  We investigate stall force and polymerization kinetics of rigid
protofilaments in a microtubule or interacting filaments in bundles under an
external load force in the framework of a discrete growth model. We introduce
the concecpt of polymerization cycles to describe the stochastic growth
kinetics, which allows us to derive an exact expression for the stall force. We
find that the stall force is independent of ensemble geometry and load
distribution. Furthermore, the stall force is proportional to the number of
filaments and increases linearly with the strength of lateral filament
interactions. These results are corroborated by simulations, which also show a
strong influence of ensemble geometry on growth kinetics below the stall force.
