Single-Molecule and Molecular Dynamics Study of the Dimerization of Myosin VI Medial Tail Domain

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Myosin VI is an actin-based molecular motor with a large step size despite its short lever arm. We recently showed that the medial tail domain of myosin VI, likely a ∼70-residue long alpha-helix, is a dimerization region when two monomers of myosin VI are brought into close proximity. This proximity mimics the effect of cargo binding onto myosin VI monomers.The medial tail domain has peculiar characteristics. Although there are a few hydrophobic residues in the lower amino acid number region, it has a distinct motif alternating between four positively charged residues and four negatively charged residues.Single-molecule experiments show that a mutant construct of the myosin VI medial tail domain, designed to disrupt the hydrophobic interactions of myosin VI, can still dimerize, therefore ruling out the hypothesis that hydrophobic interaction is the only dimerization mechanism of the medial tail domain. Additionally, it was observed that increasing ionic strength reduces the percentage of myosin VI dimerization. To unveil the molecular mechanisms involved in the dimerization of the medial tail domain, we employed three different molecular dynamics (MD) methodologies, namely: 1) coarse-grained MD facilitating microsecond timescale sampling; 2) the flexible fitting method enabled reversion of coarse-grained to all-atom descriptions; 3) all-atom equilibrium MD. The coarse-grained MD simulation showed the medial-tail domain dimerized, and the all-atom flexible-fitting method identified interactions between two medial-tail domain helices. We used the all-atom equilibrium MD simulation to make sure that the interactions involved in dimerization were stable over time.This set of simulations suggests that an array of salt bridges between positive and negative residues participates in the dimerization process. These results could have an important implication on myosin VI dimerization.