CROWDING-INDUCED ORGANIZATION OF CYTOSKELETAL ELEMENTS .1. SPONTANEOUS DEMIXING OF CYTOSOLIC PROTEINS AND MODEL FILAMENTS TO FORM FILAMENT BUNDLES

The theory for the effects of crowding on the behavior of reversibly s elf-assembling solutes is extended to mixtures containing nonassemblin g solutes. The theory predicts that excluded volume will cause dramati c demixing into domains of long, tightly packed, highly aligned fibers coexisting with an isotropic solution of unaggregated species. It sug gests that the bundling of fibers in cells is entropically driven and that accessory binding proteins in the cytoplasm, serve to modulate th e process rather than create it.