The data presented here were also obtained from models in which the contacting surface between the microtubule
and the substrate was modeled using a minimum of immobilized
contact area. To determine the effect of contact
surface on the radial stiffness, we also modeled a series of
microtubules in which up to three of the bottom protofilaments
were fixed in position, simulating microtubules that
are more strongly immobilized to the underlying surface
(see Fig. S1 in the Supporting Material). Even in these
cases, the scaling described by Eq. I is maintained, albeit
with values of n and C that are significantly higher than
expected for thin shells. Thus,all of our models suggest
that the dominant factor in determining microtubule radial
stiffness in each of these experiments is the tube dimension
(t/R) and not skew.