Actin and myosin and cell movement

Studies on the mechanisms of neurulation in the chick: morphometric analysis of the relationship between regional variations in cell shape and sites of motive force generation

Microfilaments, which are organized into bundles in the apical ends of neuroepithelial cells, are generally thought to play a major role in generating the driving forces for neural tube closure. Because of their proximity to the luminal surface, the contractile activity of these microfilament bundles results in conspicuous changes in the overall shape of neuroepithelial cells, most notably apical constriction and apical surface folding. In the present study, we have used morphometric methods and computer-assisted image analysis to reveal the distribution of microfilament-mediated forces in the developing midbrain during initial contact of apposing neural folds in chick embryos at Hamburger and Hamilton stage 8+ of development (Hamburger and Hamilton (1951) J. Morphol., 88:49-92). The degree of apical constriction, apical surface folding, and bending of the neuroepithelium was used as a barometer of local microfilament activity. Results indicate that cells forming the floor and midlateral walls of the developing midbrain consistently show a higher degree of apical constriction and surface folding than those at other locations. These same regions of the neuroepithelium also exhibit the greatest degree of bending. We conclude that the principal driving forces for closure of the neural tube, at the level of the midbrain, are concentrated in certain regions of the neuroepithelium (i.e., the floor and midlateral walls of the forming neural tube) rather than uniformly distributed.

Interorgan metabolism of amino acids, glucose, lactate, glycerol and uric acid in the domestic fowl (Gallus domesticus)

Arterial--venous differences for metabolites across liver, kidney and hindquarters were measured in fed or starved, artificially ventilated chickens. The results indicate that the liver takes up amino acids under both conditions. Urate and glucose are released by the liver in both the fed and the starved state. Lactate and amino acids are extracted from blood by the kidneys, and this increases in the starved chicken. Urate is removed from the circulation by the kidney in the fed and starved state and excreted. In the fed bird there is no significant arteriovenous difference of glucose across the kidney, but in the starved state the kidney releases glucose into the circulation. The hindquarters take up glucose in the fed but not in the starved state. The branched-chain amino acids valine and leucine were taken up by the hindquarters in the fed, but not the starved, chicken. Glycerol is released by the hindquarter of fed and starved chickens. In the starved state, alanine and glutamine represent 57% of the amino acids released by the hindquarter. Lactate is released by the hindquarter of starved chickens and represents the major gluconeogenic carbon source released by the hindquarter and taken up by kidney and liver. Although the liver is the major gluconeogenic organ in the starved chicken, the kidney accounts for approx. 30% of the glucose produced.

Fluorescence visualization of the distribution of microfilaments in gonads and early embryos of the nematode Caenorhabditis elegans

Several intracellular motility events in the Caenorhabditis elegans zygote (pseudocleavage, the asymmetric meeting of the pronuclei, the segregation of germ line-specific granules, and the generation of an asymmetric spindle) appear to depend on microfilaments (MFs). To investigate how MFs participate in these manifestations of zygotic asymmetry, the distribution of MFs in oocytes and early embryos was examined, using both antibodies to actin and the F-actin-specific probe rhodamine-phalloidin. In early-stage zygotes, MFs are found in a uniform cortical meshwork of fine fibers and dots or foci. In later zygotes, concomitant with the intracellular movements that are thought to be MF mediated, MFs also become asymmetrically rearranged; as the zygote undergoes pseudocleavage and as the germ line granules become localized in the posterior half of the cell, the foci of actin become progressively more concentrated in the anterior hemisphere. The foci remain anterior as the spindle becomes asymmetric and the zygote undergoes its first mitosis, at which time fibers align circumferentially around the zygote where the cleavage furrow will form. A model for how the anterior foci of actin may participate in zygotic motility events is discussed. Phalloidin and anti-actin antibodies have also been used to visualize MFs in the somatic tissues of the adult gonad. The myoepithelial cells that surround maturing oocytes are visibly contractile and contain an unusual array of MF bundles; the MFs run roughly longitudinally from the loop of the gonad to the spermatheca. Myosin thick filaments are distributed along the MFs in a periodic manner suggestive of a sarcomere-like configuration. It is proposed that these actin and myosin filaments interact to cause sheath cell contraction and the movement of oocytes through the gonad.

Complete nucleotide and encoded amino acid sequence of a mammalian myosin heavy chain gene. Evidence against intron-dependent evolution of the rod

The complete nucleotide sequence and exon/intron structure of the rat embryonic skeletal muscle myosin heavy chain (MHC) gene has been determined. This gene comprises 24 X 10(3) bases of DNA and is split into 41 exons. The exons encode a 6035 nucleotide (nt) long mRNA consisting of 90 nt of 5' untranslated, 5820 nt of protein coding and 125 nt of 3' untranslated sequence. The rat embryonic MHC polypeptide is encoded by exons 3 to 41 and contains 1939 amino acid residues with a calculated Mr of 223,900. Its amino acid sequence displays the structural features typical for all sarcomeric MHCs, i.e. an amino-terminal "globular" head region and a carboxy-terminal alpha-helical rod portion that shows the characteristics of a coiled coil with a superimposed 28-residue repeat pattern interrupted at only four positions by "skip" residues. The complex structure of the rat embryonic MHC gene and the conservation of intron locations in this and other MHC genes are indicative of a highly split ancestral sarcomeric MHC gene. Introns in the rat embryonic gene interrupt the coding sequence at the boundaries separating the proteolytic subfragments of the head, but not at the head/rod junction or between the 28-residue repeats present within the rod. Therefore, there is little evidence for exon shuffling and intron-dependent evolution by gene duplication as a mechanism for the generation of the ancestral MHC gene. Rather, intron insertion into a previously non-split ancestral MHC rod gene consisting of multiple tandemly arranged 28-residue-encoding repeats, or convergent evolution of an originally non-repetitive ancestral MHC rod gene must account for the observed structure of the rod-encoding portion of present-day MHC genes.

Physiological and ultrastructural analysis of elongating mitotic spindles reactivated in vitro

We have developed a simple procedure for isolating mitotic spindles from the diatom Stephanopyxis turris and have shown that they undergo anaphase spindle elongation in vitro upon addition of ATP. The isolated central spindle is a barrel-shaped structure with a prominent zone of microtubule overlap. After ATP addition greater than 75% of the spindle population undergoes distinct structural rearrangements: the spindles on average are longer and the two half-spindles are separated by a distinct gap traversed by only a small number of microtubules, the phase-dense material in the overlap zone is gone, and the peripheral microtubule arrays have depolymerized. At the ultrastructural level, we examined serial cross-sections of spindles after 1-, 5-, and 10-min incubations in reactivation medium. Microtubule depolymerization distal to the poles is confirmed by the increased number of incomplete, i.e., c-microtubule profiles specifically located in the region of overlap. After 10 min we see areas of reduced microtubule number which correspond to the gaps seen in the light microscope and an overall reduction in the number of half-spindle microtubules to about one-third the original number. The changes in spindle structure are highly specific for ATP, are dose-dependent, and do not occur with nonhydrolyzable nucleotide analogues. Spindle elongation and gap formation are blocked by 10 microM vanadate, equimolar mixtures of ATP and AMPPNP, and by sulfhydryl reagents. This process is not affected by nocodazole, erythro-9-[3-(2-hydroxynonyl)]adenine, cytochalasin D, and phalloidin. In the presence of taxol, the extent of spindle elongation is increased; however, distinct gaps still form between the two half-spindles. These results show that the response of isolated spindles to ATP is a complex process consisting of several discrete steps including initiation events, spindle elongation mechanochemistry, controlled central spindle microtubule plus-end depolymerization, and loss of peripheral microtubules. They also show that the microtubule overlap zone is an important site of ATP action and suggest that spindle elongation in vitro is best explained by a mechanism of microtubule-microtubule sliding. Spindle elongation in vitro cannot be accounted for by cytoplasmic forces pulling on the poles or by microtubule polymerization.

Myosin at the apical pole of ciliated epithelial cells as revealed by a monoclonal antibody

A monoclonal antibody (CC-212), obtained in a fusion experiment in which basal bodies from quail oviduct were used as immunogen, has been shown to label the apical pole of ciliated cells and to react with a 200-kD protein. This monoclonal antibody was demonstrated to be an anti-myosin from smooth muscle or from nonmuscular cells using the following criteria: On Western blots it reacted with the myosin heavy chains from gizzard and platelet extracts and from cultured cell line extracts, but did not react with striated muscle myosin heavy chains. By immunofluorescence it decorated the stress fibers of well-spread cells with a characteristic striated pattern, while it did not react with myotubes containing organized myofibrils. On native ciliated cells as well as on Triton-extracted ciliated cortices from quail oviduct, this monoclonal antibody decorated the apical pole with a stronger labeling of the periphery of the apical area. Ultrastructural localization was attempted using the immunogold technique on the same preparation. Myosin was associated with a filamentous material present between striated rootlets and the proximal extremities of the basal bodies. No labeling of the basal body itself or of axoneme was observed.

Nucleotide sequence of the human gamma cytoskeletal actin mRNA: anomalous evolution of vertebrate non-muscle actin genes

Two distinct, but iso-coding, gamma non-muscle actin cDNAs were isolated from an SV40-transformed human fibroblast library. The complete nucleotide sequence of the human gamma non-muscle actin cDNAs indicates that they may have arisen from polymorphic alleles. By using genomic DNA and cellular RNA transfer blots, we demonstrate that the 3' untranslated region (UTR) of the gamma actin mRNA consists of an evolutionarily conserved 5' and more divergent 3' segments. In fact, the conserved segment of the 3' UTR detects a single-copy sequence in the chicken genome and a 20S RNA transcript in chicken non-muscle tissues. The coding regions of these cDNAs were compared with those of other vertebrate non-muscle actin genes. Surprisingly, the percentage of silent base substitutions between the human beta and gamma actin coding regions is anomalously low and indicates greater sequence conservation than would be expected for a gene pair which arose during pre-avian evolution. We discuss gene conversion and recent selective pressure as possible explanations of the apparently anomalous evolution of the gamma non-muscle actin gene.

A kinetic comparison between Mg-actin and Ca-actin

The kinetics of the elongation reaction in the polymerization of actin containing tightly-bound Mg2+ (Mg-actin) or tightly-bound Ca2+ (Ca-actin) have been studied. The reaction was monitored using the increase in fluorescence intensity of N-(1-pyrenyl)iodoacetamide-labelled monomeric actin as a measure of polymer formation. The actin nucleation reaction was circumvented by the addition of phalloidin-stabilized actin nuclei. Elongation rates were obtained at various actin concentrations and at various temperatures for polymerization induced by the presence of different monovalent and divalent salt concentrations. Values for the relative rate constant of forward polymerization (mk+) for Mg-actin were found to be larger than those for Ca-actin under similar conditions (m = number of polymer ends). The critical actin concentration (Cc) of Mg-actin is lower than the Cc for Ca-actin, as were estimates of the relative rate constant of depolymerization (mk-). The temperature dependence of Cc, mk+ and mk- for Mg-actin was different from that for Ca-actin, further suggesting a difference in monomeric properties due to the type of divalent cation tightly bound to actin. Estimates of the activation enthalpy change for the forward reaction in the G in equilibrium F transformation were similar for both types of actin, but the activation enthalpy change for the depolymerization of Mg-actin was significantly larger than that for Ca-actin.

Regulation in vitro of brush border myosin by light chain phosphorylation

Myosin was purified from chicken brush border cells to greater than 95% homogeneity and in a predominantly non-phosphorylated state. The effects of light chain phosphorylation by a Ca2+-calmodulin-dependent myosin light chain kinase on the conformational, enzymatic and filament assembly properties of this myosin were investigated. The actin-activated MgATPase activity of the non-phosphorylated myosin was low, and upon light chain phosphorylation an eight- to ninefold increase in this activity was observed, which was further potentiated by tropomyosin. Light chain phosphorylation was shown to control the assembly and disassembly of brush border myosin filaments. For example, turbidity measurements and electron microscopy demonstrated that MgATP disassembled non-phosphorylated myosin filaments; the disassembled myosin could reassemble when the light chains were phosphorylated, and could be disassembled again by dephosphorylating the light chains with phosphatase. In the electron microscope, the disassembled non-phosphorylated myosin molecules appeared in a folded conformation, and they were extended when phosphorylated. Proteolytic digestion was used to probe further the conformation of these folded and extended molecules, and their subunit organizations were characterized by a gel overlay technique. Quantitative analysis further demonstrated that light chain phosphorylation alters dramatically the monomer/polymer equilibrium of brush border myosin, shifting it towards filament formation. Comparison of analogous data for myosin from gizzard and thymus shows that each myosin has distinct solubility properties.

Effect of transformation by Rous sarcoma virus on the character and distribution of actin in Rat-1 fibroblasts: a biochemical and microscopical study

Actin has been measured in subcellular fractions from Rat-1 fibroblasts and in Rous sarcoma virus-transformed Rat-1 cells (VIT), using the DNase 1 inhibition assay. The transformed cells showed a significant shift in the actin monomer (G)in equilibrium with polymer (F) equilibrium within the cell cytosol, and a significant increase in actin in the Triton-insoluble cytoskeletal core in comparison with untransformed cells. This incorporation of actin into the cytoskeletal core fraction is associated with a change in filamentous actin assemblies from 'stress fibre' patterns to punctate filament aggregates. These differences have been correlated with changes in morphology, in actin, vinculin and alpha-actinin distribution, in adhesion plaque formation and with the production of pp60v-src-associated protein kinase activity in the transformed cells. Changes in actin distribution and its polymerization in response to src-gene expression may play an important role in the determination of the transformed cell characteristics.

Comparison of three actin-coding sequences in the mouse; evolutionary relationships between the actin genes of warm-blooded vertebrates

We have determined the sequences of three recombinant cDNAs complementary to different mouse actin mRNAs that contain more than 90% of the coding sequences and complete or partial 3' untranslated regions (3'UTRs): pAM 91, complementary to the actin mRNA expressed in adult skeletal muscle (alpha sk actin); pAF 81, complementary to an actin mRNA that is accumulated in fetal skeletal muscle and is the major transcript in adult cardiac muscle (alpha c actin); and pAL 41, identified as complementary to a beta nonmuscle actin mRNA on the basis of its 3'UTR sequence. As in other species, the protein sequences of these isoforms are highly (greater than 93%) conserved, but the three mRNAs show significant divergence (13.8-16.5%) at silent nucleotide positions in their coding regions. A nucleotide region located toward the 5' end shows significantly less divergence (5.6-8.7%) among the three mouse actin mRNAs; a second region, near the 3' end, also shows less divergence (6.9%), in this case between the mouse beta and alpha sk actin mRNAs. We propose that recombinational events between actin sequences may have homogenized these regions. Such events distort the calculated evolutionary distances between sequences within a species. Codon usage in the three actin mRNAs is clearly different, and indicates that there is no strict relation between the tissue type, and hence the tRNA precursor pool, and codon usage in these and other muscle mRNAs examined. Analysis of codon usage in these coding sequences in different vertebrate species indicates two tendencies: increases in bias toward the use of G and C in the third codon position in paralogous comparisons (in the order alpha c less than beta less than alpha sk), and in orthologous comparisons (in the order chicken less than rodent less than man). Comparison of actin-coding sequences between species was carried out using the Perler method of analysis. As one moves backward in time, changes at silent sites first accumulate rapidly, then begin to saturate after -(30-40) million years (MY), and actually decrease between -400 and -500 MY. Replacements or silent substitutions therefore cannot be used as evolutionary clocks for these sequences over long periods. Other phenomena, such as gene conversion or isochore compartmentalization, probably distort the estimated divergence time.

Isolation and purification of actomyosin ATPase from mammalian brain

A simple technique for the isolation and purification of mammalian brain actomyosin, based on extraction of whole brains in low ionic-strength buffer, is described. The final preparation of brain actomyosin is obtained in good yield, has relatively high K+-EDTA and Ca2+-ATPase activities, and is substantially free of other ATPases and tubulin. The preparation is useful for initial enzymatic studies and/or as an enrichment step toward purification of the individual protein components. The Mg2+-ATPase and K+-EDTA ATPase activities are strongly inhibited by the sulfhydryl blocking reagent, pHMB. Interaction between the actin and myosin components can be demonstrated. Brain actomyosin had a distinct electrophoretic profile and enzymatic activity when compared with smooth muscle actomyosin from the aorta.

Isolation and characterization of sarcomeric actin genes expressed in Xenopus laevis embryos

A Xenopus laevis complementary DNA (cDNA) library prepared from messenger RNAs extracted from embryos has been screened for actin-coding sequences. Two cDNA clones corresponding to an alpha cardiac and an alpha skeletal muscle actin mRNA have been identified and characterized. From a genomic library, we have furthermore isolated the genes that correspond to the characterized cDNAs. In addition we have identified an actin processed gene which seems to be derived from a second type of skeletal muscle actin gene. Southern blot analysis of X. laevis DNA reveals that each of the three genes is present in at least two copies. In Xenopus tropicalis, a similar Southern blot analysis demonstrates that the three alpha actin genes exist as single copy. This result correlates with the genome duplication that has been proposed to have occurred recently in a X. laevis ancestor. A sequence comparison of the X. laevis cardiac and skeletal muscle actin cDNAs shows that the encoded peptides are highly conserved. Nevertheless, the numerous nucleotide changes at silent mutation sites suggest that the genes originated before the amphibia/reptile-bird divergence, more than 350 million years ago. Comparison of the promoters of the cardiac and skeletal actin genes, which are co-expressed in embryos, reveals a few common structural sequence elements.

The mechanism of muscle contraction

Knowledge of the mechanism of contraction has been obtained from studies of the interaction of actin and myosin in solution, from an elucidation of the structure of muscle fibers, and from measurements of the mechanics and energetics of fiber contraction. Many of the states and the transition rates between them have been established for the hydrolysis of ATP by actin and myosin subfragments in solution. A major goal is to now understand how the kinetics of this interaction are altered when it occurs in the organized array of the myofibril. Early work on the structure of muscle suggested that changes in the orientation of myosin cross-bridges were responsible for the generation of force. More recently, fluorescent and paramagnetic probes attached to the cross-bridges have suggested that at least some domains of the cross-bridges do not change orientation during force generation. A number of properties of active cross-bridges have been defined by measurements of steady state contractions of fibers and by the transients which follow step changes in fiber length or tension. Taken together these studies have provided firm evidence that force is generated by a cyclic interaction in which a myosin cross-bridge attaches to actin, exerts force through a "powerstroke" of 12 nm, and is then released by the binding of ATP. The mechanism of this interaction at the molecular level remains unknown.

Bovine serum brevin. Purification by hydrophobic chromatography and properties

Brevin, an actin-severing protein present in serum from numerous mammals, has been purified to homogeneity from bovine serum, using hydrophobic chromatography as the last purification step. The physicochemical parameters of brevin have been established and some of them studied in the absence and presence of Ca2+. Brevin exhibits an apparent Stokes radius, Rs, of 3.4 nm, an intrinsic sedimentation coefficient S degrees 20, W, of 4.8 S and 4.4 S in the absence and presence of Ca2+ respectively, indicative of calcium-induced conformational change. The native molecular mass of brevin was found to be 68 kDa and the hydrodynamic data suggest that the protein is an asymmetric molecule. Sedimentation equilibrium studies demonstrated that Ca2+ affects the shape (asymmetry) of brevin without altering its molecular mass. Limited tryptic and chymotryptic digestion of brevin distinguishes the Ca2+-induced conformation from the EGTA one. No change in the electrophoretic migration of brevin was seen upon Ca2+ addition. Several isoforms were detected by two-dimensional gel electrophoresis. Brevin increases the rate of nucleation of actin but decreases the rate of elongation of the filaments and the steady-state viscosity of F-actin in substoichiometric amounts, as measured by viscometric assays under high shear conditions. Electron microscopic examination documents these effects. Brevin produces shorter actin filaments and binds to the 'barbed' end of filaments to which monomers add preferentially during elongation, as demonstrated by indirect immunogold staining of antibodies against brevin. Filament elongation occurs only at the slowly growing end. An enzyme-linked immunosorbent assay was developed and used to detect and quantify brevin and related proteins in extracts of different bovine cells and tissues. Liver and smooth muscles were found to contain the highest amounts of the severing protein.

Can oligomeric myosin participate in smooth muscle contraction?

An experimental model was constructed to examine the possible fact that myosin oligomers take a part in smooth muscle contraction. Single fibres prepared from glycerinated rabbit skeletal muscle were emptied of myosin and then irrigated with purified chicken gizzard myosin. This was a good preparation in which to see the structural change of gizzard myosin and its interaction with actin filaments under the electron microscope. The structure of gizzard myosin was thoroughly changed by varying the concentration of free Mg2+ in the bathing solution. Myosin thick filaments were formed at the position of the A-band longitudinally bridging adjacent I-segments at high concentration of free Mg2+, while most of them disappeared from the A-band and some localized in the I-segment at low concentration of free Mg2+. When this preparation was induced to contract isometrically, it showed quite different sarcomere patterns at high and low concentrations of free Mg2+. Tensions developed by this preparation were of the same magnitude at high and low concentrations of free Mg2+, which were approximately 5% of that developed by glycerinated skeletal muscle fibres. The shortening of the prepared fibre in a contracting medium was almost the same at varying concentrations of free Mg2+. The relation between the structural organization and the contracting nature of the prepared fibre as the concentration of free Mg2+ was varied is discussed in this paper with respect to the significance of oligomeric myosin in smooth muscle contraction.

Organization of the actin multigene family of Dictyostelium discoideum and analysis of variability in the protein coding regions

There are 17 to 20 actin genes in the genome of the cellular slime mold Dictyostelium discoideum. Genomic clones of 15 of the genes have been isolated. Extensive nucleotide sequence within the protein-coding regions has been determined, including the complete nucleotide sequence of four genes representing the three distinct evolutionary groups of Dictyostelium actin genes. All are similar to mammalian cytoplasmic actins at diagnostic amino acid positions, and there is generally less variability among Dictyostelium actin genes than among Drosophila actin genes. Two genes, Actins 3-sub 1 and 3-sub 2 differ substantially from all the rest in terms of replacement amino acid substitutions and probably encode actin-related proteins rather than bona fide actins. Each contains several amino acid substitutions that should alter the secondary structure of the resulting proteins, and Actin 3-sub 2 encodes four additional amino acids at the C terminus. This gene is as divergent from other Dictyostelium actin genes as is the yeast or a soybean actin gene. At present, evidence suggests that all 15 genes examined are expressed, except the previously identified Actin 2-sub 2. We suggest that Dictyostelium might maintain a high number of functional actin genes for the purpose of regulating the level of actin synthesis within narrow limits, rather than because most genes perform different functions.

Gene-specific expression of the actin multigene family of Dictyostelium discoideum

We have investigated the expression of 14 cloned genes of the 20-member actin multigene family of Dictyostelium discoideum using gene-specific mRNA complementary probes and an RNase protection assay. Actin gene expression was studied in vegetative cells and in cells at a number of developmental stages chosen to represent the known major shifts in actin mRNA and protein synthesis. At least 13 of these genes are expressed. A few genes are expressed very abundantly at 10% or more of total actin mRNA; however, the majority are maximally expressed at 1 to 5% of actin message. Although all of the genes are transcribed in vegetative cells, most genes appear to be independently regulated. Actin 8 appears to be transcribed at constant, high levels throughout growth and development. Actin 12 mRNA is maximally expressed in vegetative cells but the level is reduced appreciably by the earliest stage of development examined, while Actin 7 mRNA is specifically induced approximately sevenfold at this time. The rest of the genes appear to be induced 1.5 to 2-fold early in development, coincident with the increase in total actin mRNA. Since 12 of the genes code for extremely homologous proteins, it is possible that the large number of actin genes in Dictyostelium is utilized for precise regulation of the amount of actin produced at any stage of development, even though individual gene expression appears in some cases to be very stage-specific. In addition to these 13 actin genes, at least two and possibly four more genes are known to be expressed, because they are represented by complementary DNA clones, and an additional one or two expressed genes are indicated by primer extension experiments. Only one known gene, Actin 2-sub 2, is almost certainly a pseudogene. Thus the vast majority of Dictyostelium actin genes are expressed.

Actin paracrystalline sheets formed at the surface of positively charged liposomes

Paracrystalline aggregates of F-actin spontaneously assemble at the surface of positively charged liposomes. This single-layered paracrystalline array is made up of parallel and juxtaposed actin filaments aligned in register and showing the typical 36-nm periodicity which corresponds to the half-pitch of the double helix strand. This crystallization of pure actin results from a direct interaction between actin and positively charged lipids and does not occur with negative or neutral lipids.

Analysis of pharmacokinetic data using parametric models. II. Point estimates of an individual's parameters

This is the second in a series of tutorial articles discussing the analysis of pharmacokinetic data using parametric models. In this article the basic issue is how to estimate the parameters of such models. Primary emphasis is placed on point estimates of the parameters of the structural (pharmacokinetic) model. All the estimation methods discussed are least squares (LS) methods: ordinary least squares, weighted least squares, iteratively reweighted least squares, and extended least squares. The choice of LS method depends on the variance model. Some discussion is also provided of computer methods used to find the LS estimates, identifiability, and robust LS-based estimation methods.

Effects of cytochalasin B on cell movements and chemoattractant-elicited actin changes of Dictyostelium

The actin-binding drug cytochalasin B (CB) was employed to study the stability and role of cytoskeletal actin following chemotactic stimulation of Dictyostelium discoideum. Intact amoebae were found to be impermeable to this drug, as shown by lack of inhibition of chemotactic movement in its presence and failure of [3H]CB to bind to intact amoebae. However, there were approx. 150 000 high affinity CB-binding sites per cell detectable after cell breakage and preparation of Triton-insoluble cytoskeletons. The effect of CB on cytoskeletons was to destabilize the second (25-45 sec) and third (60 sec) chemotactically-induced peaks of cytoskeletal actin accumulation and to reduce the actin levels to the low prestimulus amount. In contrast, the drug had no such action on the rapid (3-5 sec) actin peak. This peak appeared to be stable in the presence of CB added before or simultaneously with lysis of the cell. It was also observed that the instability of the second and third peaks to CB gradually decreased after cell lysis (as did the number of CB binding sites) such that if CB was added 5 min after lysis of the chemotactically stimulated amoebae it had no destabilizing effect. Evidence was obtained from experiments employing centrifugation of cytoskeletons at 100 000 g and from the use of the DNase I inhibition assay for G-actin, that the first (3-5 sec) actin peak of accumulation involved polymerization rather than just cross-linking of short filamentous actin fragments. The significance of these actin accumulation peaks is discussed and their timing correlated with events involved in chemotaxis.

Binding of [3H]serotonin to skeletal muscle actin

We previously observed that the neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) binds with high- and low-affinity interactions to an actin-like protein prepared from rat brain synaptosomes. In this study, we examined its binding to highly purified actin obtained from rabbit skeletal muscle. Monomeric G-actin bound serotonin with high and low affinities, exhibiting equilibrium dissociation constants (KD values) of 5 X 10(-5) M and 4 X 10(-3) M, respectively. The serotonin binding site on actin was distinct from those sites previously characterized for divalent cations, nucleotides, and cytochalasin alkaloids. The binding of serotonin (1 microM) to G-actin was increased as much as 26-fold by divalent cations. Potassium iodine (KI) increased the affinity of G-actin for serotonin, KD values for this binding being 3 X 10(-7) M and X 10(-5) M. Serotonin bound with even higher affinity to polymerized F-actin, with KD values of 2 X 10(-8) M and 2 X 10(-5) M. However, the total number of binding sites on F-actin was only about 4% of the number of G-actin. The binding of serotonin (0.1 microM) to G-actin could be inhibited by phenothiazines (1 microM) or reserpine (10 microM), but not by classical antagonists of serotonin receptors or by drugs that release serotonin or inhibit its uptake. The binding of serotonin to actin in vivo may participate in a contractile process related to neurotransmitter release.

Organization of the Dictyostelium discoideum actin multigene family. Flanking sequences show subfamily homologies and unusual dyad symmetries

Sequences flanking the protein-coding regions of 15 of the 17 to 20 actin genes in the cellular slime mold Dictyostelium discoidium have been determined. Comparison of sequences among genes shows that they contain extensive homologies at both the 5' and 3' ends of the coding regions. On the basis of these homologies, actin genes fall into three groups. Group I consists of Actin 8 alone. Group II consists of the two closely linked genes Actin 3-sub1 and Actin 3-sub2. These two genes differ from all other actin genes in the location of their TATA box and oligo(dT) run, and diverge substantially in their coding sequence as well. Group III contains all the rest of the genes we have studied. Within this group, there are two subgroups of genes, IIIA (Actins 5, 9 and 10) and IIIB (Actins M6, 2-sub1 and 2-sub2, 4, 6, 7, 11 and 12). Two actin cDNA clones, ITL-1 and III-12/A1, which have no cloned genomic counterparts, are members of groups IIIA and IIIB, respectively. Homologies at the 3' ends of genes do not extend beyond a short genomic poly(A) sequence, the probable termination of transcription. Homologies at the 5' ends may extend about 300 base-pairs 5' to the ATG but, in most cases, extend only about 150 base-pairs 5' to the ATG. We have identified a group of short, relatively G + C-rich sequences within the extremely A + T-rich sequence at the 5' ends of actin-coding regions, which are shared among different actin genes. Many of these sequences exhibit dyad symmetry, and their general location and order is conserved among the different actin genes. We suggest that they may have a role in regulation of the transcriptional patterns of individual actin genes.

Isolation and characterization of six different chicken actin genes

Genes representing six different actin isoforms were isolated from a chicken genomic library. Cloned actin cDNAs as well as tissue-specific mRNAs enriched in different actin species were used as hybridization probes to group individual actin genomic clones by their relative thermal stability. Restriction maps showed that these actin genes were derived from separate and nonoverlapping regions of genomic DNA. Of the six isolated genes, five included sequences from both the 5' and 3' ends of the actin-coding area. Amino acid sequence analysis from both the NH2- and COOH-terminal regions provided for the unequivocal identification of these genes. The striated isoforms were represented by the isolated alpha-skeletal, alpha-cardiac, and alpha-smooth muscle actin genes. The nonmuscle isoforms included the beta-cytoplasmic actin gene and an actin gene fragment which lacked the 5' coding and flanking sequence; presumably, this region of DNA was removed from this gene during construction of the genomic library. Unexpectedly, a third nonmuscle chicken actin gene was found which resembled the amphibian type 5 actin isoform (J. Vandekerckhove, W. W. Franke, and K. Weber, J. Mol. Biol., 152:413-426). This nonmuscle actin type has not been previously detected in warm-blooded vertebrates. We showed that interspersed, repeated DNA sequences closely flanked the alpha-skeletal, alpha-cardiac, beta-, and type 5-like actin genes. The repeated DNA sequences which surround the alpha-skeletal actin-coding regions were not related to repetitious DNA located on the other actin genes. Analysis of genomic DNA blots showed that the chicken actin multigene family was represented by 8 to 10 separate coding loci. The six isolated actin genes corresponded to 7 of 11 genomic EcoRI fragments. Only the alpha-smooth muscle actin gene was shown to be split by an EcoRI site. Thus, in the chicken genome each actin isoform appeared to be encoded by a single gene.

Human plasma actin-depolymerizing factor. Purification, biological activity and localization in leukocytes and platelets

The plasma and serum of humans and various animal species exert an actin-depolymerizing activity. Human actin-depolymerizing factor (ADF) has been purified by ammonium sulfate fractionation, DEAE-cellulose and blue-Sepharose chromatography. It is a single polypeptide of approximately 90 kDa, with a pI between 6.0 and 6.5. ADF is heat and trypsin-sensitive, inactivated by EGTA, not stained by HIO4/Schiff on sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE), and not retained on a concanavalin-A-Sepharose column. Incubation of ethanol-fixed cultured cells or unfixed cryostat tissue sections with ADF abolishes immunofluorescent actin staining, by a mechanism which involves extraction of actin from the preparations. ADF promotes fragmentation and depolymerization of actin filaments as shown by electron microscopy, differential ultracentrifugation and DNAse I inhibition assay. This depolymerized actin retains its mobility on SDS/PAGE and is able to repolymerize in the presence of EGTA. Human white blood cells and platelets (but neither human fibroblasts nor white blood cells and platelets from pig, rat and rabbit) contain a 90-kDa protein reacting with an antibody raised in rabbit against human ADF as judged by immunofluorescence and immunoblotting techniques. Immunoblots of human granulocyte subcellular fractions suggest that the protein reacting with ADF antibody is present in the soluble cytoplasmic fraction. ADF may play a role in solubilization of plasma actin and in the intracellular organization of actin, and should be useful for the evaluation of the relative stability of cytoplasmic actin filaments in various physiological and pathological processes.

Human erythrocyte myosin: identification and purification

Human erythrocytes contain an Mr 200,000 polypeptide that cross-reacts specifically with affinity-purified antibodies to the Mr 200,000 heavy chain of human platelet myosin. Immunofluorescence staining of formaldehyde-fixed erythrocytes demonstrated that the immunoreactive myosin polypeptide is present in all cells and is localized in a punctate pattern throughout the cell. Between 20-40% of the immunoreactive myosin polypeptide remained associated with the membranes after hemolysis and preparation of ghosts, suggesting that it may be bound to the membrane cytoskeleton as well as being present in the cytosol. The immunoreactive myosin polypeptide was purified from the hemolysate to approximately 85% purity by DEAE-cellulose chromatography followed by gel filtration on Sephacryl S-400. The purified protein is an authentic vertebrate myosin with two globular heads at the end of a rod-like tail approximately 150-nm long, as visualized by rotary shadowing of individual molecules, and with two light chains (Mr 25,000 and 19,500) in association with the Mr 200,000 heavy chain. Peptide maps of the Mr 200,000 heavy chains of erythrocyte and platelet myosin were seen to be nearly identical, but the proteins are distinct since the platelet myosin light chains migrate differently on SDS gels (Mr 20,000 and 17,000). The erythrocyte myosin formed bipolar filaments 0.3-0.4-micron long at physiological salt concentrations and exhibited a characteristic pattern of myosin ATPase activities with EDTA, Ca++, and Mg++-ATPase activities in 0.5 M KCl of 0.38, 0.48, and less than 0.01 mumol/min per mg. The Mg++-ATPase activity of erythrocyte myosin in 0.06 M KCl (less than 0.01 mumol/min per mg) was not stimulated by the addition of rabbit muscle F-actin. The erythrocyte myosin was present in about 6,000 copies per cell, in a ratio of 80 actin monomers for every myosin molecule, which is an amount comparable to actin/myosin ratios in other nonmuscle cells. The erythrocyte myosin could function together with tropomyosin on the erythrocyte membrane (Fowler, V.M., and V. Bennett, 1984, J. Biol. Chem., 259:5978-5989) in an actomyosin contractile apparatus responsible for ATP-dependent changes in erythrocyte shape.

Improved negative staining of microfilament arrangements in detergent-extracted Physarum amoeboflagellates

A motile, lamellipodium-like structure, the ridge, forms as amoeboflagellate cells of Physarum polycephalum release from a substratum and begin swimming in fluid. Actin microfilaments form a distinct laminar core within the ridge; they are seen as a sparse, disordered meshwork in cytoskeletons prepared by conventional methods using uranyl acetate negative staining [10]. Preservation and visualization of these filaments and their arrangements improved considerably when cytoskeletons were imaged with phosphotungstic acid buffered with ammonium hydroxide (PTA(NH4]. Microfilaments within ridge cytoskeletons were found to form loose bundles and criss-crossing, 'meshwork' arrays several layers deep. Differences could be detected in morphology and detailed arrangement of microfilaments within cytoskeletons prepared in the presence of phalloidin. PTA(NH4) may be useful for studies of cytoskeletal elements and their rearrangements in dynamic, motile regions of cells.

DNA rearrangements of the actin gene cluster in Caenorhabditis elegans accompany reversion of three muscle mutants

We present evidence that associates dominant mutations in Caenorhabditis elegans that disrupt muscle structure and motility with a cluster of three actin genes mapped in the same region of linkage group V. We examined spontaneous and mutagen-induced wild-type revertants of these dominant alleles for alterations in the DNA of the actin gene cluster. Four of 73 revertants contain detectable DNA rearrangements within the cluster of actin genes including an insertion, a deletion and gene fusions. We postulate that these rearrangements inactivate or delete at least one gene in the cluster and consequently the original mutations are within the actin gene cluster.

The relationship between stress fiber-like structures and nascent myofibrils in cultured cardiac myocytes

The topographical relationship between stress fiber-like structures (SFLS) and nascent myofibrils was examined in cultured chick cardiac myocytes by immunofluorescence microscopy. Antibodies against muscle-specific light meromyosin (anti-LMM) and desmin were used to distinguish cardiac myocytes from fibroblastic cells. By various combinations of staining with rhodamine-labeled phalloidin, anti-LMM, and antibodies against chick brain myosin and smooth muscle alpha-actinin, we observed the following relationships between transitory SFLS and nascent and mature myofibrils: (a) more SFLS were present in immature than mature myocytes; (b) in immature myocytes a single fluorescent fiber would stain as a SFLS distally and as a striated myofibril proximally, towards the center of the cell; (c) in regions of a myocyte not yet penetrated by the elongating myofibrils, SFLS were abundant; and (d) in regions of a myocyte with numerous mature myofibrils, SFLS had totally disappeared. Spontaneously contracting striated myofibrils with definitive Z-band regions were present long before anti-desmin localized in the I-Z-band region and long before morphologically recognizable structures periodically link Z-bands to the sarcolemma. These results suggest a transient one-on-one relationship between individual SFLS and newly emerging individual nascent myofibrils. Based on these and other relevant data, a complex, multistage molecular model is presented for myofibrillar assembly and maturation. Lastly, it is of considerable theoretical interest to note that mature cardiac myocytes, like mature skeletal myotubes, lack readily detectable stress fibers.

Cell type-specific activation of actin genes in the early amphibian embryo

Muscle actin genes are the earliest yet described to show cell type-specific activation in amphibian embryos. Gene-specific probes show that alpha-skeletal and alpha-cardiac actin genes start to be transcribed simultaneously at the end of gastrulation, but only in those regions of the mesoderm that subsequently form embryonic muscle. Their expression provides a molecular marker for early cell determination.

Isolation and characterization of six different chicken actin genes

Genes representing six different actin isoforms were isolated from a chicken genomic library. Cloned actin cDNAs as well as tissue-specific mRNAs enriched in different actin species were used as hybridization probes to group individual actin genomic clones by their relative thermal stability. Restriction maps showed that these actin genes were derived from separate and nonoverlapping regions of genomic DNA. Of the six isolated genes, five included sequences from both the 5' and 3' ends of the actin-coding area. Amino acid sequence analysis from both the NH2- and COOH-terminal regions provided for the unequivocal identification of these genes. The striated isoforms were represented by the isolated alpha-skeletal, alpha-cardiac, and alpha-smooth muscle actin genes. The nonmuscle isoforms included the beta-cytoplasmic actin gene and an actin gene fragment which lacked the 5' coding and flanking sequence; presumably, this region of DNA was removed from this gene during construction of the genomic library. Unexpectedly, a third nonmuscle chicken actin gene was found which resembled the amphibian type 5 actin isoform (J. Vandekerckhove, W. W. Franke, and K. Weber, J. Mol. Biol., 152:413-426). This nonmuscle actin type has not been previously detected in warm-blooded vertebrates. We showed that interspersed, repeated DNA sequences closely flanked the alpha-skeletal, alpha-cardiac, beta-, and type 5-like actin genes. The repeated DNA sequences which surround the alpha-skeletal actin-coding regions were not related to repetitious DNA located on the other actin genes. Analysis of genomic DNA blots showed that the chicken actin multigene family was represented by 8 to 10 separate coding loci. The six isolated actin genes corresponded to 7 of 11 genomic EcoRI fragments. Only the alpha-smooth muscle actin gene was shown to be split by an EcoRI site. Thus, in the chicken genome each actin isoform appeared to be encoded by a single gene.

Characterization of the ATPase activities of myosins isolated from the membrane and the cytoplasmic fractions of human platelets

Myosin was purified from the membrane fraction and the cytoplasm of human platelets, and the K+(EDTA)- and Ca2+-dependent ATPase activities were studied under various experimental conditions. The ATPase activity of the myosin from the membrane fraction was slightly lower than that of its cytoplasmic counterpart, regardless of the different assay conditions (pH, ionic strength, and temperature). Both myosins showed the same pH optima and a similar ionic strength dependence for the two ATPase activities measured. In addition, they exhibited the same substrate specificity using ATP, CTP, and GTP as substrates. The activation energy of the Ca2+-dependent ATPase activity was essentially the same for the two myosins, while the activation energy of the K+(EDTA)-dependent ATPase activity of the membrane myosin was higher than that of the cytoplasmic myosin. The ATPase activity of the membrane myosin was found to be more sensitive to freezing and thawing than the cytoplasmic myosin. The alkylation of the thiol groups by N-ethylmaleimide or N-iodoacetyl-N-(5-sulfo-1-naphtyl)ethylenediamine, and the trinitrophenylation of the lysyl residues by 2,4,6-trinitrobenzenesulfonate caused a significant decrease in the K+(EDTA)-dependent ATPase activity of the two myosins. However, the membrane myosin was much less affected than the cytoplasmic myosin. Actin induced inhibition of the K+ (EDTA) ATPase of both myosins, and much smaller quantities of actin were needed to inhibit the cytoplasmic myosin ATPase compared to quantities needed to inhibit the myosin ATPase from the membrane fraction. This indicates that the membrane myosin has a lower affinity toward actin. The observed variations in the ATPase activity of the myosins isolated from the membrane and the cytoplasm fractions of human platelets may reflect differences in their respective physiological functions.

Interaction of fluorescently-labeled contractile proteins with the cytoskeleton in cell models

To determine if a living cell is necessary for the incorporation of actin, alpha-actinin, and tropomyosin into the cytoskeleton, we have exposed cell models to fluorescently labeled contractile proteins. In this in vitro system, lissamine rhodamine-labeled actin bound to attachment plaques, ruffles, cleavage furrows and stress fibers, and the binding could not be blocked by prior exposure to unlabeled actin. Fluorescently labeled alpha-actinin also bound to ruffles, attachment plaques, cleavage furrows, and stress fibers. The periodicity of fluorescent alpha-actinin along stress fibers was wider in gerbil fibroma cells than it was in PtK2 cells. The fluorescent alpha-actinin binding in cell models could not be blocked by the prior addition of unlabeled alpha-actinin suggesting that alpha-actinin was binding to itself. While there was only slight binding of fluorescent tropomyosin to the cytoskeleton of interphase cells, there was stronger binding in furrow regions of models of dividing cells. The binding of fluorescently labeled tropomyosin could be blocked by prior exposure of the cell models to unlabeled tropomyosin. If unlabeled actin was permitted to polymerize in the stress fibers in cell models, fluorescently labeled tropomyosin stained the fibers. In contrast to the labeled contractile proteins, fluorescently labeled ovalbumin and BSA did not stain any elements of the cytoskeleton. Our results are discussed in terms of the structure and assembly of stress fibers and cleavage furrows.

Role of actin in the responses of adrenal cells to ACTH and cyclic AMP: inhibition by DNase I

Erythrocyte ghosts were loaded with pancreatic DNase I and fused with Y-1 adrenal tumor cells to test the possibility that this enzyme might inhibit the steroidogenic responses of the cells to ACTH and cyclic AMP. Fusion of erythrocyte ghosts loaded with DNase I, but not those containing albumin, ovalbumin, boiled DNase I, or DNase I with excess G-actin, inhibited the increase in production of 20 alpha-dihydroprogesterone produced by ACTH and dibutyryl cyclic AMP; inhibition was concentration-dependent with 50% inhibition by 3 X 10(7) molecules of DNase I per cell. It was found that inhibition by DNase I was exerted at the step in the steroidogenic pathway at which cholesterol is transported to mitochondria where steroidogenesis begins. This was shown by measuring transport of cholesterol into the inner mitochondrial membrane, by measuring the production of pregnenolone by isolated mitochondria and by demonstrating that DNase I was without effect on the conversion of pregnenolone to 20 alpha-dihydroprogesterone (an end-product of steroid synthesis). The actin content of Y-1 cells was measured by two methods based upon inhibition of DNase I and by SDS gels following centrifugation. The cells were found to contain 2-3 X 10(7) molecules of actin per cell of which two-thirds is present as G-actin. Since DNase I is known to bind to G-actin to give a one to one complex, these and other findings suggest that at least some of the G-actin in the cells may be necessary for the steroidogenic responses to ACTH and cyclic AMP.

The effect of 5-bromodeoxyuridine on mouse embryos during neurulation in vitro

Mouse embryos explanted at various stages during neurulation were cultured for 20-28 h in the presence of 25-900 micrograms/ml of 5-bromodeoxyuridine (BUdR). BUdR strongly inhibited closure of the cranial neural tube, which was found to be stage-dependent. When mouse embryos were exposed to BUdR after development of the concave curvature in the neuroepithelium of the midbrain to the upper hindbrain regions, they became insensitive to the drug-induced open cranial neural tube. Histological observations showed that BUdR interfered with interkinetic migration and cytokinesis of the neuroepithelial cells. These cellular abnormalities were not dependent on the morphological development of the cranial neural folds. The 3H-BUdR experiment confirmed that the label was mostly incorporated into the DNA fraction.