Intermediate filament mechanics in vitro and in the cell: from coiled coils to filaments, fibers and networks
Section snippets
‘Nanofilaments’: fibrous protein assemblies that comprise a major cytoskeletal moiety and the nuclear lamina
The fibrous intermediate filament (IF) proteins constitute the nuclear lamina network as well as a 10-nm-diameter filament system in the cytoplasm of metazoan cells [1]. Supposedly, they all originate from a common ancestor, most probably a kind of a ‘primordial nuclear lamin’ [2]. All IF proteins follow a common structural principle including a central α-helical ‘rod’ of conserved size that is flanked by non-α-helical N-terminal (‘head’) and C-terminal (‘tail’) end domains both of highly
IF proteins form filaments, fibers and networks
The dynamic nature of IF proteins reflected in the assembly process is accompanied by extreme stability; IF filaments are notoriously insoluble under physiological conditions and therefore have to be solubilized with chaotropic agents (e.g., 8 M urea or 6 M guanidine-HCl) to employ them for in vitro assembly [4]. In cells, IF structures retain this remarkable resilience, and contribute considerably to mechanical stability. Generally, individual IF proteins can be renatured without the help of
Dissecting the molecular mechanism of the ‘head-to-tail’ interaction
According to the assembly scenarios presented above, for both cytoplasmic and nuclear IFs the principal interaction for elongation is the head-to-tail association of dimers as shown by the use of ‘half-minilamins’ [13]. These represent N-terminal and C-terminal fragments of lamins with a truncated central α-helical rod domain. Accordingly, the IF consensus motifs residing at either end of the α-helical rod domain (see Figure 1a in blue), including conserved sequence motifs of the flanking
Mechanical properties and molecular architecture of single IFs
The persistence length of IF is in the range of a few hundred nm to a few μm [17, 18] classifying them — in a physical sense — as semiflexible biopolymers. Thus, eukaryotes are equipped with three filaments systems spanning nm to mm persistence lengths. Confinement in microchannels has been used to measure the equilibrium persistence length of freely fluctuating vimentin IFs in solution, that is, unadsorbed (Figure 2b). Whereas the resulting bending rigidity is only one order of magnitude smaller
IFs form bundles and networks
Like other filamentous biopolymers, IFs exhibit polymeric and polyelectrolyte properties [24••, 25••]. The interplay between their intrinsic mechanical bending rigidity and interactions between the filaments leads to the formation of superstructures. Distinct bundling and cross-bridging proteins such as plectin link IFs to one-another and to actin filaments and microtubules [26, 27]. Hence, within a cell the three principal cytoskeletal filament systems — actin filaments, intermediate filaments
Whole cell experiments
In the cell, the situation is much more complex than in the test tube due to a largely unknown environment, which also makes it much more difficult to unambiguously dissect whole cell scenarios. Despite these challenges, it has been found important to approach the problem ‘bottom-up’, as described above, and ‘top-down’ looking at cell experiments, and to eventually combine both complementary approaches.
To test the contribution of individual proteins to the deformability of cell components such
Conclusions
In most metazoan cells, the three cytoskeletal filament systems form networks that are linked to one another by so-called cytolinker proteins, thereby allowing ‘cross-talk’. This cross-talk, in turn, provides the link that is necessary to couple cell mechanics, in particular cell stiffness, deformation and stability, to intracellular transport and cell locomotion. Thus, we would like to stress the nanocomposite nature of the cytoskeleton as a whole. This sophisticated architecture allows the
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
S.K. thanks the German Research Foundation (DFG, KO 3752/5-1 and SFB 755 B07 and C10). H.H. received support from the German Research Foundation (DFG, HE 1853, FOR1228 and 1853/11-1) and from COST. R.D.G. was supported by NIH PO1GM096971 and Hannah's Hope Fund. D.A.W. acknowledges support from the NIH (PO1GM096971), the Harvard Materials Research Science and Engineering Center (DMR-0820484), and the NSF (DMR-1310266).
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