Myosin in Polymorphonuclear Leukocytes

Neuronal Nitric Oxide Synthase (nNOS) in Neutrophils: An Insight

NO (nitric oxide) is an important regulator of neutrophil functions and has a key role in diverse pathophysiological conditions. NO production by nitric oxide synthases (NOS) is under tight control at transcriptional, translational, and post-translational levels including interactions with heterologous proteins owing to its potent chemical reactivity and high diffusibility; this limits toxicity to other cellular components and promotes signaling specificity. The protein-protein interactions govern the activity and spatial distribution of NOS isoform to regulatory proteins and to their intended targets. In comparison with the vast literature available for endothelial, macrophages, and neuronal cells, demonstrating neuronal NOS (nNOS) interaction with other proteins through the PDZ domain, neutrophil nNOS, however, remains unexplored. Neutrophil's key role in both physiological and pathological conditions necessitates the need for further studies in delineating the NOS mediated NO modulations in signaling pathways operational in them. nNOS has been linked to depression, schizophrenia, and Parkinson's disease, suggesting the importance of exploring nNOS/NO-mediated neutrophil physiology in relation to such neuronal disorders. The review thus presents the scenario of neutrophil nNOS from the genetics to the functional level, including protein-protein interactions governing its intracellular sequestration in diverse cell types, besides speculating possible regulation in neutrophils and also addressing their clinical implications.

Regulation of polymorphonuclear leukocyte phagocytosis by myosin light chain kinase after activation of mitogen-activated protein kinase

Polymorphonuclear leukocyte (PMNL) phagocytosis mediated by FcγRII proceeds in concert with activation of the mitogen-activated protein (MAP) kinase, extracellular signal-regulated kinase ERK2. We hypothesized that myosin light chain kinase (MLCK) could be phosphorylated and activated by ERK, thereby linking the MAP kinase pathway to the activation of cytoskeletal components required for pseudopod formation. To explore this potential linkage, PMNLs were challenged with antibody-coated erythrocytes (EIgG). Peak MLCK activity, 3-fold increased over controls, occurred at 4 to 6 minutes, corresponding with the peak rate of target ingestion and ERK2 activity. The MLCK inhibitor ML-7 (10 μmol/L) inhibited both phagocytosis and MLCK activity to basal values, thereby providing further support for the linkage between the functional response and the requirement for MLCK activation. The MAPK kinase (MEK) inhibitor PD098059 inhibited phagocytosis, MLCK activity, and ERK2 activity by 80% to 90%. To directly link ERK activation to MLCK activation, ERK2 was immunoprecipitated from PMNLs after EIgG ingestion. The isolated ERK2 was incubated with PMNL cytosol as a source of unactivated MLCK and with MLCK substrate; under these conditions ERK2 activated MLCK, resulting in phosphorylation of the MLCK substrate or of the myosin light chain itself. Because MLCK activates myosin, we evaluated the effect of directly inhibiting myosin adenosine triphosphatase using 2,3-butanedione monoxime (BDM) and found that phagocytosis was inhibited by more than 90% but MLCK activity remained unaffected. These results are consistent with the interpretation that MEK activates ERK, ERK2 then activates MLCK, and MLCK activates myosin. MLCK activation is a critical step in the cytoskeletal changes resulting in pseudopod formation.

Regulation of polymorphonuclear leukocyte phagocytosis by myosin light chain kinase after activation of mitogen-activated protein kinase

Polymorphonuclear leukocyte (PMNL) phagocytosis mediated by FcγRII proceeds in concert with activation of the mitogen-activated protein (MAP) kinase, extracellular signal-regulated kinase ERK2. We hypothesized that myosin light chain kinase (MLCK) could be phosphorylated and activated by ERK, thereby linking the MAP kinase pathway to the activation of cytoskeletal components required for pseudopod formation. To explore this potential linkage, PMNLs were challenged with antibody-coated erythrocytes (EIgG). Peak MLCK activity, 3-fold increased over controls, occurred at 4 to 6 minutes, corresponding with the peak rate of target ingestion and ERK2 activity. The MLCK inhibitor ML-7 (10 μmol/L) inhibited both phagocytosis and MLCK activity to basal values, thereby providing further support for the linkage between the functional response and the requirement for MLCK activation. The MAPK kinase (MEK) inhibitor PD098059 inhibited phagocytosis, MLCK activity, and ERK2 activity by 80% to 90%. To directly link ERK activation to MLCK activation, ERK2 was immunoprecipitated from PMNLs after EIgG ingestion. The isolated ERK2 was incubated with PMNL cytosol as a source of unactivated MLCK and with MLCK substrate; under these conditions ERK2 activated MLCK, resulting in phosphorylation of the MLCK substrate or of the myosin light chain itself. Because MLCK activates myosin, we evaluated the effect of directly inhibiting myosin adenosine triphosphatase using 2,3-butanedione monoxime (BDM) and found that phagocytosis was inhibited by more than 90% but MLCK activity remained unaffected. These results are consistent with the interpretation that MEK activates ERK, ERK2 then activates MLCK, and MLCK activates myosin. MLCK activation is a critical step in the cytoskeletal changes resulting in pseudopod formation.

The damaging potential of leukocyte activation in the microcirculation

Traditionally leukocytes have been regarded as beneficial cells, owing to their immunologic and antimicrobial activity. Recent evidence suggests, however, an additional role of granulocytes and monocytes as mediators of cardiovascular complications, such as ischemia, reperfusion, diabetes, physiologic shock, venous ulceration, and other conditions. Granulocytes and monocytes have a large volume, and stiff cytoplasm; they have the ability to adhere to endothelium and to other substrates; and they exert several forms of cytotoxicity. Granulocytes and monocytes may be trapped in the microcirculation, may obstruct capillaries and thereby induce a no-reflow phenomenon, and may initiate organ dysfunction via oxygen free radical production and proteolytic cleavage. Few organs seem to be spared from the potential destructive actions of these cells, and novel approaches are required to interfere with leukocyte accumulation in local regions of the peripheral circulation.

Leukocyte biophysics. An invited review

The biophysical properties of leukocytes in the passive and active state are discussed. In the passive unstressed state, leukocytes are spherical with numerous membrane folds. Passive leukocytes exhibit viscoelastic properties, and the stress is carried largely by the cell cytoplasm and the nucleus. The membrane is highly deformable in shearing and bending, but resists area expansion. Membrane tension can usually be neglected but plays a role in cases of large deformation when the membrane becomes unfolded. The constant membrane area constraint is a determinant of phagocytic capacity, spreading of cells, and passage through narrow pores. In the active state, leukocytes undergo large internal cytoplasmic deformation, pseudopod projection, and granule redistribution. Several different measurements for assessment of biophysical properties and the internal cytoplasmic deformation in form of strain and strain rate tensors are presented. The current theoretical models for active cytoplasmic motion in leukocytes are discussed in terms of specific macromolecular reactions.

Rheology of leukocytes

The rheological properties of human leukocytes (WBCs) have been studied by micropipette aspiration and filtration tests. A small aspiration pressure applied via a micropipette (diameter approximately equal to 3 micron) causes the WBC to undergo a rapid elastic deformation followed by a slow creep. The data can be analyzed with a viscoelastic model: an elastic element K1 in parallel with a Maxwell element (elastic element K2 in series with viscous element mu). Neutrophils and B lymphocytes are similar in K1, K2, and mu, but these values are higher for T lymphocytes. Treatment of neutrophils with colchicine decreases K2 and mu without changing K1, whereas cytochalasin B decreases all three coefficients; these results indicate the importance of cytoskeletal microtubules and microfilaments in WBC rheology. In the presence of Ca2+, WBCs form protopods which have increased viscoelastic coefficients. Inhibition of protopod formation with pentoxifylline is associated with an increase in WBC deformability. The ruffled surface of the apparently round WBC provides an area about twice that needed to enclose a smooth sphere of the same volume; this geometric feature plays an important role in whole WBC deformability tested through 4-5 micron filter pores or micropipettes. Because of its larger volume and higher cellular viscosity, each WBC is equivalent to approximately 700 erythrocytes in its tendency to block 5-micron channels. The rheology of WBCs has significant implications in their functional behavior, including flow through the microcirculation and interaction with endothelial cells.

The organization of actin filaments in human polymorphonuclear leukocytes

Actin constitutes a major component of the cytoskeleton of human polymorphonuclear leukocytes (PMNs). In this study, we present a comprehensive view of the organization of actin in various PMN regions and functional states. Transmission electron microscopic observations were made on whole mount, migrating, and phagocytizing PMNs. Positive identification of actin filaments was made through S-1 myosin subfragment labeling. In all PMNs studied, actin filaments were primarily organized as a three-dimensional meshwork. The density of this meshwork was greatest within the cell cortex. At peripheral regions of nonpolarized (viz., no distinct head or tail region) and polarized PMNs, actin filaments organized into parallel bundles or overlapping arcs. These bundles or arcs were oriented either perpendicular or parallel to the cell periphery. At the base of the PMN, actin filaments converged upon dense, plaquelike condensations. This latter pattern of actin organization was also observed in some pseudopods at the cell front and in phagocytic processes engulfing bacteria. In areas of internalized bacteria, the surrounding actin appeared as a loose meshwork. Treatment of PMNs with the antiactin drug, cytochalasin B, revealed shearing of the peripheral actin meshwork, condensation of the meshwork around the nuclear region, and dissolution of the basal plaquelike condensations.

Studies on the antigenic sites of actin: a comparative study of the immunogenic crossreactivity of invertebrate actins

The structural homologies of invertebrate actins with cytoplasmic vertebrate actins have recently been substantiated by comparative sequence analyses. This suggests that cytoplasmic actin is the ancestral precursor of smooth and striated muscle actin in vertebrates. We have raised antibodies in rabbits against a number of invertebrate muscle actins and have characterized the antisera by means of the highly sensitive ELISA method, which allows quantitation of nanomolar amounts of actin. Despite the fact that the invertebrate actins examined are very similar in primary structure, our results indicate that antibodies raised against them clearly distinguish between only a few amino-acid substitutions, and that the immunoreactivities quantitatively reflect the genetic divergence of this ubiquitous conservative protein. Examination of several proteolytic fragments of scallop actin for immunoreactivity with the homologous antiserum suggests that the major antigenic sites of actin are located within the amino terminal region of the molecule, while a carboxy terminal fragment comprising residues 69-372 exhibits very weak crossreactivity. Immunoadsorption experiments further indicate that species-specific antibodies are directed to antigenic determinants in the N-terminal region. This finding is supported by an examination of the effects of chemical modifications to Tyr, His, Arg, and Cys residues on the immunoreactivity of actin. Interaction with DNAase I markedly decreases the immunoreactivity of actin. This is consonant with the finding that the amino terminal peptide comprising residues 1-207 inhibits DNAase I, whilst a tryptic fragment fails to bind to the enzyme. The interaction is abolished by EDTA and the removal of the tightly bound cation is accompanied by a conformational change, shown by shifts in circular dichroic spectra. The possible involvement of the amino terminal peptide of actin in cation binding is discussed.

Effects of leukocyte inhibitory factor (LIF) on human neutrophil function

The effects of the lymphokine, leukocyte inhibitory factor (LIF), on human neutrophil function were studied. This soluble mediator, which is defined by its specific inhibition of neutrophil locomotion, does not interfere with chemotactic factor binding and does not affect basal or stimulated superoxide generation by neutrophils. In contrast, phagocytosis of opsonized Staphylococcus aureus is markedly inhibited by LIF, and degranulation is stimulated by this lymphokine. The possible mechanisms of LIF action on neutrophils are discussed.

Ca2+-stimulated, Mg2+-dependent ATPase in bovine thyroid plasma membranes

An isolated plasma membrane fraction from bovine thyroid glands contained a Ca2+-stimulated, Mg2+-dependent adenosine triphosphatase ((Ca2+ + Mg2+)-ATPase) activity which was purified in parallel to (Na+ + K+)-ATPase and adenylate cyclase. The (Ca2+ + Mg2+)-ATPase activity was maximally stimulated by approx. 200 microM added calcium in the presence of approx. 200 microM EGTA (69.7 +/- 5.2 nmol/mg protein per min). In EGTA-washed membranes, the enzyme was stimulated by calmodulin and inhibited by trifluoperazine.

Subcellular localization and properties of adenosine triphosphatase in human polymorphonuclear leucocytes

Magnesium-dependent adenosine triphosphatase (Mg2+-ATPase) activities wee studied in human neutrophilic polymorphonuclear leucocytes. Kinetic studies on whole leucocyte homogenates produced curvilinear kinetics suggesting the presence of at least two forms of Mg2+-ATPase. Neutrophils were homogenized in isotonic sucrose and, after low-speed centrifugation, the supernatant was subjected to analytical subcellular fractionation. Gradient fractions were assayed for Mg2+-ATPase and for principal organelle marker enzymes. Mg2+-ATPase was distributed between the plasma membrane, mitochondrial and cytosol fractions. Kinetic and inhibitor studies on Mg2+-ATPase from each localization indicated the presence of three forms of the enzyme. The plasma membrane and mitochondrial activities had a Km value of 0.2 mmol/l for ATP, whilst the Km for the cytosolic enzyme was 1.8 mmol/l. Inhibitor studies showed further differences between the three enzymes. Neutrophils were isolated from control subjects, patients with chronic granulocytic leukaemia and patients in the third trimester of pregnancy. The specific activities (mUnits/mg protein) of Mg2+-ATPase, in contrast to those of alkaline phosphatase, were similar in all three patient groups. This result, together with the fractionation experiments and inhibitor studies, strongly suggest that the ATPase is not attributable to neutrophil alkaline phosphatase.

Cytoplasmic and plasma membrane adenosine triphosphatase of polymorphonuclear neutrophils: comparison of their enzymatic properties and attempt for a direct determination of myosin ATPase activity using polymorphonuclear neutrophil extract

Enzymatic properties of the ATPase of the plasma membrane and cytoplasmic myosin B from guinea-pig polymorphonuclear neutrophils were compared. In the plasma membrane, Mg2+- and Ca2+-activated ATPases showed the same dependence pattern on KCl concentration and pH, i.e., both ATPases increased with decreasing KCl concentration and with rising pH until pH 9.0. The maximum activation of Mg2+-ATPase was observed at 1 . 10(-3) M Mg2+. On the other hand, EDTA-activated ATPase activity was so low that no clear dependence curve was obtained. In myosin B, Mg2+-ATPase activity was below one-tenth that of the plasma membrane ATPase with the maximum activation at 1 . 10(-2) M Mg2+ and pH 9.0 EDTA- and Ca2+-activated ATPase exhibited almost the same activity and the same KCl-dependence curve, i.e., both ATPases increased and increasing KCl concentration. With regard to pH-dependence, Ca2+-ATPase showed a U-shaped curve with the minimum at pH 7.0, wherease EDTA-activated ATPase indicated a bell-shaped curve with the maximum at pH 9.0. Based on the findings that the EDTA-activated ATPase activity was hardly detected in the plasma membrane but high in myosin B, the distribution of ATPase activity on subcellular fractions was studied and the results obtained that the myosin-ATPase activity could be directly measured using the polymorphonuclear neutrophil extract if the EDTA-activated ATPase activity was used as an enzymatic marker for myosin.

Effects of sulfhydryl reagents on phagocytosis and exocytosis in rabbit polymorphonuclear leukocytes

The effect of sulfhydryl reagents on phagocytosis and concomitant enzyme release and on ionophore A 23187 + Ca2+-induced exocytosis in rabbit polymorphonuclear leukocytes (PMN's) was studied. Membrane-penetrating sulfhydryl reagents such as cytochalasin A and N-naphthylmaleimide in micromolar concentrations inhibit both phagocytosis and exocytosis. Poorly penetrating reagents such as p-chloromercuribenzene sulfonate (pCMBS) and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), inhibit only in high concentrations (pCMBS), or they are ineffective as inhibitors (DTNB). Inhibition by pCMBS is not reversed by glutathione or dithiothreitol; this suggests that some pCMBS probably enters the cell. Specific intracellular sulfhydryl compounds appear to be essential in the cellular apparatus involved in phagocytosis and exocytosis; various possibilities are considered. A concentration of N-naphthylmaleimide which completely inhibits phagocytosis and exocytosis leaves cellular ATPase activity intact.

A study of invertebrate actins by isoelectric focusing and immunodiffusion

Actin isolated from various invertebrate phyla comigrates with the beta-form of vertebrate smooth muscle actin. However, invertebrate actins are not identical, since antibodies to insect-actin will not crossreact with the other species.

Contractile proteins in epidermis. Isolation and properties of guinea-pig epidermal myosin

Proteins of apparent molecular weights between 10 000 and 250 000 could be solubilized from guinea pig epidermis using a Tris/sucrose/ATP buffer. When the ionic concentration of the solubilized extract was made 75 mM with respect to KCl and 2 mM with respect to MgCl2, a protein complex precipitated which on SDS-polyacrylamide gel electrophoresis resolved into bands corresponding in migration to myosin, actin and a number of low molecular weight proteins. Myosin was dissociated from the complex with 0.6 M KI and purified by gel filtration chromatography on an agarose column. The purified epidermal myosin fraction contained a polypeptide of 200 000 molecular weight andtwo low molecular weight polypeptides of 16 500 and 13 000. The amino acid composition of the epidermal myosin heavy chain was similar to that of muscle myosin. At high ionic strength epidermal myosin had high specific (K+ + Ca2+)- and (K+ + EDTA)-ATPase activities and low specific (K+ + Mg2+)-ATPase activity. The pH activity curves of the (K+ + Ca2+)- and (K+ + EDTA)-ATPase were different. ATP was hydrolyzed faster than other nucleoside triphosphates. At low ionic strength, the (K+ + Mg2+)-ATPase activity of epidermal myosin was stimulated two fold by skeletal muscle actin. The myosin formed bipolar filaments in 50 mM KCl in the presence of 5 mM Mg2+.

A Ca2+-stimulated ATPase activity in rabbit neutrophil membranes

An ATPase activity specifically stimulated by micromolar Ca2+ concentrations has been identified in association with rabbit neurophil membranes. These studies provide the basis of further characterization of the Ca2+-ATPase activity with regard to neutrophil function.

Isolation and surface labeling of murine polymorphonuclear neutrophils

Methods for the induction of an exudate of polymorphonuclear neutrophilic leukocytes (PMN) in the peritoneal cavity of C57BL, BALB/c, SJL and CBA mice were analysed. Peritoneal exudates in male mice were highly enriched for PMN (80-90%) three hours after a single injection of calcium caseinate whereas eosinophils comprised less than 1% of the exudate population. Female mice were a less satisfactory source of PMN because the proportion of eosinophilis in the exudate was variable. Purification of PMN from peritoneal exudate cells was performed on the basis of light scattering using a Becton-Dickinson cell sorter or by density gradient centrifugation with graded polyvinylpyrrolidone-coated silica particles (Percoll). Both techniques yielded approximately 97% pure PMN preparations. Electrophoretic analysis of the PMN proteins revealed an abundance of lactoferrin and actin, but several other proteins were also present in high concentrations. Proteolytic degradation of several high molecular weight proteins (greater than 90,000) was prevented by the addition of phenylmethylsulphonyl fluoride (PMSF) and ethylene diamine tetracetic acid (EDTA). Surface iodination, using diphenyl, tetrachloroglycouril (IODO-DEN), indicated that there were six tyrosine-containing proteins present on the external cell membrane. The apparent molecular weights of these surface proteins ranged from 185,000 to 90,000 and the major 125I-labeled protein had an apparent molecular weight of 90,000. Neither actin nor lactoferrin was labeled with 125I unless cell viability was lost during the iodination procedure. Standard conditions for labeling the cell surface only, required low iodide and IODO-GEN concentrations. Biosynthetic labeling of PMN using S-methionine increased the sensitivity of detection for most of the proteins, but some of the granule storage proteins (such as lactoferrin) were not effectively labeled within three hours.

Phagocytosis: a review

Primitive unicellular organisms depend greatly on internalization of particulate matter for nourishment. In metazoa, this process is further developed to play a major role in mechanisms of defense. This review analyzes, mainly in mammalian systems, the various phenomena surrounding the phagocytic act. Much of the emphasis is placed on experimental work which has recently elucidated some of its features. Both the structural and functional aspects of phagocytosis are considered throughout the review, which is subdivided into an examination of chemotaxis and the various agents inducing it, the mode of recognition of particles to be phagocytized, and the mechanisms of ingestion. The last includes a discussion of the possible means whereby recognition is translated into ingestion, the modes of adhesion of particles onto the surface of phagocytes, the formation and fusion of pseudopodia during engulfment and ingestion, and process and significance of degranulation. In addition, the metabolic changes in phagocytes during the processes of chemotaxis, ingestion, and digestion are described. A discussion of the various ways phagocytes may destroy microorganisms incorporates an appreciation of the importance of the microbicidal action of the acidic environment of the phagosome, the various lysosomal contents, hydrogen peroxide, superoxide, singlet oxygen, and chemiluminescence. The interdependence and interrelationship of the induction and cooperation of these mechanisms are examined.

AMg2+ -dependent class of thick filaments and correlated nuclear chromatin condensation in catfish photoreceptors

Photoreceptor cells of excised catfish retinae show morphological differences when incubated in Ringer's solutions of varying ionic composition. Two striking changes were observed in photoreceptor cells incubated in a high Mg2+ (25 mM) Ringer's: (1) Thick filaments appeared in the cytoplasm of receptor terminals and myoids; (2) A pronounced condensation of nuclear chromatin occurred in certain nuclei in the outer nuclear layer. The filaments occurred in lattices or bundles. The bundles had a diameter of approximately 0.05--0.2 micrometer and had either tapered or frayed ends. They were observed with somewhat higher incidence in tissue incubated in a 25 mM Mg2+ Ringer's with EGTA added to chelate Ca2+. A common basis for the cytoplasmic and nuclear changes may lie in a redistribution of fibrous protiens brought about by the increased Mg2+ concentration.

Reversible superprecipitation and bundle formation of plasmodium actomyosin

Synthetic actomyosin from plasmodium was found to undergo reversible superprecipitation upon addition of ATP. According to electronmicroscopic investigation upon clearing, short myosin filaments of about 0.2 micron in length appeared predominantly coexisting with actin filaments, and after superprecipitation, bundles of actin filaments were formed where short myosin filaments or myosin molecules were bound to the side of the bundle, making a whisk-like structure. The turbidity and the ATPase activity of actomyosin were measured at various ATP concentrations clamped by using an ATP-regenerating system. The turbidity was high below 1 . 10(-6) M ATP, corresponding to the state of superprecipitation, and with increasing ATP concentration it dropped in the range of 1 . 10(-6)--1 . 10(-5) M ATP. On the other hand, the ATPase activity was low below 1 . 10(-6) M ATP and increased above 1 . 10(-5) M after the turbidity dropped. Characteristic features of superprecipitation of plasmodium actomyosin observed here were discussed in relation to the mechanism of motility in vivo.

Defective neutrophil function in myotonic dystrophy

Polymorphonuclear leukocytes (PMNs), obtained from 10 patients with myotonic dystrophy and 39 age-matched controls, were tested for chemotactic activity, chemiluminescence (CL), and phagocytosis. PMNs from 8 of the 10 patients had depressed chemotactic indexes (30 +/- 23) as compared to controls (61 +/- 18) (P less than 0.0003), and 6 of the 10 patients' PMNs produced lower peaks in CL (less than 90%) than those of controls. Phagocytosis and bactericidal activity were evaluated in 7 patients and were normal. Control PMNs incubated in patient serum produced normal chemotactic activity and CL. These results indicate defects in 2 PMN membrane-associated activities in myotonic dystrophy and suggest that the PMN is affected by the generalized membrane abnormality underlying myotonic dystrophy.

Granule release by polymorphonuclear leukocytes treated with the ionophore A23187

Polymorphonuclear leukocytes (PMN's) incubate three to eight minutes at 37 degrees C in medium containing 1 X 10(-6) M of the ionophore antibiotic A23187 released their cytoplasmic granules into the extracellular medium. Transmission electron microscopy of treated cells showed microfilament bundles extending between adjacent granules within the cytoplasm and between granules and the plasma membrane. Tiny dense projections (beads) 8-12 nm in diameter were observed along segments of the cytoplasmic surface of the plasma membrane with a periodicity of 20-30 nm. These beads were observed on the plasma membrane only in the vicinity of intra- or extracytoplasmic granules. The structural relationships of the beads with the plasma membrane microfilaments suggest they play a role in the process of ionophore-induced granule release from polymorphonuclear leukocytes.

Myosin and paramyosin of Caenorhabditis elegans: biochemical and structural properties of wild-type and mutant proteins

Myosin and paramyosin have been purified from the nematode, Caenorhabditis elegans. The properties of the myosin in general resemble those of other myosins. The native molecule is a dimer of heavy (210,000 dalton) polypeptide chains and contains 18,000 and 16,000 dalton light chains. When rapidly precipitated from solution, it forms small, bipolar aggregates, about 150 nm long, consistent with the expected molecular structure of a rigid rod with a globular head region at one end. Its ATPase activity is stimulated by Ca2+ and EDTA. The myosin binds to F actin in a polar and ATP-sensitive manner, and the Mg2+-ATPase is activated by either F actin or nematode thin filaments. Dialysis of myosin to low ionic strength produces very long filaments. When a myosin-paramyosin mixture is dialyzed under the same condtions, co-filaments form which consist of a myosin cortex, surrounding a paramyosin core. Some properties of myosin from the mutants E675 and E190, which have functionally and structurally altered body wall muscles, are compared with those of wild-type myosin. These myosins of these results are discussed in terms of the myosin heavy chain composition.

Isolation and characterization of an actin-like protein from membranes of human red cells

The existence of an actin-like protein in human red-cell membranes has been confirmed. The protein was extracted from acetone-treated ghosts and purified by (NH4)2SO4 fractionation. The protein undergoes G-F transformation and forms filaments in the presence of 0.1 M KCl. The filaments can be "decorated" by muscle heavy meromyosin. The protein has the same molecular weight as muscle actin and interacts with muscle myosin. All these properties show that the protein closely resembles muscle actin.