Table 1.

Effect of myosin loop-4 on the interaction energy and salt bridge formation between actin and tropomyosin

	Interaction energies (kcal/mol)		Salt bridges
	Coulombic	van der Waals	# Residues involved
Actin-tropomyosin (WT-S1)	−617 ± 43	−34 ± 5	5
Actin-tropomyosin (7G-S1)	−1062 ± 52	−14 ± 10	8
Actin-tropomyosin (S1-free control)	−1073 ± 53	−3 ± 8	10

Using MDFF molecular dynamics flexible fitting protocols (Trabuco et al., 2008; Trabuco et al., 2009) tropomyosin (PDB ID 7UTI) and actin (PDB ID 5JLF) were fitted to their corresponding densities in reconstructions of actin-tropomyosin decorated with WT (EMDB ID EMD28083) and 7G S1 (EMDB ID EMD28270) as well as to undecorated, S1-free actin-tropomyosin (EMDB ID EMD8162). Values (±SD) for interaction energies were measured between the central three tropomyosin pseudo-repeats (repeats 3, 4, 5) and F-actin over the last 25 MDFF frames of each trajectory and then averaged. Potential salt bridges between actin and tropomyosin residues were counted when the distance between oppositely charged side chains was <2.5 Å.