Mixed and active membranes

In this proposal I describe my plan to attack several important open problems facing our understanding of multi-component, non-equilibrium fluid membranes. In a biological context these often interact with the cytoskeleton of a living cell. I will study how membrane microphase separation is induced by applied forces, such as might arise from coupling to the active cytoskeleton. This model is desperately needed to understand recent data, e.g. for large variations in the local membrane diffusion constant.I also plan to study the regulation and transient adaptation of the membrane tension and pressure of a living cell. I will construct an appropriate non-equilibrium model for the creation of transient bonds between the membrane and the cytoskeleton and use this to establish a self-consistent theory for the steady state, and time variation, of these quantities. Forces generated by molecular motors anchored to the cytoskeleton act on these bonds. The lifetime of the bonds depends on the membrane compliance, which in turn depends on it tension and pressure in a way that can be computed self-consistently. The transient response of the cell following rapid changes in volume or membrane area, can be probed by modern micropipette or tether-pulling experiments. We thereby hope to construct a theory for the active mechanical properties of the cell membrane.I will also work to rigorously test the long standing but untested Saffman-Delbruck theory for membrane diffusion. I will analyse experimental data from Prof Bassereau's group at the Institut Curie. These ongoing experiments have been motivated by our recent theoretical analysis of flows on cylindrical membrane tubes in which we suggest that the controlled variation of tube radius in this geometry may provide the most discriminatory test of the Saffman-Delbruck theory yet proposedI will also study the assembly and dynamics of the FtsZ contractile ring. This protein, a bacterial analogue of tubulin, assembles into fibers that form a ring around dividing bacterial cells. This process involves molecular force generation, fibre self assembly, depolymerisation and membrane-mediated forces, all of which are fields in which I have significant expertise. Very recently model in vitro experiments have studied the role of FtsZ in generating membrane deformation. We plan to repeat similar experiments in membrane vesicles but with the addition of additional vital molecular components, such as FtsA, which plays an important role in associating the FtsZ ring with the membrane. A testable physical model for the action of these dividing rings in generating controlled membrane deformation would be most useful in a field which is of the very highest contemporary interest. There is the hope that this may also have pharmaceutical relevance, e.g. in new antibacterial treatments that target the cell division apparatus.There is an additional element to this proposal which is not included here for reasons of confidentiality.

This proposal offers substantial opportunities for commercial, as well as academic impact. In this summary I will focus first on those with the greatest potential commercial impact. Our proposed study on the FtsZ ring and how it generates force is significant because of the physical role this ring plays in the bacterial cell division process. This system is of significant contemporary interest to cell biologists and biophysicists. FtsZ is also a proven antibiotic target. If its activity is disrupted bacteria can no longer divide. A theoretical model of the FtsZ cell division apparatus is therefore a potentially useful tool for the refinement of existing drugs or the development of new treatment strategies. The FtsZ ring is known to contain a number of cofactors whose role we aim to study in the project. There is the opportunity for commercial exploitation based on new discoveries about how bacterial cell division is controlled by these. We expect the ultimate beneficiaries to be UK pharmaceutical companies although we may also become involved in small scale commercialisation, e.g. through spin out companies and patents. To ensure that this benefit accrues I will remain in close contact with Warwick Ventures, the University's business development arm. I am familiar with Warwick Ventures having recently established another small (medical device related) spin out company. I will also rely on contacts through my project partner Dr Dafforn, who is already contact with several pharmaceutical companies. Dr Dafforn has also developed an approach for solubilising small (10nm) membrane disks. These will be important in the experimental work proposed here because they provide a convenient way to manipulate membrane proteins (some of which are known cofactors for FtsZ). The development of a theoretical model with which to understand the formation and stability of these disks is also proposed here. This may result in further refinements to the technology. Intruigingly, the small size of these disks also makes them unusually sensitive to the Gaussian rigidity of the membrane, a property which is notoriously difficult to measure but which can drive dramatic changes in the macroscopic structure of the membrane. This may make these disks a perfect Gaussian rigidity sensor, perhaps allowing us to calibrate these membrane rigidities or even sort membrane into disks according to its gaussian rigidity. There is another theme in this project that offers unproven but exciting opportunities for commercial investment but is not listed here for reasons of confidentiality. The overall objective of this work is to develop new theoretical models to enable us to better understand the biophysics of living cells. I expect that our work will be of substantial interest, not just to researchers at the physical-life sciences interface but also in soft condensed matter physics, biological chemistry, computer science, engineering and cell and molecular biology. We are committed to timely publication in multi-disciplinary journals such as Nature, Science, Biophysical Journal, Physical Review Letters, PLoS J. Computational Biology etc. The PI and PDRA will ensure impact by attending, and giving presentations at, international meetings, such as the American Society for Cell Biology meetings, the US Biophysical Society meetings the European Conferences on Mathematical and Theoretical Biology etc. It is also a priority to engage engineers, e.g. by attending the IEEE conferences. The PI, with the help of the PDRA, will establish a website for the free dissemination of code and results. This website will also include sections providing lay summaries of the work for the general public. We will engage the E-learning team at Warwick, which specialises in developing internet video and audio for the promotion of the University's research, to communicate our scientific activities.