Structural basis of the super- and hyper-relaxed states of myosin II

Figures

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  Figure 1.. ATP turnover rates of myosin and proposed relation to head organization. (A) Logarithmic plot of rates, varying from very slow (hyper-relaxed), to slow (super-relaxed), to fast (disordered-relaxed), to very fast (actin-activated). (B) Expanded plot of relaxed rates and proposed relation to head configuration and interactions. The cartoons show the IHM as found in single molecules (upper) and thick filaments (lower); in filaments, only molecules in a single crown are shown. The colored ellipses are the interacting motor domains. The smaller, gray domains are the ELCs and RLCs, and the gray lines are the proximal region of the myosin tail (S2). Molecules with a sufficient length of S2 (25 heptads [25 hep]; ∼250 Å) form an IHM, while single heads and two-headed molecules with only two heptads of tail show no interactions. Molecules in filaments have SRX and DRX turnover rates similar to those of isolated molecules (25-heptad IHM; cf. vertebrate IHM) or more inhibited (HRX) rates. Heads turn over ATP at fast (F; noninteracting), slow (S; interacting), or very slow (VS; more interactions) rates. Slow (attached) FHs of the IHM can detach and sway out (red, double-headed arrows) and,…

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  Figure 2.. Structural basis of SRX. (A) Proposed conformations of myosin heads (S1), in equilibrium with each other, underlying SRX (left, bent) and DRX (right, straight) ATP turnover rates (Anderson et al., 2018). MD, motor domain. (B) IHM of cardiac HMM showing BH and FH (based on Protein Data Bank accession no. 5TBY). Ellipses show regions of interaction between BH and FH motor domains (black ellipse), FH and S2 (black circle), and BH and S2 (white ellipse; interaction occurs on rear side of BH). (C) Thick filament (tarantula; EM Data Bank accession no. 1950) showing IHMs lying along four coaxial helices (three on front marked with arrows) creating intermolecular interactions (ellipses) between FH of one IHM and regulatory domain of IHM above. M-line would be at the top of the image. The reconstruction shows the average positions of the heads in the filament; however, the FHs are thought to be dynamic, leaving and returning to the IHM (Fig. 1 B; see text). Models in this figure were created with UCSF Chimera (Pettersen et al, 2004).

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  Figure 3.. Comparison of intermolecular interactions in 3-D reconstructions of different thick filaments. Reconstructions are fitted with IHM atomic models (Protein Data Bank accession nos. 3JBH or 5TBY) in each case. All thick filaments (except insect flight muscle; Hu et al., 2016) show interactions between IHMs along the helical tracks (arrows), involving the FH motor domain at one level and the BH lever arm at the next. These interactions are the same at every level in tarantula and scallop (true helical structures) but different at different levels in vertebrates, which are quasi-helical. Additional interactions vary between species. (A) Tarantula shows interaction of S2 from one crown of heads with SH3 and converter domains of the next level (circles; see Fig. 4, A and B). IHM (Protein Data Bank accession no. 3JBH) was fitted to four levels of heads in cryo-EM reconstruction (EMD accession no. 1950; gray). (B) Similar fitting of IHM (Protein Data Bank accession no. 5TBY) to vertebrate (human) cardiac negative stain reconstruction of C-zone (EMD accession no. 2240) shows well-ordered IHMs at two of every three levels of heads (strong map density for pink and cyan IHMs), while the third…

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  Figure 4.. Comparison of tail interactions with the SH3 and converter domains in atomic models of tarantula thick filaments and 10S smooth muscle myosin. (A and B) Tarantula thick filament (Protein Data Bank accession no. 3JBH). (C and D) 10S myosin (Protein Data Bank accession no. 6XE9). (A and C) Front views at same scale. (B and D) End views (same scale) obtained by rotating A and C 90° around the x axis so that pink IHM in A is closest to viewer (i.e., looking toward M-line). S2 in tarantula filament and segment 2 (Seg2) in 10S myosin both pass through a groove formed by the SH3 and converter (Cnv) domains of the BH motor domain (MD). The looser fit to the groove in tarantula may be due to the lower resolution of the reconstruction used to obtain the atomic model (20 Å resolution; cf. 4.3 Å for 10S myosin). Models in this figure were created with UCSF Chimera (Pettersen et al, 2004).