Actin decoration

Schroder R R, Jahn W, Manstein D, Hoimes K C and Spudich J A 1993 Three-dimensionai atomic modei of F-actin decorated with Dictyostelium myosin SI Nature 364 171-4... 

Schroeder, R.R., et. al. Three-dimensional atomic model of F-actin decorated with Dictyostelium myosin SI. Nature 364 171-174, 1993. 

Schroder, R.R., Manstein, D.J., Jahn, W., Holden, H., Rayment, L, Holmes, K.C., Spudich, J.A. (1993). Three-dimensional atomic model of F-actin decorated with Dictyostelium myosin SI. Nature 364, 171-174. 

Figure Bl.17.6. A protein complex (myosin SI decorated filamentous actin) embedded in a vitrified ice layer. Shown is a defociis series at (a) 580 mn, (b) 1130 mn, (c) 1700 mn and (d) 2600 mn underfocus. The pictures result from averagmg about 100 individual images from one electron micrograph the decorated filament length shown is 76.8 nm.

The myosin head has long been shown to induce, even in low ionic strength buffers, polymerization of G-actin into decorated F-actin-S i filaments that exhibit the classical arrowhead structure (Miller et al., 1988 and older references therein). However, to date, the molecular mechanism of this polymerization process remains unknown. 

S-1 (molecular mass approximately 115 kDa) does exhibit ATPase activity, binds L chains, and in the absence of ATP will bind to and decorate actin with arrowheads (Figure 49-5). Both S-1 and HMM exhibit ATPase activity, which is accelerated 100- to 200-fold by complexing with F-actin. As discussed below, F-actin greatly enhances the rate at which myosin ATPase releases its products, ADP and Pj. Thus, although F-actin does not affect the hydrolysis step per se, its ability to promote release of the products produced by the ATPase activity greatly accelerates the overall rate of catalysis. 

Figure 49-5. The decoration of actin fiiaments with the S-1 fragments of myosin to form "arrowheads."...

The diffraction data were also used to guide the selection of the best preserved e.m. images of decorated actin which were then used for a 3-D reconstruction (Amos et al., 1982). In this work, it was suggested that a myosin head interacts with two actin monomers (while still retaining a 1 1 stoichiometry), but this point has not been proved definitively. 

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... 

Moore, P. B., Huxley, H. E., and De Rosier, D. J. (1970). Three-dimensional reconstruction of F-actin, thin filaments and decorated thin filaments. /. Mol. Biol. 50, 279-295. 

When compared with post-rigor or pre-powerstroke states the structural effects of cleft closure appear to include the movement of SW1, which opens the nucleotide-binding pocket, together with a twist of the central /Lsheet, which is associated with a large movement of the P-loop that considerably modifies the nucleotide binding site. Partial closure of the actin-binding cleft and a very similar twisting of the /3-sheet were also seen in the nucleotide-free structure of Dictyostelium myosin II reported by Reubold et al. (2003). The myosin V atomic model can be fitted without deformation into the electron microscope three-dimensional (3D) reconstruction of decorated actin (Holmes et al., 2004). For this and other... 

The protein myosin (Fig. 5-32) interacts specifically with each actin molecule in a filament and, as a result, the filament becomes decorated with a pattern of arrow-heads all pointing the same way. Because of this pattern, one end of the filament is known as the pointed end while the other is called the barbed end. 

Without the atomic resolution afforded by x-ray crystallography, the cleft in an actin subunit and therefore the polarity of a filament are not detectable. However, the polarity of actin filaments can be demonstrated by electron microscopy in decoration experiments, which exploit the ability of myosin to bind specifically to actin filaments. In this type of experiment, an excess of myosin SI, the globular head domain of myosin, is mixed with actin filaments and binding is permitted to take place. Myosin attaches to the sides of a filament with a slight tilt. When all the actin subunits are bound by myosin, the filament appears coated ( decorated ) with arrowheads that all point toward one end of the filament (Figure 19-4). Because... 

A EXPERIMENTAL FIGURE 19-4 Decoration demonstrates the polarity of an actin filament. Myosin SI head domains bind to actin subunits in a particuiar orientation. When bound to aii the subunits in a fiiament, SI appears to spirai around the fiiament. This coating of myosin heads produces a series of arrowhead-iike decorations, most easiiy seen at the wide views of the fiiament. The poiarity in decoration defines a pointed (-) end and a barbed (-f) end the former corresponds to the top of the model in Figure 19-3c. [Courtesy of R. Craig.]... 

A EXPERIMENTAL FIGURE 19-8 Myosin decoration and capping proteins demonstrate unequal growth rates at the two ends of an actin filament, (a) When short myosindecorated filaments are the nuclei for actin polymerization, the resulting elongated filaments have a much longer undecorated (-t) end than (-) end. This result indicates that G-actin monomers are added much faster at the (-t) end than at the (-) end. 

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