The acrosomal process of the sperm of the horseshoe crab (Limulus
polyphemus) is a unique crystalline actin bundle, consisting of multiple
actin filaments cross-linked by the actin-bundling protein, scruin. For successful fertilization, the acrosomal bundle must penetrate through a
30 mm thick jelly coat surrounding the egg and thus it must be sufficiently
stiff. Here, we present two measurements of the bending stiffness of a
single crystalline bundle of actin. Results from these measurements
indicate that the actin:scruin composite bundle has an average elastic
modulus of 2 GPa, which is similar to that of a single actin filament, and
a bending stiffness that is more than two orders of magnitude larger
than that of a bundle of uncross-linked actin filaments due to stiffening
by the scruin matrix.