Dynamic properties of protein shells: Manipulating
microtubules and bacteriophage capsids with a scanning force probe
Polymeric macromolecular assemblies play crucial roles in biology,
from DNA to the cytoskeleton or the cell membrane. We are generally
interested in the mechanical physical properties of such structures.
I will report in particular on recent measurements of the elastic
properties of two types of 2D-crystalline protein shells which we
have probed at the nanometer scale by indentation with a scanning
force microscope (SFM) tip.
Microtubules are cylindrical shells and we find a linear elastic
regime that can be described by both thin-shell theory and finite
element methods, in which microtubules are modeled as homogeneous
hollow tubes. We also find a non-linear regime and catastrophic
collapse of the microtubules under large loads. The main physics of
protein shells at the nanometer scale shows simultaneously aspects of
continuum elasticity in their linear response, as well as molecular
graininess in their non-linear behavior.
Bacteriophages use highly ordered proteinaceous shells to protect
their genome from the environment and, interestingly, also to store
elastic energy for the injection process. We have studied empty and
filled bacteriophage f29 shells, again by SFM indentation. These
shells are approximately ellipsoidal. We again find a regime of
linear elastic response, followed by non-linear response and
break-down. The linear regime can again be described by thin shell
theory, assuming a homogeneous material, but we observe, already in
the linear regime, signatures of the substructure of the shells.