  Inspired by the complex influence of the globular crosslinking proteins on
the formation of biofilament bundles in living organisms, we study and analyze
a theoretical model for the structure and thermodynamics of bundles of helical
filaments assembled in the presence of crosslinking molecules. The helical
structure of filaments, a universal feature of biopolymers such as filamentous
actin, is shown to generically frustrate the geometry of crosslinking between
the "grooves" of two neighboring filaments. We develop a coarse-grained model
to investigate the interplay between the geometry of binding and mechanics of
both linker and filament distortion, and we show that crosslinking in parallel
bundles of helical filaments generates {\it intrinsic torques}, of the type
that tend to wind bundle superhelically about its central axis. Crosslinking
mediates a non-linear competition between the preference for bundle twist and
the size-dependent mechanical cost of filament bending, which in turn gives
rise to feedback between the global twist of self-assembled bundles and their
lateral size. Finally, we demonstrate that above a critical density of bound
crosslinkers, twisted bundles form with a thermodynamically preferred radius
that, in turn, increases with a further increase in crosslinking bonds. We
identify the {\it stiffness} of crosslinking bonds as a key parameter governing
the sensitivity of bundle structure and assembly to the availability and
affinity of crosslinkers.
