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 level (yellow) is poorly ordered (weak map density, suggesting substantial IHM mobility; Zoghbi et al., 2008; AL-Khayat et al., 2013). S2 cannot be fitted unambiguously due to low resolution of map and lack of internal detail with negative stain; within these limitations, there is no obvious S2–head interaction between crowns (circles; see Video 1; cf. Video 2 for 3-D view). (C) In scallop cryo-EM reconstruction, S2 is not resolved, but the tight azimuthal crowding of IHMs around the circumference at each crown suggests potential intermolecular interaction between the SH3 domain of a BH and the motor domain of the neighboring FH (circles). Filaments are oriented with M-line at top; their different symmetries (fourfold, threefold, and sevenfold rotational symmetry, respectively) cause the varying views of the IHMs in the different filaments. All reconstructions are at the same scale. The human has a smaller diameter due to radial shrinkage occurring during negative staining and to the smaller number of molecules (n = 3) at each level. The scallop and tarantula cryo-reconstructions also have different diameters: scallop has seven molecules at each level forming a shell above the filament backbone (Woodhead et al., 2013), while tarantula has four molecules at each level, closer to the backbone (Woodhead et al., 2005), leading to a smaller diameter. Models in this figure were created with UCSF Chimera (Pettersen et al, 2004).

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 level (yellow) is poorly ordered (weak map density, suggesting substantial IHM mobility; Zoghbi et al., 2008; AL-Khayat et al., 2013). S2 cannot be fitted unambiguously due to low resolution of map and lack of internal detail with negative stain; within these limitations, there is no obvious S2–head interaction between crowns (circles; see Video 1; cf. Video 2 for 3-D view). (C) In scallop cryo-EM reconstruction, S2 is not resolved, but the tight azimuthal crowding of IHMs around the circumference at each crown suggests potential intermolecular interaction between the SH3 domain of a BH and the motor domain of the neighboring FH (circles). Filaments are oriented with M-line at top; their different symmetries (fourfold, threefold, and sevenfold rotational symmetry, respectively) cause the varying views of the IHMs in the different filaments. All reconstructions are at the same scale. The human has a smaller diameter due to radial shrinkage occurring during negative staining and to the smaller number of molecules (n = 3) at each level. The scallop and tarantula cryo-reconstructions also have different diameters: scallop has seven molecules at each level forming a shell above the filament backbone (Woodhead et al., 2013), while tarantula has four molecules at each level, closer to the backbone (Woodhead et al., 2005), leading to a smaller diameter. Models in this figure were created with UCSF Chimera (Pettersen et al, 2004).

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