F-actin filament

The diversity of these subcellular actin structures is remarkable and appears to be determined by the interactions of many actin-binding proteins (ABPs) as well as by changes in the concentrations of intracellular signaling molecules such as Ca and cAMP, by small GTP-binding proteins, and by signals arising from mechanical stress. Approximately 50% of the actin molecules in most animal cells are unpolymerized subunits in the cytosolic pool and exist in a state of dynamic equilibrium with labile F-actin filamentous structures (i.e., new structures are formed while existing structures are renewed) (Hall, 1994). 

Blood platelets are key players in the blood-clotting mechanism. These tiny fragments of cytoplasm are shed into the circulation from the surface of megakaryocytes located in the bone marrow. When the lining of a blood vessel is injured, activated platelets release clotting factors, adhere to each other and to damaged surfaces, and send out numerous filopodia. The shape changes that occur in activated platelets are the result of actin polymerization. Before activation, there are no microfilaments because profilin binds to G-actin and prevents its polymerization. After activation, profilin dissociates from G-actin, and bundles and networks of F-actin filaments rapidly appear within the platelet. 

Polymer growth J(c) showed nonlinear monomer concentration dependence in the presence of ATP (Carrier et al., 1984), while in the presence of ADP, the plot of J(c) versus monomer concentration for actin was a straight line, as expected for reversible polymerization. The data imply that newly incorporated subunits dissociate from the filament at a slower rate than internal ADP-subunits in other words, (a) the effect of nucleotide hydrolysis is to decrease the stability of the polymer by increasing k and (b) nucleotide hydrolysis is uncoupled from polymerization and occurs in a step that follows incorporation of a ATP-subunit in the polymer. Newly incorporated, slowly dissociating, terminal ATP-subunits form a stable cap at the ends of F-actin filaments. 

The kinetics of F-actin-Si assembly from G-actin and Si via nucleation of actin filaments, followed by Si binding are not observed in a low ionic strength medium. Instead, the mechanism involves condensation of high affinity (G-actin)2 S complexes rapidly preformed in solution. Assembly of F-actin-Si in the presence of Si > G-actin is a quasi-irreversible process. This mechanism is therefore different from that involving the assembly of F-actin filaments, which is characterized by the initial, energetically unfavorable formation of a small number of nuclei representing a minute fraction of the population of actin molecules, followed by endwise elongation from G-actin subunits. 

Yanagida, T., Nakase, M., Nishiyama, K., Oosawa, F. (1984). Direct observation of motion of single F-actin filaments in the presence of myosin. Nature 307. 58-60. 

How can hydrolysis of ATP produce macroscopic movement Muscle contraction essentially consists of the cychc attachment and detachment of the S-1 head of myosin to the F-actin filaments. This process can also be referred to as the making and breaking of cross-bridges. The attachment of actin to myosin is followed by conformational changes which are of particular importance in the S-1 head and are dependent upon which nucleotide is present (ADP or ATP). These changes result... 

G-actin is present in most if not all cells of the body. With appropriate concentrations of magnesium and potassium chloride, it spontaneously polymerizes to form double helical F-actin filaments like those seen in muscle. There are at least two types of actin in nonmus-... 

Abbreviations CNT, Carbonnanotube F-actin,Filamentous actin TNT, Tunneling nanotube... 

Actin, the most abundant protein in eukaryotic cells, is the protein component of the microfilaments (actin filaments). Actin occurs in two forms—a monomolecular form (C actin, globular actin) and a polymer (F actin, filamentous actin). G actin is an asymmetrical molecule with a mass of 42 kDa, consisting of two domains. As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical homopolymer. G actin carries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form ADR Actin therefore also has enzyme properties (ATPase activity). 

Two early reports introduced the first biological demonstrations of hydrophilic QDs [12, 13], In the first demonstration, Bruchez et al. used the ubiquitous biotin-avidin chemistry to label cellular F-actin filaments [12], In the second demonstration, Chan and Nie reported attachment of transferrin to... 

The two ends of the F-actin filaments have different surfaces of the monomer exposed and grow at different rates. This has been demonstrated by allowing the myosin fragment called heavy meromyosin (HMM see Fig. 19-10) to bind to (or "decorate") an actin filament. The... 

Figure 19-12 (A) Stereoscopic views of computer-assisted reconstructions of images of myosin heads attached to an F-actin filament centered between two thick filaments. Atomic structures of actin (Fig. 7-10) and of myosin heads (Fig. 19-15) have been built into the reconstructed images obtained by electron microscopy. (A) With the nonhydrolyzable ATP analog ATPPNP bound in the active sites. (B) Rigor. Two myosin heads are apparently bound to a single actin filament in (A). If they belong to the same myosin molecule the two C-terminal ends must be pulled together from the location shown here. In (B) a third head is attached, presumably from another myosin rod. This configuration is often seen in rigor. From Winkler et al.13i Courtesy of K. A. Taylor.

Figure 1. The muscle dystrophin-glycoprotein complex. The dystrophin-glycoprotein complex normally spans the plasma membrane of the skeletal muscle cell and may stabilize the sarcolemma and cytoskeleton to allow force transduction between the intracellular cytoskeleton (F-actin filaments) and the extracellular matrix. The molecules indicated are core components of the dystrophin-glycoprotein complex. Laminin 2 is the predominant laminin isoform in skeletal muscle basement membranes. Modified from McNeil and Steinhardt (2003)...

Janmey, P.A., Tang, J.X., and Schmidt, C.F. Actin Filaments, In Biophysics Textbook, On-Line (V. Bloomeld, Ed.), Sponsored by the Biophysical Society (1999), http //www.biophysics. org/education/ janmey.pdf. 

Freezing and storage after addition of sodium glutamate decreased the rate of denaturation. The solubility did not decrease and the F-actin filaments kept their fine structures during frozen storage (Figure 6). 

Figure 6. Electron micrographs of F-actin filaments of carp before and after frozen storage in 0.05M KCl at —20°C. A, B and C, no additives D, E and F, 0.2M sodium glutamate added. A and D, before freezing B and E, after 1 week of frozen storage C and F, after 4 weeks of frozen storage. Negatively stained...

Nature produces the stiffest worm-like chains the long persistence length of DNA (Xp = 0.053 ixm) is short compared to that of F-actin filaments, Xp 17 /xm (Ott et al. 1993) the latter are components of the cellular cytoskeleton. Self-assembled cellular microtubules are even stiffer than this, with Xp on the order of millimeters ... 

Bruce Goode and coworkers started to reveal the nature of this partnership when they found that Cm Ip bound Arp2/3 via its coiled-coil and recruited it to the sides of pre-existing F-actin filaments. In the absence of pre-existir filaments they found, however, that Crnlp inhibited the actin-nucleating activity of the complex. Therefore, they proposed that coronin inhibits Arp2/3-mediated polymerization in the cytoplasm, but promotes nucleation and branching at the cell cortex. ... 

Research into the control of glycolytic rate in muscle has revealed that enzyme activity may also be controlled by reversible formation of enzyme-F-actin complexes. F-actin is a polymer of actin molecules and makes up one of the two muscle filaments that participate in muscle contraction. It can be shown that enzymes, such as PK, readily bind to F-actin filaments under conditions of low ionic strength in vitro (Chan et al., 1986). Extrapolation of the conditions in the test tube to conditions found in cells suggests that a significant proportion of PK may be bound in vivo (Brooks and Storey, 1991 a). In the case of PK, binding decreases the enzyme activity by increasing the Km value for PEP. F-actin, therefore, acts like ATP and alanine in allosterically inhibiting the enzyme. 

Actin exists as a globular monomer called G-actin and as a filamentous polymer called F-actin, which is a linear chain of G-actin subunits. (The microfilaments visualized in a cell by electron microscopy are F-actin filaments plus any bound proteins.) Each actin molecule contains a Mg " ion complexed with either ATP or ADR Thus there are four states of actin ATP-G-actin, ADP-G-actin, ATP-F-actin, and ADP-F-actin. Two of these forms, ATP-G-actin and ADP-F-actin, predominate in a cell. The importance of the interconversion between the ATP and the ADP forms of actin in the assembly of the cytoskeleton is discussed later. 

A FIGURE 19-3 Structures of monomeric G-actin and F-actin filament, (a) Model of a p-actin monomer from a nonmuscle cell shows It to be a platellke molecule (measuring... 

The addition of ions—Mg, K, or Na —to a solution of G-actin will Induce the polymerization of G-actin into F-actin filaments. The process is also reversible F-actln depolymerlzes into G-actln when the ionic strength of the solution is lowered. The F-actln filaments that form in vitro are Indistinguishable from mlcrofllaments Isolated from cells, indicating that other factors such as accessory proteins are not required for polymerization in vivo. The assembly of G-actln into F-actln is accompanied by the hydrolysis of ATP to ADP and Pc however, as discussed later, ATP hydrolysis affects the kinetics of polymerization but is not necessary for polymerization to take place. 

As is evident from viscometry and electron microscopy, these so-called "modulator" proteins inhibit G-actin polymerization and also depolymerize assembled F-actin. The modulator proteins isolated from stomach and aorta required Ca + for their activity and formed a 1 2 complex with actin they apparently could interact along the length of F-actin filaments. In contrast, activity of the gizzard protein(s) was independent of the Ca2+ concentration and appeared to... 

Marriott and Heidecker reported a Cys-caged heavy meromyosin (HMM) using DMNB-Br and evaluated the capacity of photoactivated HMM to couple the energy of calcium/actin-activated ATP hydrolysis to the movement of F-actin filaments in an in vitro motility assay [29, 86], It was known from labeling studies with the thiol-reactive fluorophore tetramethylrhodamine... 

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