Assembly of Simple Epithelial Keratin Filaments: Decipheringthe Ion Dependence in Filament Organization.

The intermediate filament proteinskeratin K8 and K18 constitutean essential part of the cytoskeleton in simple epithelial cell layers,structurally enforcing their mechanical resistance. K8/K18 heterodimersform extended filaments and higher-order structures including bundlesand networks that bind to cell junctions. We study the assembly ofthese proteins in the presence of monovalent or divalent ions by small-angleX-ray scattering. We find that both ion species cause an increaseof the filament diameter when their concentration is increased; albeit,much higher values are needed for the monovalent compared to the divalentions for the same effect. Bundling occurs also for monovalent ionsand at comparatively low concentrations of divalent ions, very differentfrom vimentin intermediate filaments, a fibroblast-specific cytoskeletoncomponent. We explain these differences by variations in charge andhydrophobicity patterns of the proteins. These differences may reflectthe respective physiological situation in stationary cell layers versussingle migrating fibroblasts. [ABSTRACT FROM AUTHOR]