'Macromolecular crowding' is a primary factor in the organization of the cytoskeleton.

We propose that, in the cell, the reversible conversion of actin filaments into actin bundles is controlled by the concentration of the macromolecules [we have employed poly(ethylene glycol) 6000 to mimic the macromolecules of the cell] as well as by the nature of the ancillary cytoskeletal proteins that decorate actin filaments. The proposal is based on the following evidence. (1) Under our experimental conditions the transition from filaments into bundles occurs at increasing concentrations of poly(ethylene glycol), with the following sequence: caldesmon-actin, 3%; filamin-actin, 4-5%; caldesmon-tropomyosin-actin, 5-7%; actin, 6-7%; tropomyosin-actin, 9-10%. (2) Under conditions of low osmoelastic stress [3% poly(ethylene glycol)], preformed caldesmon-actin bundles are dissociated by the addition of either tropomyosin or tropomyosin-decorated actin. The dissociation of the bundles promoted by the addition of tropomyosin-decorated actin is faster than that promoted by the addition of tropomyosin.