Concentrated Tris solutions for the preparation, depolymerization, and assay of actin: application to erythroid actin

Regulation of actin by ion-linked equilibria

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

Release of an ~55kDa fragment containing the actin-binding domain of β-spectrin by caspase-8 during FND-induced apoptosis depends on the presence of protein 4.1

Analyses of the status of the membrane spectrin-based skeleton during fludarabine/mitoxantrone/dexamethasone-induced (FND-induced) apoptosis revealed proteolytic degradation of β-spectrin, with the prevalent appearance of a specific fragment with a molecular weight of ~55kDa, containing the actin-binding domain (ABD). Appearance of this fragment was dependent on induction of apoptosis. In silico proteolysis of spectrin identified caspase-8 as a candidate protease responsible for the generation of this ~55kDa ABD-containing fragment. Analyses of spectrin and procaspase-8 localization during early apoptosis indicated temporary (<30-120min) submembranous colocalization of both proteins. Proteolytic release of the N-terminal ~55kDa fragment of purified spectrin by recombinant caspase-8 does not occur in normal cells, but does occur in isolated membrane, such as red blood cell ghosts, or in vitro in the presence of apoptotic cell extracts. Surprisingly, proteolysis of purified spectrin by recombinant caspase-8 resulted in the generation of the ~55kDa fragment only in the presence of purified protein 4.1. This suggests that only the appropriate spatial arrangement of the spectrin-based membrane skeleton or the appropriate conformational state of spectrin, which are both known to be induced by 4.1, can sensitize β-spectrin to cleavage by caspase-8 at the N-terminal ABD-containing region.

The human erythrocyte plasma membrane: a Rosetta Stone for decoding membrane-cytoskeleton structure

The mammalian erythrocyte, or red blood cell (RBC), is a unique experiment of nature: a cell with no intracellular organelles, nucleus or transcellular cytoskeleton, and a plasma membrane with uniform structure across its entire surface. By virtue of these specialized properties, the RBC membrane has provided a template for discovery of the fundamental actin filament network machine of the membrane skeleton, now known to confer mechanical resilience, anchor membrane proteins, and organize membrane domains in all cells. This chapter provides a historical perspective and critical analysis of the biochemistry, structure, and physiological functions of this actin filament network in RBCs. The core units of this network are nodes of ~35-37 nm-long actin filaments, interconnected by long strands of (α1β1)₂-spectrin tetramers, forming a 2D isotropic lattice with quasi-hexagonal symmetry. Actin filament length and stability is critical for network formation, relying upon filament capping at both ends: tropomodulin-1 at pointed ends and αβ-adducin at barbed ends. Tropomodulin-1 capping is essential for precise filament lengths, and is enhanced by tropomyosin, which binds along the short actin filaments. αβ-adducin capping recruits spectrins to sites near barbed ends, promoting network formation. Accessory proteins, 4.1R and dematin, also promote spectrin binding to actin and, with αβ-adducin, link to membrane proteins, targeting actin nodes to the membrane. Dissection of the molecular organization within the RBC membrane skeleton is one of the paramount achievements of cell biological research in the past century. Future studies will reveal the structure and dynamics of actin filament capping, mechanisms of precise length regulation, and spectrin-actin lattice symmetry.

Biochemical and cell biological analysis of actin in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has long been a useful model organism for muscle research. Its body wall muscle is obliquely striated muscle and exhibits structural similarities with vertebrate striated muscle. Actin is the core component of the muscle thin filaments, which are highly ordered in sarcomeric structures in striated muscle. Genetic studies have identified genes that regulate proper organization and function of actin filaments in C. elegans muscle, and sequence of the worm genome has revealed a number of conserved candidate genes that may regulate actin. To precisely understand the functions of actin-binding proteins, such genetic and genomic studies need to be complemented by biochemical characterization of these actin-binding proteins in vitro. This article describes methods for purification and biochemical characterization of actin from C. elegans. Although rabbit muscle actin is commonly used to characterize actin-binding proteins from many eukaryotic organisms, we detect several quantitative differences between C. elegans actin and rabbit muscle actin, highlighting that use of actin from an appropriate source is important in some cases. Additionally, we describe probes for cell biological analysis of actin in C. elegans.

Phosphoinositide-specific phospholipase C in oat roots: association with the actin cytoskeleton

Phosphoinositide-specific phospholipase C (PI-PLC) activities are involved in mediating plant cell responses to environmental stimuli. Two variants of PI-PLC have been partially purified from the roots of oat seedlings; one cytosolic and one particulate. Although the cytosolic enzyme was significantly purified, the activity still co-migrated with a number of other proteins on heparin HPLC and also on size-exclusion chromatography. The partially purified PI-PLC was tested by Western blotting, and we found that actin and actin-binding proteins, profilin and tropomyosin, co-purified with cytosolic phospholipase C. After a non-ionic detergent (Triton X-100) treatment, PI-PLC activities still remained with the actin cytoskeleton. The effects of phalloidin and F-buffer confirmed this association; these conditions, which favor actin polymerization, decreased the release of PI-PLC from the cytoskeleton. The treatments of latrunculin and G-buffer, the conditions that favor actin depolymerization, increased the release of PI-PLC from the cytoskeleton. These results suggest that oat PI-PLC associates with the actin cytoskeleton.

Dual roles of Gln137 of actin revealed by recombinant human cardiac muscle alpha-actin mutants

The actin filament is quite dynamic in the cell. To determine the relationship between the structure and the dynamic properties of the actin filament, experiments using actin mutants are indispensable. We focused on Gln(137) to understand the relationships between two activities: the conformational changes relevant to the G- to F-actin transition and the activation of actin ATPase upon actin polymerization. To elucidate the function of Gln(137) in these activities, we characterized Gln(137) mutants of human cardiac muscle alpha-actin. Although all of the single mutants, Q137E, Q137K, Q137P, and Q137A, as well as the wild type were expressed by a baculovirus-based system, only Q137A and the wild type were purified to high homogeneity. The CD spectrum of Q137A was similar to that of the wild type, and Q137A showed the typical morphology of negatively stained Q137A F-actin images. However, Q137A had an extremely low critical concentration for polymerization. Furthermore, we found that Q137A polymerized 4-fold faster, cleaved the gamma-phosphate group of bound ATP 4-fold slower, and depolymerized 5-fold slower, as compared with the wild-type rates. These results suggest that Gln(137) plays dual roles in actin polymerization, in both the conformational transition of the actin molecule and the mechanism of ATP hydrolysis.

Reduction of thymosin beta4 and actin in HL60 cells during apoptosis is preceded by a decrease of their mRNAs

Thymosin beta4 (Tbeta4) is the most prominent representative of the beta-thymosins, a family of highly conserved polar 5 kDa peptides. This peptide is now regarded to be the main G-actin sequestering peptide in mammals and therefore plays an important role in organization of the microfilamental system. During apoptosis of cells this microfilamental system is disrupted. Therefore we studied changes in thymosin beta4 and actin content of HL60 cells after induction of apoptosis using cytosine arabinoside (araC). Thymosin beta4 content decreased to about 30% of the control value after incubation for 48 h in the presence of araC. Also the amount of total actin decreased to about half of the control, while total cellular protein remained constant. To further elucidate if the changes of thymosin beta4 and actin content correlate with the gene expression the relative mRNA content of thymosin beta4 and beta-actin was determined using the ribonuclease protection assay (RPA). Already after 24 h the relative amount of mRNA of thymosin beta4 and beta-actin was greatly reduced to 71 and 58%, respectively. Upon a 48 h araC treatment, the mRNA of these two proteins decreased to 15 and 10% compared to the control, whereas the content of total RNA and protein per cell was nearly unchanged. According to our data araC has a significant influence on the transcriptional level of thymosin beta4 and actin.

Cloning and characterization of betaCAP73, a novel regulator of beta-actin assembly

In non-muscle cells, the isoactins are differentially localized, with beta-actin specifically enriched at the cell cortex within motile structures, such as lamellae, while gamma-actin shows no specific localization. To understand the sorting and regulation of beta-actin within moving cells, we previously isolated betaCAP73, a novel beta-actin-specific binding protein (Cell Motil. Cytoskel. 35 (1996) 175). Here, we have cloned and characterized the 4718 nucleotide betaCAP73 cDNA from an endothelial cell library. betaCAP73 cDNA contains six predicted ankyrin-like repeats at the amino terminus and is partially homologous to three previously reported sequences of unknown function. Northern analysis reveals betaCAP73 expression in all tissues tested, with highest levels in skeletal muscle. Consistent with previously demonstrated interactions between native betaCAP73 and beta-actin filament barbed-ends, recombinant betaCAP73 inhibits pyrene-actin assembly in an isoactin-specific manner. Compared to stationary cells betaCAP73 mRNA is down regulated in crawling cells. Similarly, motility-defective cells have increased betaCAP73 protein. Overexpression of full-length betaCAP73 induces the formation of novel membrane protrusions that are enriched in betaCAP73, while overexpression of betaCAP73 domains alters cell morphology. Combined, these results indicate that betaCAP73 modulates isoactin dynamics to regulate the morphological alterations required for cell growth and motility.

The role of ATP, ADP and divalent cations in the formation of binary and ternary complexes of actin, cofilin and DNase I

Actin is the major cytoskeletal protein of virtually all eukaryotic cells. Actin assembly/disassembly is involved in a variety of cellular processes and actin-binding proteins are essential in regulation of the pool of actin monomers. Cofilin and DNase I are actin-binding proteins, which form both binary (actin-DNase 1, cofilin-actin) and ternary (cofilin-actin-DNase I) complexes with actin. Here we use native gel electrophoresis to examine the roles of ATP, ADP, Ca2+ and Mg2+ in the formation of these complexes as well as on the ability of actin to self-assemble. Conditions which favour actin polymerisation are: ATP (no Me2+) > or = ADP (no Me2+) > ADP-Ca2+ = ADP-Mg2+ > ATP-Mg2+ > ATP-Ca2+. Preferential conditions for the formation of the binary actin-cofilin complex are: ADP-Mg2+ > or = ADP-Ca2+ >> ATP-Ca2+ approximately equals ATP-Mg2+ approximately equals ADP-No Me2+ approximately equals ATP-No Me2+. Actin forms a very tight complex with DNase I in the order: ATP-Ca2+ > or = ATP-Mg2+ approximately equals ADP-Mg2+ approximately equals ADP-Ca2+ > or = ADP-(no Me2+) > ATP-(no Me2+). Effectively, the complex does not form in the presence of ATP and the absence of free Me2+. Finally, the conditions which favour the formation of a ternary complex of cofilin-actin-DNase I resemble the actin-DNase I, namely: ATP-Ca2+ approximately equals ADP-Ca2+ approximately equals ADP-Mg2+ approximately equals ATPMg2+ ADP (no Me2+) > ATP-(no Me2+).

Purification and biochemical characterization of actin from Caenorhabditis elegans: its difference from rabbit muscle actin in the interaction with nematode ADF/cofilin

Biochemical analysis of cytoskeletal proteins of the nematode Caenorhabditis elegans can be combined with a vast resource of genetic information in order to understand the regulation and function of the cytoskeleton in vivo. Here, I report an improved and efficient method to purify actin from wild-type C. elegans and characterization of its biochemical properties. The purified actin was highly pure and free of several known actin-binding proteins. G-actin was polymerized into F-actin in a similar kinetic process to rabbit muscle actin. G-actin interacted with bovine DNase I and inhibited its activity. However, UNC-60B, an isoform of ADF/cofilin in C. elegans, showed a marked depolymerizing activity on C. elegans actin but not on rabbit muscle actin. The results indicate that C. elegans actin shares common biochemical properties with rabbit muscle actin, while actin-binding proteins can interact with C. elegans actin in a distinct manner from rabbit muscle actin.

Self-regulated polymerization of the actin-related protein Arp1

The actin-related protein Arp1 (or centractin, actin RPV) is the major subunit of dynactin, a key component of the cytoplasmic dynein motor machinery [1] [2] [3]. Of the ubiquitously expressed members of the Arp superfamily, Arp1 is most similar to conventional actin [4] [5] [6] and, on the basis of conserved sequence features, is predicted to bind ATP and possibly polymerize. In vivo, all cytosolic Arp1 sediments at 20S [7] suggesting that it assembles into oligomers, most likely dynactin - a multiprotein complex known to contain eight or nine Arp1 monomers in a 37 nm filament [8]. The uniform length of Arp1 polymers suggests a novel assembly mechanism that may be governed by a 'ruler' activity. In dynactin, the Arp1 filament is bounded by actin-capping protein at one end and a heterotetrameric protein complex containing the p62 subunit (D.M. Eckley, S.R. Gill, J.B.B., J.E. Heuser, T.A.S., unpublished observations) at the other [8]. In the present study, we analyzed the behavior of highly purified, native Arp1. Arp1 was found to polymerize rapidly into short filaments that were similar, but not identical, in length to those in dynactin. With time, these filaments appeared to anneal to form longer assemblies but never attained the length of conventional actin filaments.

In vitro and in vivo evidence for actin association of the naphthylphthalamic acid-binding protein from zucchini hypocotyls

The N-1-naphthylphthalamic acid (NPA)-binding protein is part of the auxin efflux carrier, the protein complex that controls polar auxin transport in plant tissues. This study tested the hypothesis that the NPA-binding protein (NBP) is associated with the actin cytoskeleton in vitro and that an intact actin cytoskeleton is required for polar auxin transport in vivo. Cytoskeletal polymerization was altered in extracts of zucchini hypocotyls with reagents that stabilized either the polymeric or monomeric forms of actin or tubulin. Phalloidin treatment altered actin polymerization, as demonstrated by immunoblot analyses following native and denaturing electrophoresis. Phalloidin increased both filamentous actin (F-actin) and NPA-binding activity, while cytochalasin D and Tris decreased both F-actin and NPA-binding activity in cytoskeletal pellets. The microtubule stabilizing drug taxol increased pelletable tubulin, but did not alter either the amount of pelletable actin or NPA-binding activity. Treatment of etiolated zucchini hypocotyls with cytochalasin D decreased the amount of auxin transport and its regulation by NPA. These experimental results are consistent with an in vitro actin cytoskeletal association of the NPA-binding protein and with the requirement of an intact actin cytoskeleton for maximal polar auxin transport in vivo.

Rapid and efficient purification of actin from nonmuscle sources

The need for biochemical quantities of nonmuscle actin has been increased by observations that actin isoform composition of a cell influences the cell's motile and structural properties. In addition, the number of actin binding proteins that exhibit different binding interactions with beta- and gamma-actin compared to alpha-actin from skeletal muscle is growing. We report a procedure designed to purify actin from nonmuscle tissues employing extraction of monomeric actin from tissues with high concentrations of Tris, chromatography on DE-53 cellulose, and affinity chromatography of DNase I-agarose. The preparation is easy to perform and yields quantities of nonmuscle actin sufficient for biochemical and cell biological assays. Actin from bovine erythrocytes and from brains of adult and embryonic chickens was obtained using this method, which can be readily used with other sources of tissue. Coomassie-Blue-stained SDS gels of the purified actin show no contaminants; capping protein, a common contaminant of actin preparations, is absent by immunoblotting. This method for purifying nonmuscle actin will be useful to investigate functional differences in the biology of actin isoforms or their regulating proteins.