Landscape-Based View on the Stepping Movement of Myosin VI

Myosin VI dimer walks toward the minus end of the actin
filament
with a large and variable step size of 25–36 nm. Two competing
models have been put forward to explain this large step size. The
Spudich model assumes that the myosin VI dimer associates at a distal
tail near the cargo-binding domain, which makes two full-length single
α-helix (SAH) domains serve as long legs. In contrast, the Houdusse–Sweeney
model assumes that the association occurs in the middle (between residues
913 and 940) of the SAH domain and that the three-helix bundles unfold
to ensure the large step size. Their consistency with the observation
of stepping motion with a large and variable step size has not been
examined in detail. To compare the two proposed models of myosin VI,
we computationally characterized the free energy landscape experienced
by the leading head during the stepping movement along the actin filament
using the elastic network model of two heads and an implicit model
of the SAH domains. Our results showed that the Spudich model is more
consistent with the 25–36 nm step size than the Houdusse–Sweeney
model. The unfolding of the three-helix bundles gives rise to the
free energy bias toward a shorter distance between two heads. Besides,
the stiffness of the SAH domain is a key factor for giving strong
energetic bias toward the longer distance of stepping. Free energy
analysis of the stepping motion complements the visual inspection
of static structures and enables a deeper understanding of underlying
mechanisms of molecular motors.