Rigor state actin binding

D. Rigor State Actin Binding Closes the 50K Cleft. 171... 

Fig. 3. Details of the seven-stranded /1-sheet and associated structures (A and B) in the post-rigor conformation and (C and D) in the pre-powerstroke conformation. The orientation of A and C is at right angles to that shown in Fig. 2. When attached to actin, it corresponds to that shown in Fig. 5B. The colors are as in Fig. 2. The views shown in B and D are at right angles to A and C looking out radially from the axis of the actin helix. Note the kink in the relay helix shown in C and D that leads to a 60° rotation of the converter domain. This in turn rotates the lever arm 60°. The P-loop (which constitutes the ATP-binding site) and the adjoining a-helix are shown in yellow. The flanking switch sequences (1 and 2) are also shown. The strands of the /1-sheet are numbered from the N-terminal (distal) end of the sheet. The lower part of strand 5 (light blue) constitutes switch 2. In the post-rigor state, switch 2 lies out of the plane of the /1-sheet (open) and in the pre-powerstroke state switch 2 is in the plane of the /1-sheet (closed).

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 strongly bound pre-powerstroke state or top-of-powerstroke state is the transitory state labeled 4 in Fig. 1. It is experimentally difficult to characterize this either kinetically or structurally. At present, the structure can only be guessed at by an extrapolation of the properties of the adjoining structures. It seems very likely that the actin-binding cleft closes on strong binding in the top-of-powerstroke state. Comparison of the structures of the pre-powerstroke and post-rigor states with the nucleotide-free... 

Fig. 7. The strongly bound top-of-powerstroke state. Shown is the truncated myosin crossbridge without the lever arm. The orientation is as in Fig. 5A. (A) Pre-powerstroke state with the upper 50K domain shown in yellow. (B) The rigor-like state with the upper 50K domain of the pre-powerstroke state (yellow) superimposed on the upper 50K domain of the rigor-like state (red). (C) The model produced by taking the superimposed orientation of the upper 50K domain and combining it with the original pre-powerstroke coordinates. This generates a pre-powerstroke state with a shut actin-binding cleft that serves as a model of the ephemeral strongly bound top-of-powerstroke state.

The relay/converter conformation is the readout of the result of these inputs. In going from the post-rigor to pre-power states, the only signal is SW2 going from open to closed, which produces the relay kink and the converter up. To go from the pre-power to rigor states requires the /i-twist, which is triggered by actin binding and cleft closure. 

Squire, J. M., and Harford, J. J. (1988). Actin filament organisation and myosin head labelling patterns invertebrate skeletal muscles in the rigor and weak-binding states. / Mus. Res. Cell Motil. 9, 344-358. 

Figure 8.56. Stereo view in space-filling representation of a scallop cross-bridge (SI) view of the actin binding site on myosin cross-bridge, with continuity of the cleft from the ADP-BeF, binding site to the actin binding site that dissociates on ATP binding (A) Near-rigor state without nucleotide in the binding...

Figure E.5. Stereo view (cross-eye) in space-filling representation of the scallop muscle cross-bridge(Sl) viewed approximately from the side of the actin binding site on myosin cross-bridge for the purpose of locating the narrow clefts that direct the apolar-polar repulsive force. (A) The hydrophobic association of the near-rigor state. (Prepared using...

Figure 17.3 Structural events during the Lymn-Taylor muscle contraction cycle. In the rigor state (state I), myosin is strongly bound to actin in the absence of ATP. During rigor dissociation, that is, conformational transition from the rigor state to the prerecovery state (state II), myosin binds to ATP and dissociates from actin. As a result of the recovery stroke transition, myosin attains...

The modeled structure may also be used to generate the attached top-of-powerstroke state. This is shown in Fig. 9A compared with the rigor conformation (Fig. 9B). The same geometry for the attachment to actin has been used, as was found by electron microscopy for the binding of myosin V to actin. The lever arm from chicken skeletal myosin has been used to complete the model. 

---