  Collective dynamics and force generation by cytoskeletal filaments are
crucial in many cellular processes. Investigating growth dynamics of a bundle
of N independent cytoskeletal filaments pushing against a wall, we show that
chemical switching (ATP/GTP hydrolysis) leads to a collective phenomenon that
is currently unknown. Obtaining force-velocity relations for different models
that capture chemical switching, we show, analytically and numerically, that
the collective stall force of N filaments is greater than N times the stall
force of a single filament. Employing an exactly solvable toy model, we
analytically prove the above result for N=2. We, further, numerically show the
existence of this collective phenomenon, for N>=2, in realistic models (with
random and sequential hydrolysis) that simulate actin and microtubule bundle
growth. We make quantitative predictions for the excess forces, and argue that
this collective effect is related to the non-equilibrium nature of chemical
switching.
