Assembly kinetics determine the structure of keratin networks

Living cells exhibit an enormous bandwidth of mechanical and morphological properties that are mainly determined by the cytoskeleton. In metazoan cells this composite network is constituted of three different types of filamentous systems: actin filaments, microtubules and intermediate filaments. Keratin-type intermediate filaments are an essential component of epithelial tissues, where they comprise networks of filaments and filament bundles. However, the underlying mechanisms leading to this inherently polymorphic structure remain elusive. Here, we show that keratin filaments form kinetically trapped networks of bundles under near-physiological conditions in vitro. The network structure is determined by the intricate interplay between filament elongation and their lateral association to bundles and clusters.