Actin

Straub, a young biochemist in Szent-Györgyi's laboratory, discovered actin in 1942. Previously Szent-Györgyi has shown that brief extraction of minced rabbit muscle with an alkaline 0.6 M KCl solution in the cold room yields a myosin with low viscosity (myosin A), whereas when the muscle mince was left in the 0.6 M KCl for a day a very viscous myosin solution was extracted (myosin B). Straub thought that the difference between myosin B and A is caused by the extraction of a new protein that makes the one-day extract viscous. Accordingly, he extracted myosin A from the muscle, then left the residue in the cold room for a day. The muscle residue was washed with distilled water to remove the KCl and remaining cytoplasmic proteins, and finally the residue was dried with acetone. The protein extracted from the acetone-dried residue formed a very viscous complex with myosin A, that was similar to myosin B, "it activated myosin" and hence it was named actin. In skeletal muscle, actin comprises about 15% of the total protein.

The two forms of actin: After an improved procedure for actin preparation, Straub has found that water extraction of the acetone-dried muscle residue yielded an actin solution with low viscosity, monomeric or globular (G) actin, that upon addition of salts (at physiological concentrations) polymerized to a highly viscous gel, filamentous or fibrous (F) actin (Fig. A).

Fig. A. illustration of the two forms of actin (Courtesy of Dr. Helen Rarick).

Straub followed the polymerization of actin by viscometry, shown in Fig. A1.

 

Fig. A1. Polymerization of actin in the presence of various ions. Curve 1) 110 mM NaCl, 3 mM KCl, 3 mM CaCl2, and 10 mM MgSO4; Curve 2) same as 1 but without Mg2+; Curve 3) same as 1 but without Ca2+; Curve 4) same as 1 but without K+. Temperature 24o C (From Feuer et al., 1948).

Ionic strength, temperature, and pH affect polymerization. Optimal conditions are: 0.1 M salts concentration, 37o C, and pH 6.5-7.5.

In 1950 Straub and Feuer reported that G-actin contains bound ATP and during polymerization of actin the ATP is hydrolyzed to bound ADP and Pi. Straub postulated that the transformation of G-actin-ATP to F-actin-ADP plays a role in muscle contraction, but this was not found in skeletal muscle of live animals (Martonosi et al., 1960). However, the transformation of G-actin-ATP to F-actin-ADP was found in intact smooth muscle recently (Bárány et al., 2001), (see Smooth Muscle chapter). Furthermore, actin polymerization with concomitant ATP hydrolysis takes place in non-muscle cells and provides the mechanochemistry for motility (see Cell Motility chapter).

Electron micrograph of fibrous actin filaments reveals that the structure consists of twin strings of actin globules wound around each other in a double helix. The subunit repeat is about 55 Å and the helical repeat is about 370 Å.

Fig. A2. Electron micrograph of actin filament (From Huxley, H.E., 1972). With permission from The Structure and Function of Muscle (G.H. Bourne, Ed.),1972, Academic Press.

Janmey et al. (1999) reviewed the characteristic properties of actin filaments.

Actin-myosin binding: F-actin combines with myosin to form actomyosin. In 0.6 M KCl actomyosin forms a viscous solution; upon addition of ATP, actomyosin dissociates into its components actin and myosin, with accompanying reduction of the viscosity. At physiological ionic strength actomyosin is insoluble, the same way as in the muscle; under these conditions actin activates the myosin ATPase 50-100-fold.

F-actin also combines with the proteolytic fragments of myosin, HMM or S1. The complex formed actoheavymeromyosin or actosubfragment 1 remains soluble at low ionic strength. When HMM or S1 is added to muscle thin filament it attaches to the actin component of the filament, forming a specific "arrow head" structure (Fig. A3). This suggests a structural polarity for the thin filament.

Fig. A3. Electron micrograph of thin filament decorated with HMM. (From. Huxley, H.E., 1972). With permission from The Structure and Function of Muscle (G.H. Bourne, Ed.),1972, Academic Press.

 Based on this observation, H. E. Huxley postulated that the structural polarity of thin and thick filaments allows the sliding force to move the thin filaments toward the center of the sarcomere (Fig. A4).

Fig. A4. Diagram for the structural polarity of thin and thick filaments (From Huxley, H.E., 1972). With permission from The Structure and Function of Muscle (G.H. Bourne, Ed.),1972, Academic Press.

 
Three-dimensional structure of actin: Kabsch and collaborators (1990) were the first to crystallize G-actin and determined its structure (Fig. A5).

 

Fig. A5. Scheme for the structure of actin. (Reprinted from Current Opinion in Structural Biology, vol. 1, Holmes and Kabsch, Muscle Proteins:actin, pp. 270-280, 1991, with permission from Elsevier Science).

Folding of the actin molecule is represented by ribbon tracing of the a-carbon atoms. N and C correspond to the amino- and carboxyl-terminals, respectively. The letters followed by numbers represent amino acids in the polypeptide chain. A hypothetical vertical line divides the actin molecule into two domains "large", left side, and "small", right side. ATP and Ca2+ are located between the two domains. These two domains can be subdivided further into two subdomains each, the small domain being composed of subdomains 1 and 2, and the large domain of subdomains 3 and 4. (Subdomain 2 has significantly less mass than the other three subdomains and this is the reason of dividing actin into small and large domains). The four subdomains are held together and stabilized mainly by salt bridges and hydrogen bonds to the phosphate groups of the bound ATP and to its associated Ca2+ localized in the center of the molecule. Because of the less mass in subdomain 2, the actin molecule is distinctly polar in the direction from subdomains 1 and 3, called the "barbed end", toward subdomains 2 and 4, called the "pointed end". This polarity defines the orientation of the actin molecule in the myosin HMM decoration pattern of the thin filament, shown in Fig. A3.

The intersubunit contacts in the F-actin filament: In helical structures, such as the F-actin filament, two types of intersubunit contacts are possible in principle: those along and those between the long-pitch helical strands. In the atomic model of the F-actin filament, 24 amino acid residues per subunit are involved in contacts along the long-pitch helical strands. By contrast, only 15 residues per subunit mediate the weaker contacts between the two strands.

Modification of the actin structure: Modified actin proteins, generated by mutation, or by blocking chemically reactive amino acid side chains (mainly cysteine) may provide new aspects of the actin-actin interaction and the interaction of actin with myosin or with tropomyosin-troponin. Interestingly, relocation of charged residues in subdomain 1 of actin (Wong et al., 1999) or placing a fluorescence probe on Cys10, located in subdomain 1 of actin, (Eli-Berchoer et al., 2000), had no effect on the functional properties of actin. On the other hand, mutations in actin subdomain 2 or subdomain 3 impaired thin filament regulation by troponin and tropomyosin (Korman and Tobacman, 1999; Korman et al., 2000). It was established that residue Glu93 in actin, located near to the junction of subdomain 1 and subdomain 2, is involved in myosin binding (Razzaq et al., 1999). Fluorescence resonance energy transfer detected a conformational change around Cys374, at the C-terminal of actin, when F-actin combined with myosin (Moens and dos Remedios, 1997).

Localization of actin in the structure of muscle: Under the microscope, myosin extracted myofibrils exhibit the thin filaments, attached to the Z line. When 0.6 M KI solution, that dissolves F-actin, is added to such a myofibrillar ghost the structure disappears, indicating that the thin filaments are composed of actin. In the structure of muscle, the I band contains thin filaments whereas the A band contains both thick and thin filaments.

Structure of the thin filament: Fig. A6 shows the structure: actin molecules form two strings wound around each other, in the grove is the tropomyosin strand and at regular intervals troponin molecules are attached to tropomyosin.

Fig. A6. Model for the structure of the thin filament (From Huxley, H.E., 1972).With permission from The Structure and Function of Muscle (G.H. Bourne, Ed.),1972, Academic Press.

References

Bárány, M., Barron, J.T., Gu, L., and Bárány, K. (2001). Exchange of the actin-bound nucleotide in intact arterial smooth muscle. J. Biol. Chem., 276, 48398-48403.

Eli-Berchoer, L., Reisler, E., and Muhlrad, A. (2000). Structural implications of the chemical modification of Cys10 on actin. Biophys. J., 78, 1482-1489.

Feuer, G, Molnár, F., Pettkó, E. and Straub, F.B. (1948). Studies on the composition and polymerization of actin. Hungarica Acta Physiologica, 1, 150-163.

Holmes, K.C. and Kabsch, W. (1991). Muscle proteins: actin. Current Opinion in Structural Biology, 1, 270-280.

Huxley, H.E. (1972). Molecular basis for contraction in cross-striated muscles. In The structure and function of muscle, ( G.H. Bourne, Ed.) second edition, vol. I, Part 1, pp. 301-387.

Janmey, P.A., Tang, J.X., and Schmidt, C.F. (1999). Actin filaments. In Supramolecular Assemblies, (V. Bloomfield, Ed.), Biophysical Society Homepage.

Kabsch, W., Mannherz, E.G., Suck, D., Pai, E.F., and Holmes, K.C. (1990). Atomic structure of the actin:DNase I complex. Nature, 347, 37-44.

Korman, V.L., and Tobacman, L.S. (1999). Mutations in actin subdomain 3 that impairs thin filament regulation by troponin and tropomyoxin. J. Biol. Chem. 274, 22191-22196.

Korman, V.L., Hatch, V., Dixon, K.Y., Craig, R., Lehman, W. and Tobacman, L.S. (2000). An actin subdomain 2 mutation that impair thin filament regulation by troponin and tropomyoxin. J. Biol. Chem. 275, 22470-22478.

Martonosi, A., Gouvea, M.A., and Gergely, J. (1960). Studies on actin, III. G-F transformation of actin and muscular contraction (experiments in vivo). J. Biol. Chem. 235, 1707-1710.

Moens, P.D.J. and des Remedios, C.G. (1997). A conformational change in F-actin when myosin binds: fluorescence resonance energy transfer detects an increase in the radial coordinate of Cys-374. Biochemistry, 36, 7353-7360.

Razzaq, A., Schmitz, S., Veigel, C., Molloy, J. E., Geeves, M.A., and Sparrow, J.C. (1999). Actin residue Glu93 is identified as an amino acid affecting myosin binding. J. Biol. Chem. 274, 28321-28328.

Straub, F.B. and Feuer, G. (1950). Adenosinetriphosphate the functional group of actin. Biochim. Biophys. Acta. 4, 455-470.

Wong, W.W., Doyle, T.C., and Reisler, E. (1999). Nonspecific weak actomyosin interactions: relocation of charged residues in subdomain 1 of actin does not alter actomyosin function. Biochemistry, 38, 1365-1370.


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