  The dynamics of many macromolecular machines is characterized by
chemically-mediated structural changes that achieve large scale functional
deployment through local rearrangements of constitutive protein sub-units.
Motivated by recent high resolution structural microscopy of a particular class
of such machines, contractile injection systems (CIS), we construct a coarse
grained semi-analytical model that recapitulates the geometry and bistable
mechanics of CIS in terms of a minimal set of measurable physical parameters.
We use this model to predict the size, shape and speed of a dynamical actuation
front that underlies contraction. Scaling laws for the velocity and physical
extension of the contraction front are consistent with our numerical
simulations, and may be generally applicable to related systems.
