Cryo-EM modeling of TPM1 traversing over actin. E192 of tropomyosin moves across a positively charged surface on actin during regulatory transitions. Shown are ribbon diagrams of tropomyosin from the refined low Ca²⁺ (red) and high Ca²⁺ (green) cryo-EM structures (Yamada et al., 2020; Pavadai et al., 2020) and the cryo-EM structure of tropomyosin and actin saturated with cardiac myosin (green; Protein Data Bank accession no. 6X5Z; Doran et al., 2020). Actin is shown as a cutaway surface colored by residue type (blue for positively charged residues; red for negatively charged residues; green for polar, uncharged residues; and white for hydrophobic residues). Note that as E192 moves from the structural B state through the C state to the M state and back, it traverses a positive patch on actin formed by actin residues K215 and K238. Substitution of E192 with a lysine would be expected to create unfavorable interactions with this patch, thereby altering regulatory transitions, which in conjunction with altered persistence length would affect contraction and relaxation in mutant muscle.