Oscillations, Waves, and Interactions - GWDG

DPI60plus – a future with biophysics 441
Figure 3. AFM scans of MTs. Scale bars represent 100 nm. Because of tip-sample dilation
the MT width appears exaggerated (Schaap et al., 2004, Ref. [1]). (A) MT without tau, showing
clearly the protofilaments. (B) For MTs with tau (ratio of 1:1 of tau:tubulin monomers),
the protofilaments are still visible. The height increased by 2 nm (see also Fig. 4). Inset: this
zoom shows a loose fibre with a height of ∼ 0.5 nm that could occasionally be seen. (From
Schaap et al., J. Struct. Biol., 2007, Ref. [3]).
prepared. They are similar in size to tubulin subunits and move with velocities of up
to 1 µm/s along the microtubule at saturating ATP conentrations. We succeeded in
imaging microtubules fully decorated with kinesin and measured a significant increase
in diameter (see Fig. 4). We also observed clusters and single kinesin motors on microtubules.
Repeated scanning indicates that we are able to follow individual kinesin
motors moving along the microtubule. The technical challenge is to increase the rate
of AFM-imaging to video-rate. The biophysics group will focus on the development
of fast AFMs for this and other applications.
2.4 Viral capsids
We have further applied AFM to study the structure and mechanical properties of
viral capsids. The particular viruses we have studied are the bacteriophage Phi29 and
the plant virus cowpea chlorotic mottle virus (CCMV). Much like microtubules, viral
capsids are self-assembling structures with typical sizes of tens of nanometres. Most
viral capsids have highly regular and symmetric structures of more or less icosahedral
symmetry. The shells are assembled from a well defined number of copies of mostly
just one structural protein. Packaging of the DNA into the capsid is in the case of
bacteriophages driven by motor proteins to such packing density that the capsid
has to resist considerable outward directed forces, translated to a pressure about
60 atm [5].
Phi29 capsids deformed elastically under the AFM tip up to a force of about 1 nN
and we could model the initial linear response of the shells by a simple homogeneous
shell model. Under higher forces the shells fractured and collapsed. CCMV virus
shells have the particular property that they expand under a change of pH. We
observed that this expansion which goes along with an effective thinning of the shell
wall caused a transition between two very different elastic responses. At low pH, in
the condensed state, shells deform linearly and then buckle and fracture, whereas in