Crystal structures, myosin

Knll, F. J., Sablin, E. P, Lan, R., et al., 1996. Crystal structure of die kinesin motor domain reveals a structural similarity to myosin. Nature 380 550-555. 

The crystal structure of myosin S-1 shows how the three subdomains (20K, 50K and 27K) of the myosin heavy chain (produced by further enzymatic digestion of... 

Figure 14. Crystal structures of actin and myosin subfragment-1, shown to the same scale.

Structural information on autoinhibition is available for the twitchin kinase. The twitchin kinase is a Ser/Thr-specific protein kinase of the nematode Caenorhabditis ele-gans and is homologous to the myosin light chain kinase of mammals (see 7.4.1). The crystal structure of a catalytic fragment of twitchin kinase (Hu et al., 1994) has an auto-inhibitory element at the C-terminus, which makes specific contact with parts of the active site and the ATP binding site. The active site of twitchin kinase is blocked by the autoinhibitory structural element by ... 

The crystal structures of the complexes of myosin-Mg2+ with ADP and the two alleged substrate analogues AMP-PNP and ATPyS are all very similar, suggesting that the latter do not induce the ATP-bound state.89 AMP-PNP and ATPyS are not good substrate mimics when there are conformational transitions that depend crucially on the differences between the different nucleotides that are bound. 

Fig. 15. Stereo views of the different myosin head, SI, structures showing their variable conformations in different crystal structures. (A) The heads with their motor domains superimposed and oriented as if interacting with a vertical actin filaments in the rigor conformation, Z-band bottom and M-band top. (B) The same structures in a view down the actin filament long axis, looking from the M-band towards the Z-band. Blue is the Dominguez et al. (1998) structure of SI in chicken smooth muscle with ADP.AIF4 bound, orange is the insect flight muscle SI in the ADP.Pj state (Al-Khayat et al., 2003), yellow is scallop SI crystal structure in the ADP.VO4 state (Houdusse et al., 1999), and green is the chicken skeletal muscle with no nucleotide bound (Rayment et al., 1993a).

Dominguez, R., Freyzon, Y., Trybus, K. M., and Cohen, C. (1998). Crystal structure of a vertebrate smooth muscle myosin motor domain and its complex with the essential light chain Visualization of the pre-power stroke state. Cell 94, 559-571. 

The events associated with SW2 closure followed by ATP hydrolysis have been described above. The idea originally explored in 1999 (Geeves and Holmes, 1999), that SW2 must close onto the yP of ATP before ATP hydrolysis takes place, has been elegantly demonstrated for both Dic-tyostelium myosin II and rabbit fast muscle myosin II (Malnasi-Gsizmadia et al, 2001 Urbanke and Wray, 2001). The kinetic and spectroscopic studies are all compatible with the post-rigor and pre-powerstroke crystal structures. 

The structure of the myosin-II motor domain has been determined in various nucleotide states by crystal structure analysis using constructs originating from diverse sources (chicken skeletal and smooth muscle... 

It has long been surmised that switch-2 movement and the concomitant swinging of the lever arm must be controlled by binding to and detachment from the actin filament to avoid futile consumption of ATP. However, direct evidence was lacking because near-atomic resolution crystal structures are necessarily obtained in the absence of the filament. Now, crystal structures of Dictyostelium myosin II (Reubold et at, 2003) and chicken myosin-V (Coureux et al., 2003) have revealed that the switch-1 motif can also exist in open and closed conformations. It has been inferred that switch-1 opening may be coupled to cleft closure and tight binding of the myosin head to the actin filament. This conclusion is supported by electron microscopy (Holmes et al., 2003) and fluorescence spectroscopy (Conibear et al., 2003) studies of the acto-myosin complex, which show that the concepts derived from crystal structures of isolated myosin heads are indeed valid for the functional complex. 

Despite considerable structural information, the structure of the N-terminal residues of the RLC and the location of the phosphorylatable serine are not known. In the crystal structure of the skeletal muscle myosin head, electron density was not observed for the N terminus of the RLC, suggesting that this region is flexible (Rayment et al., 1993). Density for the N terminus of the scallop myosin RLC begins six residues beyond the serine homologous to Ser 19 in the smooth RLC (Xie et al., 1994). In addition, scallop RLC lacks the N-terminal extension before the serine that contains the basic residues necessary for myosin light chain kinase binding. 

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