Studies on actin-actin and actin-myosin interaction

Actin filaments-A target for redox regulation

Actin and its ability to polymerize into dynamic filaments is critical for the form and function of cells throughout the body. While multiple proteins have been characterized as affecting actin dynamics through noncovalent means, actin and its protein regulators are also susceptible to covalent modifications of their amino acid residues. In this regard, oxidation-reduction (Redox) intermediates have emerged as key modulators of the actin cytoskeleton with multiple different effects on cellular form and function. Here, we review work implicating Redox intermediates in post-translationally altering actin and discuss what is known regarding how these alterations affect the properties of actin. We also focus on two of the best characterized enzymatic sources of these Redox intermediates-the NADPH oxidase NOX and the flavoprotein monooxygenase MICAL-and detail how they have both been identified as altering actin, but share little similarity and employ different means to regulate actin dynamics. Finally, we discuss the role of these enzymes and redox signaling in regulating the actin cytoskeleton in vivo and highlight their importance for neuronal form and function in health and disease. © 2016 Wiley Periodicals, Inc.

Extracellular inhibitors, repellents, and semaphorin/plexin/MICAL-mediated actin filament disassembly

Multiple extracellular signals have been identified that regulate actin dynamics within motile cells, but how these instructive cues present on the cell surface exert their precise effects on the internal actin cytoskeleton is still poorly understood. One particularly interesting class of these cues is a group of extracellular proteins that negatively alter the movement of cells and their processes. Over the years, these types of events have been described using a variety of terms and herein we provide an overview of inhibitory/repulsive cellular phenomena and highlight the largest known protein family of repulsive extracellular cues, the Semaphorins. Specifically, the Semaphorins (Semas) utilize Plexin cell-surface receptors to dramatically collapse the actin cytoskeleton and we summarize what is known of the direct molecular and biochemical mechanisms of Sema-triggered actin filament (F-actin) disassembly. We also discuss new observations from our lab that reveal that the multidomain oxidoreductase (Redox) enzyme Molecule Interacting with CasL (MICAL), an important mediator of Sema/Plexin repulsion, is a novel F-actin disassembly factor. Our results indicate that MICAL triggers Sema/Plexin-mediated reorganization of the F-actin cytoskeleton and suggest a role for specific Redox signaling events in regulating actin dynamics.

Thin filament proteins and thin filament-linked regulation of vertebrate muscle contraction

Recent developments in the field of myofibrillar proteins will be reviewed. Consideration will be given to the proteins that participate in the contractile process itself as well as to those involved in Ca-dependent regulation of striated (skeletal and cardiac) and smooth muscle. The relation of protein structure to function will be emphasized and the relation of various physiologically and histochemically defined fiber types to the proteins found in them will be discussed.

The isolation and characterisation of guinea-pig polymorphonuclear leucocyte actin and myosin

The contractile proteins actin and myosin have been isolated from the soluble phase of guinea-pig polymorphonuclear leucocytes and partially characterised. Two forms of actin have been identified, designated 'Mg-actin' and 'KCl-actin'. They have different polymerising properties but their propensity to form synthetic homologous and heterologous actomyosins and to inhibit DNAase-1 does not significantly differ. Both show beta and gamma isoelectric forms in focusing gels and the Mg-actin accounts for about 5% of the soluble-phase protein and te KCl-actin around 2%. Leucocyte myosin has been isolated by affinity chromatography on N6-ADP-Sepharose with a good enrichment of both Ca2+-ATPase and the ATPase activity measured in the absence of Ca2+ or Mg2+ and in the presence of EDTA. This protein, too, has the capacity to form synthetic homologous and hybrid actomyosins with enhancement of the basal Mg2+-ATPase activity. The ratio of actin to myosin in the leucocyte calculated on a molar basis is well in excess of 100, a figure consistent with the findings from other non-muscle cells.

Dinitrophenylation of rabbit skeletal actomyosin

Dinitrophenylated reconstituted or natural actomyosin effected changes in the Ca2+ sensitivity which were dependent upon the ionic strength of the reaction medium. Dinitrophenylation of reconstituted actomyosin in 0.6 M KCl led to the incorporation of 2-6 mol of the reagent per 5-10(5) g of protein and it possessed considerable Ca2+ sensitivity. Dinitrophenylated natural actomyosin under the same conditions lost most of its Ca2+ sensitivity when 1.3-5.4 mol of the dinitrophenyl group were bound. The myosin from these modified actomyosins did not lose Ca2+ sensitivity and the myosin was labeled only with 0.4-1.7 mol of the dinitrophenyl group. Dinitrophenylation of both kinds of actomyosin in 0.06 M KCl abolished the Ca2+ sensitivity; the myosin from the modified actomyosins also lost Ca2+ sensitivity. Myosin alone was more susceptible to a loss of Ca2+ sensitivity than myosin in actomyosin. Actin protected the ability of myosin to sense Ca2+ regulated actin in modified actomyosin at 0.6 M KCl but not at 0.06 M KCl. Actomyosin dinitrophenylated in the presence of ATP lost Ca2+ sensitivity. However, the myosin from this actomyosin possessed Ca2+ sensitivity. Thiolysis of the dinitrophenylated actomyosin by 2-mercaptoethanol at low ionic strength did not restore the Ca2+ sensitivity of this actomyosin or its myosin although there was a significant loss of the dinitrophenyl group.

The reactivity and function of thiol groups in trout actin

1. Considerable differences were found between the rates and degrees of modification of native trout actin with iodo[2-(14)C]acetate and iodo[1-(14)C]acetamide. 2. With iodoacetate, G- and F-actin were both labelled in the N-terminal peptide only. This modification had little effect on the ability of the actin to polymerize. 3. Iodoacetamide labelled three cysteine residues in both G- and F-actin. The modified cysteine residues were identified from the position of the corresponding tryptic peptides on peptide ;maps'. 4. The modification had little effect on the ability of G-actin to polymerize, to bind ATP or to bind Ca(2+), or on the ability of F-actin to depolymerize. 5. It is concluded that the three cysteine residues present on the ;surface' of the native trout actin molecule have no direct role in the polymerization processes, the binding of ATP, or the binding of Ca(2+).

The amino acid sequence around four cysteine residues in trout actin

Four unique carboxymethylcysteine-containing peptides were isolated from tryptic and chymotryptic digests of trout muscle actin carboxymethylated with iodo[2-(14)C]acetic acid in 6m-guanidinium chloride. The amino acid sequences of these peptides were determined and showed a high degree of homology with the corresponding sequences from rabbit actin. One of the radioactive peptides was the C-terminal peptide and another sequence probably contained the cysteine residue from the N-terminal region of the protein.

Effects of sulphydryl inhibitors on frog sartorius muscle: N-ethylmaleimide

1. The characteristic action of the -SH inhibitor, N-ethylmaleimide (NEM), is muscle rigour. The dose-response curve indicates a biphasic effect with maximum rigour tension produced by 1.0 mM NEM; beyond 1.0 mM there was an inverse relationship between dose and response.2. NEM produces a membrane depolarization unrelated to rigour development.3. NEM causes a sustained increase in (45)Ca efflux from whole muscle. Pretreatment of a muscle with ethylenediamine tetra-acetic acid (EDTA, 5 mM) to remove membrane calcium does not alter the NEM induced (45)Ca efflux.4. It is suggested that the primary site of NEM action is inhibition of calcium uptake by the sarcoplasmic reticulum thereby producing rigour. At concentrations above 1.0 mM, NEM may affect the myofilaments.

Chemical studies on the cysteine and terminal peptides in tryptic digests of actin

1. On exhaustive digestion of carboxymethylated actin in 6m-urea solutions with carboxypeptidase A, 1 mole of phenylalanine was liberated/43000g. of protein. At a lower urea concentration and in the absence of urea, carboxymethyl-cysteine (CMCys) was also liberated. 2. Three cysteine-containing peptides were identified by the study of peptide ;maps' of tryptic digests of actin treated with thiol reagents. 3. The three peptides, each containing one residue of CMCys, were isolated from tryptic digests of carboxymethylated actin by ion-exchange chromatography. 4. One of these peptides was possibly the N-terminal peptide and contained about 17-18 residues; another was CMCys-Asp-Ile-Asp-Ile-Arg; the other, CMCys-Phe, was the C-terminal tryptic peptide. 5. The chemical evidence suggests that the actin molecule consists of a single polypeptide chain of molecular weight about 44000.