Oscillations, Waves, and Interactions - GWDG

440 S. Lakämper and C. F. Schmidt
Figure 2. (a) shows a typical SFM image of an MT before performing a set of force-distance
measurements (FZ). In (b) a hole can be observed at the spot of the microtubule where
the FZs where performed after the detection of catastrophic breakages in the force distance
curves (from Ref. [2]).
tau is one of the most abundant microtubule associated proteins and is involved in the
stabilization and bundling of axonal microtubules in neurons. Tau is also infamous as
a major component of the fibrillar structures correlated with human neurodegenerative
diseases such as Alzheimer’s. Although intense research has revealed much about
tau function and its involvement in Alzheimer’s disease, it has remained unclear how
exactly tau binds to microtubules [3].
In a recent study we used AFM to image microtubules at saturating tau concentrations
and found an increase in diameter of tau-decorated microtubules of 2 nm.
While tau slightly increased the damage threshold of microtubules, measuring the
radial stiffness of decorated microtubules revealed no difference to undecorated microtubules.
Together with the finding that tau binding leaves the proto-filament structure
well visible, this finding is consistent with the model that tau binds along the
ridge of a proto-filament. Finite-element modelling confirmed that the radial elasticity
should be unaffected by tau decoration in that way [3,4].
In contrast to tau, the MAP doublecortin (DCX) has been reported to bind on
the outside of microtubules between the protofilaments. Finite-element modelling of
that geometry predicts an increased radial stiffness of decorated microtubules. DCX
has been found to be of importance for neuronal development. DCX mutations lead
to mislocalization of nuclei in developing neurons and DCX dysfunction in humans
leads to the disorder lisenzephaly. Ongoing AFM experiments with DCX-decorated
microtubules have not yet shown a substantially increased radial stiffness.
2.3 Imaging motor proteins using AFM
We are also investigating the movement of motor proteins on microtubules by AFM.
While dynein is a rather large roughly globular protein and should therefore be well
suited for visualization by AFM, it is very difficult to prepare in pure and active
form. Furthermore the flexibility of the dynein stalk might make imaging of dynein
challenging. Kinesin motors, on the other hand, bind tightly and are relatively easily