Session 4: Technology: what do we need to know and how can we measure it?

How can nanodevices and biomimetic systems (such as GUVs and nanotubes) help us to develop more realistic and testable models of biological membranes?

Patricia Bassereau

Simple model membrane systems, such as vesicles, nanotubes or supported bilayers, have long been used by physicists as a basis for examining membrane physics. At the same time, small or large liposomes (diameter less than 1 μm) were largely used by biochemists, cell biologists and structural biologists for bulk in vitro assays. However, due to their small size, these liposomes were not suitable for optical microscopy. More recently, giant unilamellar liposomes (GUVs; diameter larger than a few μm) have become popular among biophysicists. These biomimetic systems can be designed using a minimal number of components, including various lipids¹, membrane proteins or cytoskeleton proteins². Long membrane nanotubes can be pulled out of these GUVs when a mechanical force is applied, or when molecular motors, bound to the membrane, move along microtubules³,4.

Interestingly, techniques used to study cells, such as videomicroscopy, fluorescence microscopy, fluorescence correlation spectroscopy (FCS) and fluorescence recovery after photobleaching (FRAP), are adapted to these systems, as their sizes are similar; moreover, the number of physical parameters is much more limited for liposomes than for cells, and these parameters (for instance, membrane tension and bending rigidity) can be measured and controlled¹. Because of the small number of components involved, the effect of adding extra proteins to the system can also be identified. One of the great advantages of biomimetic systems is that they can be directly compared with theoretical physical models to test hypotheses on cellular mechanisms. However, the full complexity of the cell machinery cannot be reproduced in vitro. In addition to their potential biotechnological applications, biomimetic systems are useful tools for biologists, providing that key questions on cell functions have been previously identified.

References

1. Baumgart, T., Hess, S. T. & Webb, W. W. Imaging coexisting fluid domains in biomembrane models coupling curvature and line tension. Nature 425, 821-824 (2003)

2. Giardini, P. A., Fletcher, D. A. & Theriot, J. A. Compression forces generated by actin comet tails on lipid vesicles. Proc. Natl Acad. Sci. USA 100, 6493-6498 (2003)

3. Koster, G., VanDuijn, M., Hofs, B. & Dogterom, M. Membrane tube formation from giant vesicles by dynamic association of motor proteins. Proc. Natl Acad. Sci. USA 100, 15583-15588 (2003)

4. Roux, A. et al. A minimal system allowing tubulation using molecular motors pulling on giant liposomes. Proc. Natl Acad. Sci. USA 99, 5394-5399 (2002)

Full list of key questions

Session key questions:

What techniques (both fluorescence- and non-fluorescence-based) can help us detect membrane microdomains and measure affinities of protein-lipid and protein-protein interactions within membranes?