One cellular function of intermediate filaments is to provide cells with compliance to small deformations while strengthening them when large stresses are applied. How IFs accomplish this mechanical role is revealed by recent studies of the elastic properties of single IF protein polymers and by viscoelastic characterization of the networks they form. IFs are unique among cytoskeletal filaments in withstanding large deformations. Single filaments can stretch to more than 3 times their initial length before breaking, and gels of IF withstand strains greater than 100% without damage. Even after mechanical disruption of gels formed by crossbridged neurofilaments, the elastic modulus of these gels rapidly recovers under conditions where gels formed by actin filaments are irreversibly ruptured. The polyelectrolyte properties of IFs may enable crossbridging by multivalent counterions, but identifying the mechanisms by which IFs link into bundles and networks in vivo remains a challenge.

A method to produce highly purified thrombin from salmon blood is described, and a series of biochemical, cell biologic, and biophysical assays demonstrate the functional similarities and some differences between salmon and human thrombins. Salmon thrombin with specific activity greater than 1000 units/mg total protein can be prepared by modifications of the methods used for purification of human thrombin. Using a synthetic substrate based on the human fibrinogen A-alpha polypeptide sequence as an indicator of enzymatic activity, salmon and human thrombin preparations contain similar specific activities per mass of purified protein. Salmon thrombin activates human fibrinogen and initiates the formation of fibrin clots whose structure and rheologic properties are indistinguishable from those of human fibrin clotted by human thrombin. Salmon thrombin also activates human platelets. Approximately 10 times higher activities are needed for the same rate of platelet aggregation compared to human thrombin, and some aspects of platelet activation, most notably phosphatidylserine exposure, are diminished relative to the effects of human thrombin. This latter finding suggests that salmon thrombin may not activate all of the receptors that are targets of human thrombin, although it does appear to activate signals that are sufficient to produce normal rates of activation and aggregation as measured by conventional aggregometry. Together with the recent purification of salmon fibrinogen and its application in mammalian wound healing, the availability of salmon thrombin allows the formulation of biological sealants devoid of any exogenous mammalian proteins and so may aid the design of materials with increased safety from infectious disease transmission.