Keratin filament suspensions show unique micromechanical properties

All epithelial cells feature a prominent keratin intermediate filament (IF) network in their cytoplasm. Studies in transgenic mice and in patients wit h inherited epithelial fragility syndromes showed that a major function of keratin Ifs is to provide mechanical support to epithelial cell sheets. Yet the micromechanical properties of keratin Ifs themselves remain unknown. W e used rheological methods to assess the properties of suspensions of epide rmal type I and type II keratin Ifs and of vimentin, a type III LF polymer. We find that both types of Ifs form gels with properties akin to visco ela stic solids. With increasing deformation they display strain hardening and yield relatively rapidly. Remarkably, both types of gels recover their pres hear properties upon cessation of the deformation. Repeated imposition of s mall deformations gives rise to a progressively stiffer gel for keratin but not vimentin Ifs. The visco-elastic moduli of both gels show a weak depend ence upon the frequency of the input shear stress and the concentration of the polymer, suggesting that both steric and nonsteric interactions between individual polymers contribute to the observed mechanical properties. In s upport of this, the length of individual polymers contributes only modestly to the properties of IF gels. Collectively these properties render Ifs uni que among cytoskeletal polymers and have strong implications for their func tion in vivo.