Water and the cytoskeleton

The diffusion of intracellular fluid and solutes is mainly limited by the d ensity and the geometry of crossbridges between cytoskeletal polymers media ting die formation of an integrated cytoplasmic scaffold. Evidence for spec ific relationships between water and cytoskeletal polymers arises from the effect of heavy water on their polymerization process in vitro and on the c ytoskeleton of living cells. The hydration of cytoskeletal subunits is modi fied through polymerization, a mechanism which may be involved in the direc t contribution of the cytoskeleton to the osmotic properties of cells toget her with changes of hydration of polymers within networks. The dynamic prop erties of die hydration layer of cytoskeletal polymers may reflect the repe titive distribution of the surface charges of subunits within die polymer l attice, thus inducing a local and long range ordering of the diffusion flow s of water and solutes inside polymer networks. The interactions between su bunits in protofilaments; and between protofilaments determine the specific viscoelastic properties of each type of polymer, regulated by associated p roteins, and the mechanical properties of the cell through the formation of bundles and gels. Individual polymers are interconnected into dynamic netw orks through crossbridging by sturctural associated proteins and molecular motors, die activity of which involves cooperative interactions with the po lymer lattice and likely the occurence of coordinated modifications of the hydration layer of the polymer surface. The cytoskeletal polymers are polye lectrolytes which constitute a large intracellular surface of condensed ani onic charges and form a buffering structure for the sequestration of cation s involved in the regulation of intracellular events, This property allows also the association of cytoplasmic enzymes and multimolecular complexes wi th the cytoskeleton, facilitating metabolic channelling and die localizatio n of these complexes in specific subdomains of the cytoplasm. The consequen ces of interactions between membranes and the cytoskeleton in all cellular compartments range from the local immobilizations and clustering of lipids and membrane proteins to the regulation of water and ion flows by the assoc iation of cytoskeletal subunits or polymers with transmembrane channels. Th e possibility that the polyelectrolyte properties of the cytoskeletal polym ers contribute to die modulation of membrane potentials supports the hypoth esis of a direct involvement of the cytoskeleton in intercellular communica tions.