Unpolymerized actin in fibroblasts and brain

Measuring expression heterogeneity of single-cell cytoskeletal protein complexes

Multimeric cytoskeletal protein complexes orchestrate normal cellular function. However, protein-complex distributions in stressed, heterogeneous cell populations remain unknown. Cell staining and proximity-based methods have limited selectivity and/or sensitivity for endogenous multimeric protein-complex quantification from single cells. We introduce micro-arrayed, differential detergent fractionation to simultaneously detect protein complexes in hundreds of individual cells. Fractionation occurs by 60 s size-exclusion electrophoresis with protein complex-stabilizing buffer that minimizes depolymerization. Proteins are measured with a ~5-hour immunoassay. Co-detection of cytoskeletal protein complexes in U2OS cells treated with filamentous actin (F-actin) destabilizing Latrunculin A detects a unique subpopulation (~2%) exhibiting downregulated F-actin, but upregulated microtubules. Thus, some cells may upregulate other cytoskeletal complexes to counteract the stress of Latrunculin A treatment. We also sought to understand the effect of non-chemical stress on cellular heterogeneity of F-actin. We find heat shock may dysregulate filamentous and globular actin correlation. In this work, our assay overcomes selectivity limitations to biochemically quantify single-cell protein complexes perturbed with diverse stimuli.

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.

Mammalian CAP (Cyclase-associated protein) in the world of cell migration: Roles in actin filament dynamics and beyond

Cell migration is essential for a variety of fundamental biological processes such as embryonic development, wound healing, and immune response. Aberrant cell migration also underlies pathological conditions such as cancer metastasis, in which morphological transformation promotes spreading of cancer to new sites. Cell migration is driven by actin dynamics, which is the repeated cycling of monomeric actin (G-actin) into and out of filamentous actin (F-actin). CAP (Cyclase-associated protein, also called Srv2) is a conserved actin-regulatory protein, which is implicated in cell motility and the invasiveness of human cancers. It cooperates with another actin regulatory protein, cofilin, to accelerate actin dynamics. Hence, knockdown of CAP1 slows down actin filament turnover, which in most cells leads to reduced cell motility. However, depletion of CAP1 in HeLa cells, while causing reduction in dynamics, actually led to increased cell motility. The increases in motility are likely through activation of cell adhesion signals through an inside-out signaling. The potential to activate adhesion signaling competes with the negative effect of CAP1 depletion on actin dynamics, which would reduce cell migration. In this commentary, we provide a brief overview of the roles of mammalian CAP1 in cell migration, and highlight a likely mechanism underlying the activation of cell adhesion signaling and elevated motility caused by depletion of CAP1.

F- and G-actin concentrations in lamellipodia of moving cells

Cells protrude by polymerizing monomeric (G) into polymeric (F) actin at the tip of the lamellipodium. Actin filaments are depolymerized towards the rear of the lamellipodium in a treadmilling process, thereby supplementing a G-actin pool for a new round of polymerization. In this scenario the concentrations of F- and G-actin are principal parameters, but have hitherto not been directly determined. By comparing fluorescence intensities of bleached and unbleached regions of lamellipodia in B16-F1 mouse melanoma cells expressing EGFP-actin, before and after extraction with Triton X-100, we show that the ratio of F- to G-actin is 3.2+/−0.9. Using electron microscopy to determine the F-actin content, this ratio translates into F- and G-actin concentrations in lamellipodia of approximately 500 µM and 150 µM, respectively. The excess of G-actin, at several orders of magnitude above the critical concentrations at filament ends indicates that the polymerization rate is not limited by diffusion and is tightly controlled by polymerization/depolymerization modulators.

Calponin binds G-actin and F-actin with similar affinity

Calponins are actin-binding proteins that are implicated in the regulation of actomyosin. Calponin binds filamentous actin (F-actin) through two distinct sites ABS1 and ABS2, with an affinity in the low micromolar range. We report that smooth muscle calponin binds monomeric actin with a similar affinity (K(d) of 0.15 microM). We show that the arrangement of binding is similar to that of F-actin by a number of criteria, most notably that the distance between Cys273 on calponin and Cys374 of actin is 29A when measured by fluorescent resonance energy transfer, the same distance as previously reported for F-actin.

Involvement of SipA in modulating actin dynamics during Salmonella invasion into cultured epithelial cells

Salmonella entry into epithelial host cells results from the host actin cytoskeleton reorganization that is induced by a group of bacterial proteins delivered to the host cells by the Salmonella type III secretion system. SopE, SopE2 and SopB activate CDC42 and Rac1 to intercept the signal transduction pathways involved in actin cytoskeleton rearrangements. SipA and SipC directly bind actin to modulate the actin dynamics facilitating bacterial entry. Biochemical studies have indicated that SipA decreases the critical concentration for actin polymerization and may be involved in promoting the initial actin polymerization in Salmonella-induced actin reorganization. In this report, we conducted experiments to analyze the in vivo function(s) of SipA during Salmonella invasion. SipA was found to be preferentially associated with peripheral cortical actin filaments but not stress fibres using permeabilized epithelial cells. When polarized Caco-2 cells were infected with Salmonella, actin cytoskeleton rearrangements induced by the wild-type strain had many filopodia structures that were intimately associated with the bacteria. In contrast, ruffles induced by the sipA null mutant were smoother and distant from the bacteria. We also found that the F-actin content in cells infected with the sipA mutant decreased nearly 80% as compared to uninfected cells or those infected with the wild-type Salmonella strain. Furthermore, expression of either the full-length or the SipA(459-684) actin-binding fragment induced prominent punctuate actin assembly in the cortical region of COS-1 cells. These results indicate that SipA is involved in modulating actin dynamics in cultured epithelial cells during Salmonella invasion.

The beta-thymosins, small actin-binding peptides widely expressed in the developing and adult cerebellum

The beta-thymosins are a highly conserved family of small polar peptides known to bind monomeric actin and inhibit its polymerization. The beta-thymosins show a high degree of sequence conservation among all vertebrate classes and they have been also identified in some invertebrate phyla. The most abundant beta-thymosins in mammals are thymosin beta4 (Tbeta4) and thymosin beta10 (Tbeta10), two ubiquitous small (43 amino acids) peptides sharing a high degree of sequence homology. Both beta-thymosins are present in virtually all mammalian tissues and cells studied, showing distinct patterns of expression in several tissues. The beta-thymosins are expressed in the developing and mature nervous system, indicating their participation with other actin-binding peptides in the control of actin polymerization. In the rat cerebellum the temporal and cellular patterns of expression of Tbeta4 and Tbeta10 are different, suggesting that each beta-thymosin could play a specific physiological function during cerebellum development. The possible roles of beta-thymosins in the developing mammalian cerebellum are discussed.

Actin Directly Interacts with Phospholipase D, Inhibiting Its Activity

Mammalian phospholipase D (PLD) plays a key role in several signal transduction pathways and is involved in many diverse functions. To elucidate the complex molecular regulation of PLD, we investigated PLD-binding proteins obtained from rat brain extract. Here we report that a 43-kDa protein in the rat brain, beta-actin, acts as a major PLD2 direct-binding protein as revealed by peptide mass fingerprinting in combination with matrix-assisted laser desorption ionization/time-of-flight mass spectrometry. We also determined that the region between amino acids 613 and 723 of PLD2 is required for the direct binding of beta-actin, using bacterially expressed glutathione S-transferase fusion proteins of PLD2 fragments. Intriguingly, purified beta-actin potently inhibited both phosphatidylinositol-4,5-bisphosphate- and oleate-dependent PLD2 activities in a concentration-dependent manner (IC50 = 5 nm). In a previous paper, we reported that alpha-actinin inhibited PLD2 activity in an interaction-dependent and an ADP-ribosylation factor 1 (ARF1)-reversible manner (Park, J. B., Kim, J. H., Kim, Y., Ha, S. H., Kim, J. H., Yoo, J.-S., Du, G., Frohman, M. A., Suh, P.-G., and Ryu, S. H. (2000) J. Biol. Chem. 275, 21295-21301). In vitro binding analyses showed that beta-actin could displace alpha-actinin binding to PLD2, demonstrating independent interaction between cytoskeletal proteins and PLD2. Furthermore, ARF1 could steer the PLD2 activity in a positive direction regardless of the inhibitory effect of beta-actin on PLD2. We also observed that beta-actin regulates PLD1 and PLD2 with similar binding and inhibitory potencies. Immunocytochemical and co-immunoprecipitation studies demonstrated the in vivo interaction between the two PLD isozymes and actin in cells. Taken together, these results suggest that the regulation of PLD by cytoskeletal proteins, beta-actin and alpha-actinin, and ARF1 may play an important role in cytoskeleton-related PLD functions.

Differential expression of thymosins beta(4) and beta(10) during rat cerebellum postnatal development

The beta-thymosins are a family of actin monomer-sequestering proteins widely distributed among vertebrate classes. The most abundant beta-thymosins in mammalian species are thymosin beta(4) (Tbeta(4)) and thymosin beta(10) (Tbeta(10)), two small peptides (43 amino acids) sharing a high degree of sequence homology. In the present work, we have analyzed the distribution of Tbeta(4) and Tbeta(10) in the developing and adult rat cerebellum using in situ hybridization and immunohistochemistry techniques. Our results show that the temporal and cellular patterns of expression of both beta-thymosins are different. In the young (7 and 18 postnatal days) and adult (1 and 4 months old) rat cerebellum, Tbeta(4) was mainly expressed in the glia (microglia, Golgi epithelial cells and oligodendrocytes), neurons (granule cells and Purkinje cells), and in the capillaries. In 14-month-old rats, the Tbeta(4) immunoreactivity was only detected in some microglia cells. In young and adult animals, most of the Tbeta(10) immunoreactivity was localized in several types of neuronal cells including granule cells, Golgi neurons and Purkinje cells. In old animals, a faint Tbeta(10) signal could be detected in a few Purkinje cells. Our results suggest that each beta-thymosin could play a different function in the control of actin dynamics.

Molecular mechanisms controlling actin filament dynamics in nonmuscle cells

We review how motile cells regulate actin filament assembly at their leading edge. Activation of cell surface receptors generates signals (including activated Rho family GTPases) that converge on integrating proteins of the WASp family (WASp, N-WASP, and Scar/WAVE). WASP family proteins stimulate Arp2/3 complex to nucleate actin filaments, which grow at a fixed 70 degrees angle from the side of pre-existing actin filaments. These filaments push the membrane forward as they grow at their barbed ends. Arp2/3 complex is incorporated into the network, and new filaments are capped rapidly, so that activated Arp2/3 complex must be supplied continuously to keep the network growing. Hydrolysis of ATP bound to polymerized actin followed by phosphate dissociation marks older filaments for depolymerization by ADF/cofilins. Profilin catalyzes exchange of ADP for ATP, recycling actin back to a pool of unpolymerized monomers bound to profilin and thymosin-beta 4 that is poised for rapid elongation of new barbed ends.

Role of MARCKS in regulating endothelial cell proliferation

Myristoylated alanine-rich C kinase substrate (MARCKS), as a specific protein kinase C (PKC) substrate, mediates PKC signaling through its phosphorylation and subsequent modification of its association with filamentous actin (F-actin) and calmodulin (CaM). PKC has long been implicated in cell proliferation, and recent studies have suggested that MARCKS may function as a cell growth suppressor. Therefore, in the present study, we investigated MARCKS protein expression, distribution, and phosphorylation in preconfluent and confluent bovine pulmonary microvascular endothelial cells (BPMEC) in the presence or absence of the vascular endothelial growth factor (VEGF). In addition, we examined functional alterations of MARCKS in these cells by studying the association of MARCKS with F-actin and CaM-dependent myosin light chain (MLC) phosphorylation. Our results indicate that MARCKS protein is downregulated during BPMEC proliferation. Decreased MARCKS association with F-actin, increased actin polymerization, and CaM-dependent MLC phosphorylation appear to mediate cell shape changes and motility during BPMEC growth. In contrast, VEGF stimulated MARCKS phosphorylation without alteration of protein expression during BPMEC proliferation, which may result in reduced interaction between MARCKS and actin or CaM, leading to actin reorganization and MLC phosphorylation. Our data suggest a regulatory role of MARCKS during endothelial cell proliferation.

Sequestered actin in chick embryo fibroblasts

Chick embryo fibroblasts contain about 75-100 microM unpolymerized actin and at least four proteins which can bind actin monomers, actin depolymerizing factor (ADF), gelsolin, profilin, and thymosin beta4 (Tbeta4). Fibroblast extracts are analyzed by non-denaturing polyacrylamide gel electrophoresis and immunoblotting where most of the G-actin is detected as a complex with Tbeta4. When fibroblast extracts are fractionated by gel filtration and the fractions are analyzed by PAGE and HPLC, most of the G-actin elutes in a peak that also contains Tbeta4 at an overall molar ratio of 1.9:1 relative to actin. Gelsolin, profilin, and ADF are also detectable in the gel filtration eluate and at least partly coelute with actin, and account for only a minor fraction of the soluble actin pool. These observations indicate that under the growth conditions studied, Tbeta4 is the major actin-sequestering protein in fibroblasts.

Thrombin-induced phosphorylation of MARCKS does not alter its interactions with calmodulin or actin

Myristoylated alanine-rich C kinase substrate (MARCKS) is a calmodulin (CaM)- and actin-binding protein and prominent protein kinase C (PKC) substrate. In vitro phosphorylation of MARCKS by PKC has been shown to induce the release of both CaM and actin, leading to the suggestion that MARCKS may regulate CaM availability during agonist-induced signalling. In support of this hypothesis we previously demonstrated that thrombin-induced MARCKS phosphorylation in endothelial cells (EC) parallels activation of myosin light chain kinase, a CaM-dependent enzyme. To test this theory further, we transfected CHO cells, which normally do not express significant levels of MARCKS, with a MARCKS cDNA. The thrombin-stimulated phosphorylation of myosin light chains and the sensitivity to CaM antagonists in the MARCKS overexpressing cells was the same as that in control CHO cells. MARCKS associated with the actin cytoskeleton in EC was markedly increased upon treatment with the PKC activator, PMA, but only modestly enhanced by thrombin treatment. Similarly, colocalisation of MARCKS with actin was enhanced when the EC were challenged with PMA but not thrombin. These data may be partially explained by PKC-independent phosphorylation of MARCKS in response to thrombin stimulation.

Profilin is predominantly associated with monomeric actin in Acanthamoeba

We used biochemical fractionation, immunoassays and microscopy of live and fixed Acanthamoeba to determine how much profilin is bound to its known ligands: actin, membrane PIP(2), Arp2/3 complex and polyproline sequences. Virtually all profilin is soluble after gentle homogenization of cells. During gel filtration of extracts on Sephadex G75, approximately 60% of profilin chromatographs with monomeric actin, 40% is free and none voids with Arp2/3 complex or other large particles. Selective monoclonal antibodies confirm that most of the profilin is bound to actin: 65% in extract immunoadsorption assays and 74–89% by fluorescent antibody staining. Other than monomeric actin, no major profilin ligands are detected in crude extracts. Profilin-II labeled with rhodamine on cysteine at position 58 retains its affinity for actin, PIP(2) and poly-L-proline. When syringe-loaded into live cells, it distributes throughout the cytoplasm, is excluded from membrane-bounded organelles, and concentrates in lamellapodia and sites of endocytosis but not directly on the plasma membrane. Some profilin fluorescence appears punctate, but since no particulate profilin is detected biochemically, these spots may be soluble profilin between organelles that exclude profilin. The distribution of profilin in fixed human A431 cells is similar to that in amoebas. Our results show that the major pool of polymerizable actin monomers is complexed with profilin and spread throughout the cytoplasm.

Rho-family GTPases require the Arp2/3 complex to stimulate actin polymerization in Acanthamoeba extracts

BACKGROUND

Actin filaments polymerize in vivo primarily from their fast-growing barbed ends. In cells and extracts, GTPgammaS and Rho-family GTPases, including Cdc42, stimulate barbed-end actin polymerization; however, the mechanism responsible for the initiation of polymerization is unknown. There are three formal possibilities for how free barbed ends may be generated in response to cellular signals: uncapping of existing filaments; severing of existing filaments; or de novo nucleation. The Arp2/3 complex localizes to regions of dynamic actin polymerization, including the leading edges of motile cells and motile actin patches in yeast, and in vitro it nucleates the formation of actin filaments with free barbed ends. Here, we investigated actin polymerization in soluble extracts of Acanthamoeba.

RESULTS

Addition of actin filaments with free barbed ends to Acanthamoeba extracts is sufficient to induce polymerization of endogenous actin. Addition of activated Cdc42 or activation of Rho-family GTPases in these extracts by the non-hydrolyzable GTP analog GTPgammaS stimulated barbed-end polymerization, whereas immunodepletion of Arp2 or sequestration of Arp2 using solution-binding antibodies blocked Rho-family GTPase-induced actin polymerization.

CONCLUSIONS

For this system, we conclude that the accessibility of free barbed ends regulates actin polymerization, that Rho-family GTPases stimulate polymerization catalytically by de novo nucleation of free barbed ends and that the primary nucleation factor in this pathway is the Arp2/3 complex.

A quantitative analysis of G-actin binding proteins and the G-actin pool in developing chick brain

The large G-actin pool in individual actively motile cells has been shown to be maintained primarily by the actin sequestering protein thymosin beta four (Tbeta4). It is not clear whether Tbeta4 or an isoform also plays a primary role in neural tissue containing highly motile axonal growth cones. To address this question we have made a definitive analysis of the relative contributions of all the known G-actin sequestering proteins: Tbeta4, Tbeta10, profilin, and phosphorylated (inactive) and unphosphorylated (potentially active) forms of both ADF and cofilin, in relation to the G-actin pool in developing chick brain at embryonic days 13 and 17. From our measurements we estimate the intracellular concentration of G-actin as 30-37 microM and of Tbeta4 as 50-60 microM in an 'average' brain cell in embryonic chick brain. No other beta thymosin isoforms were detected in these brain extracts. The ratio of soluble, unphosphorylated ADF to Tbeta4 is only 1:7 at 13 embryonic days, but increases to 1:4 at 17 days. Profilin and cofilin concentrations are an order of magnitude lower than Tbeta4. Combining the contributions of Tbeta4, unphosphorylated ADF and unphosphorylated cofilin, we estimate a mean G-actin critical concentration of approximately 0.45 microM and approximately 0.2 microM, respectively, in day 13 and day 17 embryonic brain extracts, suggesting a significant developmental decrease. We conclude that (a) Tbeta4 is the major actin sequestering protein in embryonic chick brain and the only beta thymosin isoform present; (b) ADF may play a significant developmental role, as its concentration changes significantly with age; (c) the known G-actin binding proteins can adequately account for the G-actin pool in embryonic chick brain.

Interactions of Acanthamoeba profilin with actin and nucleotides bound to actin

Three methods, fluorescence anisotropy of rhodamine-labeled profilin, intrinsic fluorescence and nucleotide exchange, give the same affinity, Kd = 0.1 microM, for Acanthamoeba profilins binding amoeba actin monomers with bound Mg-ATP. Replacement of serine 38 with cysteine created a unique site where labeling with rhodamine did not alter the affinity of profilin for actin. The affinity for rabbit skeletal muscle actin is about 4-fold lower. The affinity for both actins is 5-8-fold lower with ADP bound to actin rather than ATP. Pyrenyliodoacetamide labeling of cysteine 374 of muscle actin reduces the affinity for profilin 10-fold. The affinity of profilin for nucleotide-free actin is approximately 3-fold higher than for Mg-ATP-actin and approximately 24-fold higher than for Mg-ADP-actin. As a result, profilin binding reduces the affinity of actin 3-fold for Mg-ATP and 24-fold for Mg-ADP. Mg-ATP dissociates 8 times faster from actin-profilin than from actin and binds actin-profilin 3 times faster than actin. Mg-ADP dissociates 14 times faster from actin-profilin than from actin and binds actin-profilin half as fast as actin. Thus, profilin promotes the exchange of ADP for ATP. These properties allow profilin to bind a high proportion of unpolymerized ATP-actin in the cell, suppressing spontaneous nucleation but allowing free barbed ends to elongate at more than 500 subunits/second.

Role of actin in anchoring postsynaptic receptors in cultured hippocampal neurons: differential attachment of NMDA versus AMPA receptors

We used actin-perturbing agents and detergent extraction of primary hippocampal cultures to test directly the role of the actin cytoskeleton in localizing GABAA receptors, AMPA- and NMDA-type glutamate receptors, and potential anchoring proteins at postsynaptic sites. Excitatory postsynaptic sites on dendritic spines contained a high concentration of F-actin that was resistant to cytochalasin D but could be depolymerized using the novel compound latrunculin A. Depolymerization of F-actin led to a 40% decrease in both the number of synaptic NMDA receptor (NMDAR1) clusters and the number of AMPA receptor (GluR1)-labeled spines. The nonsynaptic NMDA receptors appeared to remain clustered and to coalesce in cell bodies. alpha-Actinin-2, which binds both actin and NMDA receptors, dissociated from the receptor clusters, but PSD-95 remained associated with both the synaptic and nonsynaptic receptor clusters, consistent with a proposed cross-linking function. AMPA receptors behaved differently; on GABAergic neurons, the clusters redistributed to nonsynaptic sites, whereas on pyramidal neurons, many of the clusters appeared to disperse. Furthermore, in control neurons, AMPA receptors were detergent extractable from pyramidal cell spines, whereas AMPA receptors on GABAergic neurons and NMDA receptors were unextractable. GABAA receptors were not dependent on F-actin for the maintenance or synaptic localization of clusters. These results indicate fundamental differences in the mechanisms of receptor anchoring at postsynaptic sites, both regarding the anchoring of a single receptor (the AMPA receptor) in pyramidal cells versus GABAergic interneurons and regarding the anchoring of different receptors (AMPA vs NMDA receptors) at a single class of postsynaptic sites on pyramidal cell dendritic spines.

Hydrogen peroxide-induced cytoskeletal rearrangement in cultured pulmonary endothelial cells

Although the signaling pathways leading to hydrogen peroxide (H2O2)-induced endothelial monolayer permeability remain ambiguous, cytoskeletal proteins are known to be essential for maintaining endothelial integrity and regulating solute flux through the monolayer. We have recently demonstrated that thrombin-induced actin reorganization in bovine pulmonary artery endothelial cells (BPAEC) requires activation of both myosin light chain kinase (MLCK) and protein kinase C (PKC). Therefore, the present study was designed to investigate the effects of H2O2 on actin reorganization in BPAEC. H2O2 initiated sustained recruitment of actin to the cytoskeleton and transient myosin recruitment in a time- and concentration-dependent manner. The H2O2-induced actin recruitment was significantly inhibited by the calmodulin antagonists, W7 and TFP, but not by the MLCK inhibitor, KT5926, nor the PKC inhibitors, H7 and calphostin C. H2O2 also caused actin filament rearrangement in BPAEC with disruption of the dense peripheral bands and formation of stress fibers. These alterations occurred prior to actin translocation to the cytoskeleton and are prevented by inhibition of either MLCK or PKC. High concentrations of H2O2 transiently attenuated PKC activity but slightly increased the phosphorylation of the prominent PKC substrate and actin-binding protein, myristoylated alanine-rich C kinase substrate (MARCKS), by 5 min. However, MARCKS phosphorylation was reduced to below basal levels by 30 min. On the other hand, H2O2 induced a time- and dose-dependent phosphorylation of myosin light chains which was eliminated by both MLCK and PKC inhibitors. These data suggest that MLCK contributes to H2O2-induced myosin light chain phosphorylation and actin rearrangement and that PKC may play a permissive role. Neither of these enzymes appears to be involved in the H2O2-induced recruitment of actin to the cytoskeleton.

Interpreting photoactivated fluorescence microscopy measurements of steady-state actin dynamics

A continuum model describing the steady-state actin dynamics of the cytoskeleton of living cells has been developed to aid in the interpretation of photoactivated fluorescence experiments. In a simplified cell geometry, the model assumes uniform concentrations of cytosolic and cytoskeletal actin throughout the cell and no net growth of either pool. The spatiotemporal evolution of the fluorescent actin population is described by a system of two coupled linear partial-differential equations. An analytical solution is found using a Fourier-Laplace transform and important limiting cases relevant to the design of experiments are discussed. The results demonstrate that, despite being a complex function of the parameters, the fluorescence decay in photoactivated fluorescence experiments has a biphasic behavior featuring a short-term decay controlled by monomer diffusion and a long-term decay governed by the monomer exchange rate between the polymerized and unpolymerized actin pools. This biphasic behavior suggests a convenient mechanism for extracting the parameters governing the fluorescence decay from data records. These parameters include the actin monomer diffusion coefficient, filament turnover rate, and ratio of polymerized to unpolymerized actin.

Molecular motors and cell motility in the brain

The advent of video computer-enhanced microscopy has provided a new vision of cell migrations, growth cones, and fast axonal transport in the nervous system. In images obtained in studies of fast transport in isolated axoplasm from the squid giant axon, a virtual torrent of membrane traffic could be seen moving in both directions. Similarly, examination of growth cones and cell migrations in vitro and in vivo revealed properties of cell motility that were previously unsuspected. Evidence has accumulated that many of these activities are driven by a variety of microtubule and microfilament based motors.

Thyroidal stimulation of tubulin and actin in rat brain cytoskeleton

In cultures of neonatal rat brain cells, labeled with 35S-methionine in the presence or absence of triiodothyronine (T3), the hormone promoted a significant enhancement of labeled tubulin and actin in the insoluble fraction (30,000 g pellet) of cell homogenate. To identify the specific sub-cellular fraction associated with this induction, organ cultures of 1 day rat cerebra were labelled with 35S-methionine in the presence and absence of T3 and the insoluble fraction (30,000 g pellet) was subfractionated into mitochondria, plasma membrane and cytoskeleton. Analysis of the labeled proteins by SDS-PAGE, autoradiography and densitometry revealed a T3-induced increase of 50-80% for both tubulin and actin, only in the cytoskeleton fraction without any significant effect on the other fractions. Similar results were obtained when plasma membrane or cytoskeleton were isolated directly from labeled cerebrum by conventional methods instead of fractionating from the 30,000 g pellet. Analysis of relative stimulation of labeled tubulin and actin by T3 in cytoskeleton fraction derived from primary cultures of neuronal (N) and glial (G) cells labeled with 35-methionine show that the stimulatory effect is predominantly on the N cells. Studies on the kinetics of induction of labeled tubulin and actin by T3 in the cytoskeleton fraction prepared from cerebra labeled with 35S-methionine for 2, 8 and 18 hrs revealed no significant difference at 2 hrs; at 8 hrs, an increased incorporation into both tubulin and actin was reproducibly seen in the controls relative to T3-treated samples.(ABSTRACT TRUNCATED AT 250 WORDS)

Location of profilin at presynaptic sites in the cerebellar cortex; implication for the regulation of the actin-polymerization state during axonal elongation and synaptogenesis

Profilin is a 15 kDa protein that binds actin monomers and inhibits their polymerization in vitro. The actin-profilin complex can be rapidly dissociated in vitro by phosphatidylinositol-4,5-bis-phosphate, providing a mechanism for regulating actin assembly-disassembly cycles during cell motile events. We have used a polyclonal antibody to calf spleen profilin to analyse the developmental expression and cellular distribution of profilin in the rat cerebellum and cultured cortical neurons. Immature neurons contain large amount of profilin both in vivo and in vitro. Immunofluorescence showed it to be present in developing neurites and growth cones but not in the filopodia of cortical neurons in culture. Profilin immunoreactivity was intense in the parallel fibres, the granule cell axons of the cerebellar cortex, at the time when they are elongating. Purkinje cell dendrites were not labelled. Profilin immunostaining was present in presynaptic varicosities, but not in dendritic spines within the molecular layer of juvenile and adult rats. The profilin concentration was higher in synaptosomes than in the total cerebellum during the second and third postnatal weeks, a period of intense synaptogenesis. Thus, profilin may help regulate actin polymerization and depolymerization during axonal elongation and synaptogenesis. Its restriction to the presynaptic site in the adult suggests that it may also be involved in the regulation of the release of synaptic vesicles.

The structure of crystalline profilin-beta-actin

The three-dimensional structure of bovine profilin-beta-actin has been solved to 2.55 A resolution by X-ray crystallography. There are several significant local changes in the structure of beta-actin compared with alpha-actin as well as an overall 5 degrees rotation between its two major domains. Actin molecules in the crystal are organized into ribbons through intermolecular contacts like those found in oligomeric protein assemblies. Profilin forms two extensive contacts with the actin ribbon, one of which appears to correspond to the solution contact in vitro.

Localization and dynamics of nonfilamentous actin in cultured cells

Although the distribution of filamentous actin is well characterized in many cell types, the distribution of nonfilamentous actin remains poorly understood. To determine the relative distribution of filamentous and nonfilamentous actin in cultured NRK cells, we have used a number of labeling agents that differ with respect to their specificities toward the filamentous or nonfilamentous form, including monoclonal and polyclonal anti-actin antibodies, vitamin D-binding protein (DBP), and fluorescent phalloidin. Numerous punctate structures were identified that bind poorly to phalloidin but stain positively with several anti-actin antibodies. These bead structures also stain with DBP, suggesting that they are enriched in nonfilamentous actin. Similar punctate structures were observed after the microinjection of fluorescently labeled actin into living cells, allowing us to examine their dynamics in living cells. The actin-containing punctate structures were observed predominantly in the region behind lamellipodia, particularly in spreading cells induced by wounding confluent monolayers. Time-lapse recording of cells injected with fluorescent actin indicated that they form continuously near the leading edge and move centripetally toward the nucleus. Our results suggest that at least part of the unpolymerized actin molecules are localized at discrete sites, possibly as complexes with monomer sequestering proteins. These structures may represent transient storage sites of G-actin within the cell which can be transformed rapidly into actin filaments upon stimulation by specific signals.

Analysis of the interactions of actin depolymerizing factor with G- and F-actin

Chick actin depolymerizing factor (ADF) is an actin binding protein previously shown to rapidly depolymerize actin filaments in vitro, yielding a 1:1 complex of ADF and actin monomer. Here we show that ADF protects actin monomer from denaturation by EDTA by inhibiting the exchange of actin-bound nucleotide. Under low ionic strength conditions, the approximate dissociation constant (KD) for the ADF-actin complex determined from exchange of nucleotide (1,N6-etheno-ATP) is about 150 and is calcium-independent. Addition of ADF to monomeric actin inhibits actin assembly as well as the ATP hydrolysis that normally accompanies assembly. Complex formation is demonstrated between ADF and actin containing either ATP, ADP, or AMPPNP as the bound nucleotide. A KD of 0.1-0.2 microM was calculated for both the ADF-ATP-actin and ADF-AMPPNP-actin complexes, whereas the KD for the ADF-ADP-actin complex is about 1.3 microM. ADF can either depolymerize or cosediment with F-actin in a stoichiometric fashion, but these reciprocal activities are pH-dependent. At pHs between 6.5 and 7.1, ADF cosediments with F-actin and demonstrates only weak depolymerizing activity. ADF binding is cooperative and saturates at a 1:1 ADF:actin molar ratio. At pHs between 7.1 and 7.7, ADF shows increasing depolymerizing activity and less F-actin binding. At pH 8.0, ADF depolymerizes F-actin in a stoichiometric manner. Both the F-actin binding and the depolymerizing activities of ADF are inhibited by phalloidin.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of profilin and actin expression in muscle and nonmuscle cells

Profilin is a small G-actin binding protein implicated in sequestering actin monomers in vivo. We have quantitated profilin and actin expression in human hepatoma HepG-2 cells and in two mouse myogenic cell lines, BC3H1 and C2C12, to determine whether the expression of profilin and the expression of nonmuscle isoactin or total actin are co-regulated. During differentiation of both muscle cell types, profilin and nonmuscle actin expression decrease in a coordinate manner as shown by measurements of steady state mRNA and newly synthesized protein. In human hepatoma HepG-2 cells, the twofold increase in actin synthesis observed after 24 hours of exposure to cytochalasin D did not result in an increase in profilin synthesis. Thus, profilin and actin expression are not co-regulated in all cells. To determine if there is sufficient profilin to sequester a large portion of cellular G-actin, we measured total profilin and G-actin levels in the three cell types. In each case, profilin accounted for less than 10% of the total G-actin on a molar basis. Thus, profilin is not responsible for total G-actin sequestration in these cells. Finally, using poly-L-proline affinity chromatography, we showed that, in the cell types tested, less than 20% of the poly-L-proline purified profilin existed as a complex with G-actin. The profilin in these cells may be interacting with cellular components other than actin.

Isolation of the phosphatidylinositol 4-monophosphate dissociable high-affinity profilin-actin complex

Profilin was originally discovered in a tight complex with monomeric actin from bovine spleen, leading to its description as an actin monomer sequestering protein that maintains a pool of unpolymerized actin in cells. Subsequent purifications of profilin using different methods from diverse cells have consistently yielded preparations that affect the kinetics of actin assembly but do not efficiently maintain actin monomeric at steady state in solutions containing mM magnesium. Recent evidence that profilin inhibits phospholipase C and enhances nucleotide exchange of actin has led some to question whether profilin is ever truly an actin monomer sequestering agent. Here we report that the extraction of bovine spleen with fluoride- and pyrophosphate-containing solutions facilitates isolation of monomeric actin that is bound to profilin and does not polymerize in mM magnesium ion. The integrity of this complex depends on the presence of ATP. Phosphatidylinositol 4-monophosphate (PIP), previously shown to dissociate the low-affinity profilin-actin complex (Kd = 0.4 microM in mM Mg2+), also dissociates the high-affinity profilin-actin complex (Kd less than 0.02 microM in mM Mg2+) yielding actin that is polymerization competent and profilin that functions like profilins purified by conventional methods. Although the chemical basis of these results is not known, they indicate that profilin can tightly sequester actin monomers and support the earlier suggestion that the affinity of profilin for actin may be under metabolic control.

Actin depolymerizing factor is a component of slow axonal transport

We examined the low molecular weight proteins transported with actin in the chicken sciatic nerve after injection of [35S]methionine into the lumbar spinal cord. A prominent component of slow axonal transport with apparent molecular mass 19 kDa comigrated on two-dimensional gels with chicken actin depolymerizing factor (ADF), previously shown to be a major actin-binding protein in brain. There was comparatively little radioactivity associated with the actin monomer sequestering proteins, profilin or cofilin, and examination of the rapid component of axonal transport failed to reveal appreciable quantities of actin, ADF, profilin, or cofilin. These results show that both actin and ADF are carried by slow axonal transport and raise the possibility that actin travels within the axon in an unpolymerized form in a complex with ADF.

Thrombin-stimulated phosphorylation of myosin light chain and its possible involvement in endothelin-1 secretion from porcine aortic endothelial cells

Thrombin-stimulated secretion of endothelin-1 (ET-1) from porcine aortic endothelial cells was inhibited in the presence of 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8), trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7). 1-(5-Chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9) also prevented the thrombin-stimulated secretion of ET-1 but it enhanced the accumulation of ET-1 in the endothelial cells. When the endothelial cells were treated with thrombin, the phosphorylation of a 20-kDa protein which was identified as myosin light chain (MLC) was detected. Phosphorylation was augmented in a time-dependent manner. As in the case of ET-1 secretion, MLC phosphorylation was prevented by TMB-8, trifluoperazine, W-7 and ML-9 at the same concentrations which were effective in inhibiting the ET-1 secretion. The site of phosphorylation of MLC was identified as a serine residue. Parallel to the phosphorylation of MLC, thrombin increased the amounts of the 43- and 200-kDa proteins in the Triton-insoluble fraction; these proteins were identified as actin and myosin heavy chain, respectively. These results suggest that the MLC phosphorylation elicited by MLC kinase may facilitate the formation of filamentous myosin and actin which are probably involved in ET-1 secretion, possibly in the transport of ET-1-containing vesicles in thrombin-stimulated endothelial cells.

Actin-depolymerizing factor (ADF) in the cerebellum of the developing rat: a quantitative and immunocytochemical study

A specific antiserum against actin-depolymerizing factor (ADF) was used in a quantitative and immunocytochemical study of ADF in the cerebellum of developing rats. The Triton-soluble ADF concentration remained stable throughout development. Light and electron microscopic immunocytochemistry showed that ADF was not detected in all cerebellar cells. ADF immunoreactivity was found in Purkinje cells, but not in granule cells. It was found in the Bergmann astrocytes and the astrocytes of the white matter, but not in the oligodendrocytes. The cell bodies and dendrites of Purkinje cells were immunoreactive for ADF but the axons were not. In contrast, the other axons of the white matter (mossy and climbing fibres) were labeled. Thus, ADF was not restricted to either the dendritic or axonal compartments. However, dendritic spines and postsynaptic densities were immunoreactive, whereas presynaptic varicosities were unlabeled. The immunoreactivities for ADF and actin were compared. ADF staining was uniformly distributed throughout the entire dendritic arborization of the Purkinje cell, while filamentous actin is highly concentrated in the dendritic spines, indicating that ADF activity might vary according to its cellular localization.

Primary structure of profilins from two species of Echinoidea and Physarum polycephalum

Profilin is a small G-actin-binding protein, the amino acid sequence of which was previously reported for calf, human, Acanthamoeba and yeast. Here the amino acid sequences of three profilins obtained from eggs of two species of Echinoidea, Clypeaster japonicus (order, Clypeasteroida) and Anthocidaris crassispina (order, Echinoida), and plasmodium of Physarum polycephalum were determined. Two echinoid profilins were composed of 139 amino acid residues, N-termini were acylated and the molecular mass was calculated to be 14.6 kDa, slightly larger than that of 13 kDa estimated by SDS/PAGE [Mabuchi, I. & Hosoya, H. (1982) Biomed. Res. 3, 465-476]. On the other hand, Physarum profilin was composed of 124 amino acid residues, the N-terminus was acylated, and the calculated molecular mass was 13132 Da. The sequences of C. japonicus and A. crassispina profilins were homologous (84% identical). However, the similarity of these profilins with those form other organisms was low. The sequence of Physarum profilin was homologous with Acanthamoeba profilin isoforms (51% identical) and with yeast profilin (42% identical), but not with other profilins. The relatively conservative sequence of profilins from yeast, Physarum, Acanthamoeba, echinoid eggs and mammalian cells was found in the N-terminal region, which was suggested to be a common actin-binding region. The C-terminal region was also conserved, although to a lesser extent than the N-terminal region.

Isolation of a 5-kilodalton actin-sequestering peptide from human blood platelets

Resting human platelets contain approximately 0.3 mM unpolymerized actin. When freshly drawn and washed platelets are treated with saponin, 85-90% of the unpolymerized actin diffuses out. Analysis by polyacrylamide gel electrophoresis under nondenaturing conditions shows that the bulk of this unpolymerized actin migrates with a higher mobility than does pure G-actin, profilactin, or actin-gelsolin complex. When muscle G-actin is added to fresh or boiled saponin extract, the added muscle actin is shifted to the high-mobility form. The saponin extract contains an acidic peptide having a molecular mass in the range of 5 kDa, which has been purified to homogeneity by reverse-phase HPLC. This peptide also shifts muscle actin to the high-mobility form. Addition of either boiled saponin extract or the purified peptide to muscle G-actin also strongly and stoichiometrically inhibits salt-induced polymerization, as assayed by falling-ball viscometry and by sedimentation. We conclude that this peptide binds to the bulk of the unpolymerized actin in platelets and prevents it from polymerizing.

CTP:cholinephosphate cytidylyltransferase in human and rat lung: association in vitro with cytoskeletal actin

CTP:cholinephosphate cytidylyltransferase activities were compared in saline homogenates of immature fetal (15-16 weeks gestation) and adult human lung. There were no differences in subcellular enzyme distribution, in Vmax activity, or in the phosphatidylglycerol-mediated stimulation of soluble enzyme activity. These results provide no support for a developmental translocation of cytidylyltransferase from a cytosolic to a microsomal location in human lung, such as that proposed to accompany the maturation of pulmonary surfactant phosphatidylcholine biosynthesis in rat. Soluble cytidylyltransferase activity from human but not rat lung was increased after manipulation in vitro. Resolution of human H form (greater than 10(3) kDa) and L form (200 kDa) enzyme by gel filtration led to an activity increase of 200%. Incubation at 37 degrees C for 2 h increased soluble enzyme recovery, although prior centrifugal removal of generated actin-rich aggregates was necessary in adult lung fractions. In contrast, 85% of soluble rat lung cytidylyltransferase was actin aggregate-associated after incubation. The apparent heteroassociation of rat and human lung enzyme with actin in the presence of poly(ethylene glycol) at 4 degrees C strongly suggested close in vitro and potential in vivo linkage. A partial co-purification of adult human lung cytidylyltransferase with actin was also consistent with this idea. We propose that some reported cytidylyltransferase translocation phenomena may be mediated by cytoskeletal interactions in vitro.

Action of general and alpha-smooth muscle-specific actin antibody microinjection on stress fibers of cultured smooth muscle cells

Arterial smooth muscle cells express alpha- and gamma-smooth muscle, as well as beta- and gamma-cytoplasmic actins. Two actin antibodies, one recognizing smooth muscle and cytoplasmic actin isoforms, the other recognizing specifically alpha-smooth muscle actin, were microinjected into cultured aortic smooth muscle cells. The effect of these antibodies on stress fiber organization was examined by staining with rhodamine-labeled phalloidin and by immunofluorescence with the same antibodies. Microinjection of the general actin antibody abolished most of the stress fiber staining with all reagents, but did not significantly affect the shape of the injected cells. This suggests that stress fiber integrity is not absolutely necessary for the maintenance of cell shape within the time of observation. Microinjection of the specific alpha-smooth muscle antibody abolished to various extents the staining of stress fibers with this antibody, but left practically intact their staining with rhodamine-labeled phalloidin and with the general actin antibody. This suggests that the incorporation of alpha-smooth muscle actin is not absolutely necessary for the maintenance of stress fiber integrity in cultured smooth muscle cells.

Exogenous nucleation sites fail to induce detectable polymerization of actin in living cells

Most nonmuscle cells are known to maintain a relatively high concentration of unpolymerized actin. To determine how the polymerization of actin is regulated, exogenous nucleation sites, prepared by sonicating fluorescein phalloidin-labeled actin filaments, were microinjected into living Swiss 3T3 and NRK cells. The nucleation sites remained as a cluster for over an hour after microinjection, and caused no detectable change in the phase morphology of the cell. As determined by immunofluorescence specific for endogenous actin and by staining cells with rhodamine phalloidin, the microinjection induced neither an extensive polymerization of endogenous actin off the nucleation sites, nor changes in the distribution of actin filaments. In addition, the extent of actin polymerization, as estimated by integrating the fluorescence intensities of bound rhodamine phalloidin, did not appear to be affected. To determine whether the nucleation sites remained active after microinjection, cells were first injected with nucleation sites and, following a 20-min incubation, microinjected with monomeric rhodamine-labeled actin. The rhodamine-labeled actin became extensively associated with the nucleation sites, suggesting that at least some of the nucleation activity was maintained, and that the endogenous actin behaved in a different manner from the exogenous actin subunits. Similarly, when cells containing nucleation sites were extracted and incubated with rhodamine-labeled actin, the rhodamine-labeled actin became associated with the nucleation sites in a cytochalasin-sensitive manner. These observations suggest that capping and inhibition of nucleation cannot account for the regulation of actin polymerization in living cells. However, the sequestration of monomers probably plays a crucial role.

Latrunculins--novel marine macrolides that disrupt microfilament organization and affect cell growth: I. Comparison with cytochalasin D

The latrunculins are architecturally novel marine compounds isolated from the Red Sea sponge Latrunculia magnifica. In vivo, they alter cell shape, disrupt microfilament organization, and inhibit the microfilament-mediated processes of fertilization and early development. In vitro, latrunculin A was recently found to affect the polymerization of pure actin in a manner consistent with the formation of a 1:1 molar complex with G-actin. These in vitro effects as well as previous indications that the latrunculins are more potent than the cytochalasins suggest differences in the in vivo mode of action of the two classes of drugs. To elucidate these differences we have compared the short- and long-term effects of latrunculins on cell shape and actin organization to those of cytochalasin D. Exposure of hamster fibroblast NIL8 cells for 1-3 hr to latrunculin A, latrunculin B, and cytochalasin D causes concentration-dependent changes in cell shape and actin organization. However, the latrunculin-induced changes were strikingly different from those induced by cytochalasin D. Furthermore, while initial effects were manifest with both latrunculin A and cytochalasin D already at concentrations of about 0.03 microgram/ml, latrunculin A caused complete rounding up of all cells at 0.2 microgram/ml, whereas with cytochalasin D maximum contraction was reached at concentrations 10-20 times higher. The short-term effects of latrunculin B were similar to those of latrunculin A although latrunculin B was slightly less potent. All three drugs inhibited cytokinesis in synchronized cells, but their long-term effects were markedly different. NIL8 cells treated with latrunculin A maintained their altered state for extended periods. In contrast, the effects of cytochalasin D progressed with time in culture, and the latrunculin B-induced changes were transient in the continued presence of the drug. These transient effects were found to be due to a gradual inactivation of latrunculin B by serum and were used to compare recovery patterns of cell shape and actin organization in two different cell lines. This comparison showed that the transient effects of latrunculin B were fully reversible for the NIL8 cells and not for the mouse neuroblastoma N1E-115 cells.

Analysis of cell division using fluorescently labeled actin and myosin in living PtK2 cells

Actin and the light chains of myosin were labeled with fluorescent dyes and injected into interphase PtK2 cells in order to study the changes in distribution of actin and myosin that occurred when the injected cells subsequently entered mitosis and divided. The first changes occurred when stress fibers in prophase cells began to disassemble. During this process, which began in the center of the cell, individual fibers shortened, and in a few fibers, adjacent bands of fluorescent myosin could be seen to move closer together. In most cells, stress fiber disassembly was complete by metaphase, resulting in a diffuse distribution of the fluorescent proteins throughout the cytoplasm with the greatest concentration present in the mitotic spindle. The first evidence of actin and myosin concentration in a cleavage ring occurred at late anaphase, just before furrowing could be detected. Initially, the intensity of fluorescence and the width of the fluorescent ring increased as the ring constricted. In cells with asymmetrically positioned mitotic spindles, both protein concentration and furrowing were first evident in the cortical regions closest to the equator of the mitotic spindle. As cytokinesis progressed in such asymmetrically dividing cells, fluorescent actin and myosin appeared at the opposite side of the cell just before furrowing activity could be seen there. At the end of cytokinesis, myosin and actin were concentrated beneath the membrane of the midbody and subsequently became organized in two rings at either end of the midbody.

Isolation of camel brain actin--comparison of its biochemical properties with those of camel skeletal muscle, heart muscle and rabbit skeletal muscle actins

1. Actins were purified from camel brain, skeletal muscle and heart muscle and their properties were compared. 2. Individual actins were homogeneous and comigrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). 3. Isoelectric focusing analysis of camel skeletal muscle and heart muscle actin showed a single polypeptide of the alpha-species, while camel brain actin showed two polypeptides of the beta- and gamma-species typical of non-muscle actin. 4. Actins from camel skeletal muscle and heart muscle showed a greater degree of similarity to each other and to rabbit skeletal muscle actin and showed some differences from camel brain actin, as confirmed by amino acid analysis and one-dimensional peptide mapping.

Alterations in hepatocyte cytoskeleton caused by redox cycling and alkylating quinones

Quinones may induce toxicity by a number of mechanisms, including alkylation and oxidative stress following redox cycling. The metabolism of quinones by isolated rat hepatocytes is associated with cytoskeletal alterations, plasma membrane blebbing, and subsequent cytotoxicity. The different mechanisms underlying the effects of alkylating (p-benzoquinone), redox cycling (2,3-dimethoxy-1,4-naphthoquinone), and mixed redox cycling/alkylating (2-methyl-1,4-naphthoquinone) quinones on hepatocyte cytoskeleton have been investigated in detail in this study. Analysis of the cytoskeletal fraction extracted from quinone-treated cells revealed a concentration-dependent increase in the amount of cytoskeletal protein and a concomitant loss of protein thiols, irrespective of the quinone employed. In the case of redox cycling quinones, these alterations were associated with an oxidation-dependent actin crosslinking (sensitive to the thiol reductant dithiothreitol). In contrast, with alkylating quinones an oxidation-independent cytoskeletal protein crosslinking (insensitive to thiol reductants) was observed. In addition to these changes, a dose-dependent increase in the relative abundance of F-actin was detected as a consequence of the metabolism of oxidizing quinones in hepatocytes. Addition of dithiothreitol solubilized a considerable amount of polypeptides from the cytoskeletal fraction isolated from hepatocytes exposed to redox cycling but not alkylating quinones. Our findings indicate that the hepatocyte cytoskeleton is an important target for the toxic effects of different quinones. However, the mechanisms underlying cytoskeletal damage differ depending on whether the quinone acts primarily by oxidative stress or alkylation.

Snake venom cardiotoxin induces G-actin polymerization

Snake venom cardiotoxin showed the ability to induce polymerization of G-actin from rabbit skeletal muscle in a low ionic strength buffer composed of 0.2 mM CaCl2/0.2 mM ATP/0.5 mM mercaptoethanol/2.0 mM Tris-HCl, pH 8.0. The activity was enhanced greatly when 0.4 mM MgCl2 was present in the buffer and could be inhibited if G-actin was preincubated with deoxyribonuclease I. Furthermore, the DNAase could also partially depolymerize actin polymer previously formed by the interaction of G-actin with the toxin.

A lower proportion of filamentous to monomeric actin in the developing cerebellum of thyroid-deficient rats

Using a DNase I inhibition assay, in the presence or the absence of guanidine hydrochloride (which depolymerizes the actin filaments), developmental changes in total and filamentous actin were determined in the cerebellum of normal and hypothyroid rats. The total actin content per mg protein was not modified by hypothyroidism. As in normal animals, it reached a maximum around the age of 8 days and then decreased until adulthood. In contrast, the proportion of filamentous actin, which increased after the first postnatal week during normal development, was significantly reduced in the thyroid-deficient rats, only reaching normal values at 35 days. Thyroxine treatment for at least 4 days returned the filamentous actin content to normal at 14 days. The present study shows that the morphogenetic action of thyroid hormone is exerted not only on the microtubular apparatus, as previously described, but also in part through a control of actin monomer-polymer equilibrium.

Distribution and cellular localization of actin depolymerizing factor

Actin depolymerizing factor (ADF) is a low molecular mass (19 kD) protein that forms a tightly bound dimeric complex with actin. We have raised a rabbit antiserum to chick brain ADF and used it to analyze the distribution and cellular localization of ADF. We find that ADF is a major constituent of all chick embryonic and most adult tissues examined, accounting for 0.1-0.4% of the total protein. Some tissues have as much as 0.6 mol ADF per mole actin. Adult heart and skeletal muscle are unusual in having very low levels of ADF: less than 0.02% of the soluble protein. During the development of skeletal muscle, ADF levels are maximal up to approximately 11 d in ovo and then decline to reach their adult levels by 14 d posthatching. Brain tissue and cultured cell lines from several other vertebrates, including mammals, all possess proteins of identical size to ADF that are recognized by the ADF antiserum. No proteins are specifically recognized by the ADF antiserum in extracts from Acanthamoeba castellanii or from nerve tissue of several invertebrates. Indirect immunofluorescence shows that ADF is present throughout the cytosol of most cells and at the leading edge of ruffled membranes and in the neuronal growth cone. Its abundance and widespread distribution together with its ability to sequester actin molecules, even those in an already polymerized state, suggest that ADF is a major factor in the regulation of actin filaments in many vertebrate cells.

Mobility of filamentous actin in living cytoplasm

Filamentous actin in living cultured cells was labeled by microinjecting trace amounts of rhodamine-phalloidin (rh-pha) as a specific, high-affinity probe. The microinjection caused no detectable effect on cell morphology or cell division. The distribution of rh-pha-labeled filaments was then examined in dividing cells using image-intensified fluorescence microscopy, and the exchangeability of labeled filaments along stress fibers was studied during interphase using fluorescence recovery after photobleaching. rh-pha showed a rapid concentration at the contractile ring during cell division. In addition, recovery of fluorescence after photobleaching occurred along stress fibers with a halftime as short as 8 min. These observations suggest that at least some actin filaments undergo continuous movement and reorganization in living cells. This dynamic process may play an important role in various cellular functions.