization of F-actin by molar amounts of formamide was faster and proceeded further when the polymer had been formed in the presence of KCl or CaCl2 than when it been formed in the presence of MgCl2. It is concluded that Mg2+ stabilizes the F-actin structure rendering it more resistant than either Ca2+ or K+ against the destabilizing action of formamide.

Polymerization of G-actin by hydrodynamic shear stresses

In the absence of Ca2+ G-actin can be polymerized by the application of shear stress in low ionic strength buffer. When G-actin in low ionic strength buffer containing EGTA was sheared for predetermined times under different velocity gradients, viscosity attained a maximal value, comparable to that obtained by seeding with F-actin nuclei, at a velocity gradient of 3000 s-1 after about one hour. Such flow-polymerized actin was indistinguishable from KCl-polymerized actin. Under similar conditions, EDTA which can bind both Ca2+ and Mg2+, gave a smaller effect than the Ca2+-chelating agent EGTA which binds Mg2+ weakly. When an Mg2+ salt was added to EDTA- or EGTA-containing buffer to give a free Mg2+ concentration of a few micromoles/liter, flow-induced polymerization was significantly enhanced. It appears that occupancy of only a small fraction of the high affinity binding sites by Ca2+ prevents flow-polymerization while Mg2+ may enhance this type of polymerization by replacing Ca2+. We speculate that the shear stress induces polymerization by promoting nucleation and that Ca2+ bound to the high affinity divalent cation binding site inhibits formation of the nuclei.

Crystalline actin tubes. II. The effect of various lanthanide ions on actin tube formation

The ability of skeletal muscle actin to aggregate in the form of crystalline tubular structures was examined using all 14 of the available trivalent lanthanide cations. Under conditions which normally cause F-actin formation, (0.1 M KCl, pH 6.9) only those lanthanide ions which interact with actin with a molar ratio of 5:1 (Ce3+, Pr3+, Nd3+, Sm3+ and Eu3+) or 6:1 (Gd3+, Tb3+, Dy3+ and Ho3+) can promote the formation of ordered tubular structures. Under the same conditions, the remaining ions (Er3+, Tm3+, Yb3+, Lu3+ as well as La3+) interact with a 7:1 molar ratio of lanthanide to actin, but these were unable to form actin tubes. Actin tube dimensions undergo systematic changes depending on which lanthanide ion binds. The dimensions of the actin monomers and their packing arrangement (i.e. number of rows of subunits per helical repeat and the pitch angle) determines the actin tube diameter. It is suggested that the failure of actin tube formation when 7 lanthanide ions bind is due to additional charge on the monomer, possibly conferring on it a net positive charge.