Intermediate filaments (IFs) constitute a sophisticated filament system in the cytoplasm of eukaryotes. They form bundles and networks with adapted viscoelastic properties and are strongly interconnected with the other filament types, microfilaments and microtubules. IFs are cell type specific and apart from biochemical functions, they act as mechanical entities to provide stability and resilience to cells and tissues. We review the physical properties of these abundant structural proteins including both in vitro studies and cell experiments. IFs are hierarchical structures and their physical properties seem to a large part be encoded in the very specific architecture of the biopolymers. Thus, we begin our review by presenting the assembly mechanism, followed by the mechanical properties of individual filaments, network and structure formation due to electrostatic interactions, and eventually the mechanics of in vitro and cellular networks. This article is part of a Special Issue entitled: Mechanobiology.

Biomolecular motor systems are attractive for future nanotechnological devices because they can replace nanofluidics by directed transport. However, the lack of methods to externally control motor-driven transport along complex paths limits their range of applications. Based on a thermo-responsive polymer, we developed a novel technique to guide microtubules propelled by kinesin-1 motors on a planar surface. Using electrically heated gold microstructures, the polymers were locally collapsed, creating dynamically switchable tracks that successfully guided microtubule movement.

First lab-on-chip devices based on active transport by biomolecular motors have been demonstrated for basic detection and sorting applications. However, to fully employ the advantages of such hybrid nanotechnology, versatile spatial and temporal control mechanisms are required. Using a thermo-responsive polymer, we demonstrated a temperature controlled gate that either allows or disallows the passing of microtubules through a topographically defined channel. The gate is addressed by a narrow gold wire, which acts as a local heating element. It is shown that the electrical current flowing through a narrow gold channel can control the local temperature and as a result the conformation of the polymer. This is the first demonstration of a spatially addressable gate for microtubule motility which is a key element of nanodevices based on biomolecular motors.

Intermediate filaments play a central role in the cytoskeleton of eukaryotic cells. Together with microtubules and actin filaments they determine the mechanical properties of cells. Microtubules are also the guiding tracks for molecular transport in cells while actin filaments play an essential role in cell motility. The diameter of intermediate filaments lies between the diameter of actin filaments and microtubules which led to their name. There is an enormous genetic variety of intermediate filaments. In humans there are over 70 different genes for intermediate filament proteins. The different types of intermediate filaments are celltype-specific. For example, the intermediate filament protein vimentin is found in cells of mesenchymal origin and keratins occur in epithelial cells. Mutations in genes coding for intermediate filaments are known to cause more than 80 diseases, among them Alexander disease and amyotrophic lateral sclerosis (ALS). Intermediate filaments share a common hierarchical assembly scheme. Several tetramers form unit length filaments (ULFs). The ULFs then anneal longitudinally to form elongated filaments.Studies of the assembly are necessary to understand the structural properties of cells. In this thesis we aim to further understand the assembly of intermediate filaments. We use fluorescence correlation spectroscopy (FCS) which is a versatile technique to study the motility of molecules in solution. During the thesis project, we built a setup suitable for FCS. It allows for detecting the diffusive properties of the different states during the assembly. With the setup we study both the early and the long time scales of the assembly of vimentin filaments. For the long times we employ bulk measurements. To access the early time scales of the assembly, we employ microfluidic techniques. With the microfluidic mixing device we map the temporal axis to a spatial axis. This gives us the possibility to control the interaction of the molecules in a defined manner. We aim to observe the assembly reaction of vimentin intermediate filaments upon the addition of monovalent salt ions. v vi