Salt bridges formed between Tpm and actin residue 328 in C- and B-state thin filaments
| Tropomyosin residues making salt bridges to K328 on successive actin subunits along thin filaments | Tropomyosin pseudorepeat # and chain ID | Actin–tropomyosin inter-residue distance (Å) | |
| C-state | B-state | ||
| Tpm-D20 | 1A | 4.7 | 2.3 |
| Tpm-E23 | 1A | 2.3 | 6.2 |
| Tpm-D58 | 2A′ | 4.1 | 2.1 |
| Tpm-E96 | 3A | 2.7 | 4.7 |
| Tpm-D100 | 3A | 3.9 | 5.0 |
| Tpm-E138 | 4A′ | 2.6 | 2.6 |
| Tpm-E177 | 5A | 4.8 | 3.3 |
| Tpm-E180 | 5A | 3.9 | 5.3 |
| Tpm-D219 | 6A′ | 3.3 | 3.7 |
| Tpm-E257 | 7A | 3.1 | 2.0 |
| Tropomyosin residues making salt bridges to K328 on successive actin subunits along thin filaments | Tropomyosin pseudorepeat # and chain ID | Actin–tropomyosin inter-residue distance (Å) | |
| C-state | B-state | ||
| Tpm-D20 | 1A | 4.7 | 2.3 |
| Tpm-E23 | 1A | 2.3 | 6.2 |
| Tpm-D58 | 2A′ | 4.1 | 2.1 |
| Tpm-E96 | 3A | 2.7 | 4.7 |
| Tpm-D100 | 3A | 3.9 | 5.0 |
| Tpm-E138 | 4A′ | 2.6 | 2.6 |
| Tpm-E177 | 5A | 4.8 | 3.3 |
| Tpm-E180 | 5A | 3.9 | 5.3 |
| Tpm-D219 | 6A′ | 3.3 | 3.7 |
| Tpm-E257 | 7A | 3.1 | 2.0 |
The extent of salt-bridge formation between tropomyosin and actin was assessed by examining potential interactions in atomic models of B-state (PDB ID 7UTL) and C-state (PDB ID 7UTI) thin filaments. Intermolecular links between tropomyosin and actin K328 occur on all actin subunits and involve common tropomyosin residue contacts in both B- and C-states. Tip-to-tip distances between NH3+ hydrogen atoms on lysine 328 of successive actin subunits along filaments and COO− oxygen atoms of aspartate and glutamate residues on neighboring tropomyosin (Tpm) pseudorepeats were measured in UCSF Chimera (Petterson et al., 2004) and are listed for those closely apposed residues common to both states. The linkage between tropomyosin and actin residue K326 that is maintained in both B- and C-states also occurs but is more sporadic (e.g., involving tropomyosin residues E23 and E222, data not shown) and matches those previously reported (Pavadai et al., 2020a; Pavadai et al., 2020b; also see Risi et al., 2022). Numerous linkages that are either B- or C-state specific, but not common to both models augment the interactions observed (see Fig. S2). Note that because tropomyosin is a modular two-chained helical coiled-coil, its component A and A′ chain α-helices alternate with each other to form salt bridges on the successive actin subunits along thin filaments. As designated by Brown and Cohen (2005), tropomyosin pseudorepeats 1–7 are numbered to encompass tropomyosin residues 1–46, 47–85, 86–125, 126–164, 165–204, and 205–243, 244–284.