Model for Bundling of Keratin Intermediate Filaments

Abstract

Keratin intermediate filaments form dynamic intracellular networks, which span the entire cytoplasm and provide mechanical strength to the cell. The mechanical resilience of the keratin intermediate filament network itself is determined by filament bundling. The bundling process can be reproduced in artificial conditions in the absence of any specific cross-linking proteins, which suggests that it is driven by generic physical forces acting between filaments. Here, we suggest a detailed model for bundling of keratin intermediate filaments based on interfilament electrostatic and hydrophobic interactions. It predicts that the process is limited by an optimal bundle thickness, which is determined by the electric charge of the filaments, the number of hydrophobic residues in the constituent keratin polypeptides, and the extent to which the electrolyte ions are excluded from the bundle interior. We evaluate the kinetics of the bundling process by considering the energy barrier a filament has to overcome for joining a bundle.

Keratins determine network stress responsiveness in reconstituted actin–keratin filament systems

“…This model predicts that the process of keratin bundling is determined by the electric charge of the filaments, the number of hydrophobic residues, and the exclusion of the ions from the bundle interior. 49 Compared with actin, the attractive interactions between filaments in pure keratin structures are much stronger than those in entangled actin networks. 37 Consequently, the viscous loss modulus in keratin networks during oscillatory shear is significantly reduced compared to the viscous dissipation in actin networks.…”

Topological defects produce kinks in biopolymer filament bundles

“…Looking at stiff F-actin cross-linkers like -actinin, we find that ; it is too small for braids to generate kinks. We do, however, expect braids to be associated with kinks in softer filament systems such as DNA condensed by polyvalent counter ions ( 25 , 26 ) or cross-linked intermediate filaments ( 27 , 28 ) where . Currently, our understanding of collagen bundle cross-linking is less precise; our estimate in this system is that (see SI Appendix , section 1 ).…”

Braiding Dynamics in Semiflexible Filament Bundles under Oscillatory Forcing

“…Because of this effect, the large- limit also exhibits kinks. We note that this large- limit occurs around , which we believe is obtainable in some biopolymer systems, including condensed DNA [ 9 , 10 ], cross-linked intermediate filaments [ 11 , 12 ] and perhaps for collagen [ 5 , 13 , 14 , 15 ]. We should note that we introduced the torque with the opposite sign from that used here in Ref.…”