Microtubule mechanics & AFM

Material properties of microtubules

Microtubules are prominent components of the cytoskeleton and also perform a number of special functions in the cell. They form the backbone of cilia and flagella, they support dendrites and axons in neurons, and they provide the mechanical framework for the mitotic an meiotic spindles in cell division. Microtubules are hollow tubes of 25 nm diameter with a 2D protein crystal as a wall. We have been investigating the elastic properties of microtubules with atomic force microscopy (AFM), a technique enabling us to both resolve structural details on the scale of a few nanometers and to probe the mechanical properties of the structures with pN forces. The particular structure of microtubules, made up of a 2D array of aligned protofilaments, has intriguing consequences for their (anisotropic) mechanical properties. Furthermore the intrinsically metastable and dissipative character of microtubules and its consequences for microtubule dynamics in the cell is not completely understood. We are also probing microtubules with optical traps.


Material properties and dynamics of assembly and filling of virus particles

Indentation and fracture of a bacteriophage virus shell by AFM. Tip forces above ~ 1 nN eventually lead to irreversible damage to the shells. The fracture patterns follow the basic triangular building blocks of the capsids.

Viruses are intriguing intermediates between the inanimate and the animate world. They are built very simply out of few structural protein building blocks, typically arranged as crystalline shells (icosahedra and related shapes), which protect their genetic information coded in RNA or DNA. Viruses are Nature’s ultimate “nanomachines” that far outperform any man-made nanotechnology. The combination of two-dimensional, crystalline protein-shells with DNA or RNA in confined and possible ordered states in a virus particle makes for rather complex and interesting material properties. We have pioneered the study of material properties of virus shells on the nanometer and pico-Newton scale by AFM. Depending on the interactions between the shell proteins, virus particles can be rather rigid and brittle or flexible and deformable. We have tested bacteriophages (P29) and a plant virus (CCMV). In addition to their passive mechanical properties, the assembly of certain viruses also involves active processes such as packing the DNA into the preformed precursor shells using a motor protein which we are also interested in. (collaboration with J. Carascossa, Madrid; C. Knobler, W. Gelbart, W. Klug, UCLA)