Complexes of actin

Miller CA, Cohen MD, Costa M. 1991. Complexing of actin and other nuclear proteins to DNA by cis-diamminedichloroplatinum(II) and chromium compounds. Carcinogenesis 12(2) 269-276. 

A EXPERIMENTAL FIGURE 19-6 Polymerization of G-actin in vitro occurs in three phases, (a) In the initial nucleation phase, ATP-G-actin monomers (pink) slowly form stable complexes of actin (purple). These nuclei are rapidly elongated in the second phase by the addition of subunits to both ends of the filament. 

When actin and myosin have once combined to give actomyosin, it is not possible by any known method to separate them completely on a preparative scale. There is no doubt, however, that natural actomyosin is really a complex of actin and mj osin, (a) because Straub (1942) obtained in small yield from actomyosin the same actin as obtained from the dry acetone powder of muscle, and (b) natural and artificial actomyosins react with ATP in the same typical manner (Section III, 5d). It can therefore be concluded that complex formation is thermodynamically irreversible, for by repeated fractional precipitation a preparation can be obtained from muscle extracts in which no free L-myosin can be detected by methods at present available. The ultracentrifugal peak of L-myosin reappears, however, when the actomyosin in solution by its history and its properties, e.g., disappearance of ATP-sensitivity, may be regarded as denatured (Portzehl et al., 1950 see also Johnson and Landolt, 1950). 

In a pioneering paper on the cytochalasan molecular mode of action, Spudich and Lin discovered a decrease in viscosity of actomyosin from rabbit muscle - the active protein complex of actin and myosin - when treated with cytochalasin B (1088) in micromolar concentrations. Actin was identified as direct binding partner of cytochalasin B (1088), thus the first proof on the target of this compound was provided (705). Further studies by several groups revealed that cytochalasins B (1088) and D (1091) inhibit, but not completely arrest, actin filament elongation (708, 709). A plausible mechanism was proposed by Goddette and Frieden (710). 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

Bahar et al. [46] have used this kind of approach to predict the B-factors of 12 X-ray structures. Elements in the Hessian corresponding to atom pairs separated by a distance of less than 7 A are set to zero, and the remainder have the same value dependent on a single adjustable parameter. Generally B-factor predictions for the a-carbons compare very well with the B-factors measured by X-ray crystallography. Figure 1 shows the result for the subunit A of endodeoxyribonuclease I complexed with actin. 

However, release of ADP and P from myosin is much slower. Actin activates myosin ATPase activity by stimulating the release of P and then ADP. Product release is followed by the binding of a new ATP to the actomyosin complex, which causes actomyosin to dissociate into free actin and myosin. The cycle of ATP hydrolysis then repeats, as shown in Figure 17.23a. The crucial point of this model is that ATP hydrolysis and the association and dissociation of actin and myosin are coupled. It is this coupling that enables ATP hydrolysis to power muscle contraction. 

More than 50 proteins have been discovered in the cytosol of nonmuscle cells that bind to actin and affect the assembly and disassembly of actin filaments or the cross-linking of actin filaments with each other, with other filamentous components of the cytoskeleton, or with the plasma membrane. Collectively, these are known as actin-binding proteins (ABPs). Their mechanisms of actions are complex and are subject to regulation by specific binding affinities to actin and other molecules, cooperation or competition with other ABPs, local changes in the concentrations of ions in the cytosol, and physical forces (Way and Weeds, 1990). Classifications of ABPs have been proposed that are based on their site of binding to actin and on their molecular structure and function (Pollard and Cooper, 1986 Herrmann, 1989 Pollard et al., 1994). These include the following ... 

In the three-dimensional stmcture of actin, the environment of the phosphate moiety of the nucleotide appears roughly the same when CaADP or CaATP is bound. This observation argues against two different conformations. The reason why this is so is unclear. However, it must be stressed that the three-dimensional stmcture is derived from X-ray diffraction of crystals of the DNasel-actin complex, which, like G-actin, is unable to hydrolyze ATP. The conformation obtained may therefore correspond to G-actin frozen in the G-ATP state independently of the bound nucleotide. Stmctural studies in conjunction with site-directed mutagenesis experiments should eventually solve this problem. 

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. 

In addition to its effects on enzymes and ion transport, Ca /calmodulin regulates the activity of many structural elements in cells. These include the actin-myosin complex of smooth muscle, which is under (3-adrenergic control, and various microfilament-medi-ated processes in noncontractile cells, including cell motility, cell conformation changes, mitosis, granule release, and endocytosis. 

Winter, D., Podtelejnikiov, A. V., Mann, M., and Li, R. (1997). The complex containing actin-related proteins Arp2 and Arp3 is required for the motility and integrity of yeast actin patches. Curr. Biol. 7, 519-529. 

In addition to actin and myosin, other proteins are found in the two sets of filaments. Tropomyosin and a complex of three subunits collectively called troponin are present in the thin filaments and play an important role in the regulation of muscle contraction. Although the proteins constituting the M and the Z bands have not been fully characterized, they include a-actinin and desmin as well as the enzyme creatine kinase, together with other proteins. A continuous elastic network of proteins, such as connectin, surround the actin and myosin filaments, providing muscle with a parallel passive elastic element. Actin forms the backbone of the thin filaments [4]. The thin... 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

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