Lipid control of membrane protein folding

Proteins are the 'worker molecules' in life. Cells in our bodies sense and communicate with the outside world via proteins embedded in membranes that surround the cells. These membrane proteins constitute about a third of the proteins in our bodies, and over half of the current targets for new medicines. Unfortunately, there is a limited understanding of how membrane proteins work at a molecular level. This is because they are notoriously unstable outside the membrane and it is difficult to obtain sufficient amounts for scientific study. Recent advances are, however, beginning to alter this situation. We aim to devise new methods to study a phenomenon known as 'folding'. Genes carry the code for proteins, but puzzles remain in deciphering how genetic information is translated into functional proteins. Proteins begin as a string of amino acids, which then have to fold-up to a particular shape in the right part of the body. If this folding fails, disaster strikes and the proteins malfunction. The most glaring gaps in knowledge of this folding phenomenon come with membrane proteins. We propose to forge new territory by devising new methods to understand membrane protein folding. We intend to do this by focussing on the other main component of membranes: lipids. These lipid molecules are the basic building blocks of membranes, effectively the bricks that make up these biological walls and they influence how their neighbouring proteins behave. We already know that certain lipid properties can be used to alter the protein folding and we now aim to understand the precise details of this control.

Lipids participate in many biological processes and actively influence membrane function. Elastic properties of lipid bilayers, including curvature and lateral pressure are increasingly being recognised as key controlling factors. We have shown that these lipid properties alter the folding of proteins within membranes. Our hypothesis is that lipids actively control folding. Here, we propose to elucidate the exact control mechanism and identify the lipid properties that manipulate the rate and yield of folding for helical membrane proteins. We will exploit the lipid control mechanism to study the folding of potassium and mechanosensitive channels. We aim to measure lipid elastic parameters under biological conditions; at present these parameters are only known in very artificial environments. We will measure them in the presence of buffers, membrane proteins and in asymmetric bilayers. We will also measure protein folding in lipid bilayers under the same conditions. By judicious alterations of the lipid compositions we can then determine correlations between specific lipid elastic properties and folding events. We combine state of the art approaches in Physics and Biochemistry. We have developed high throughput methods to measure lipid parameters and pioneered a range of techniques to monitor folding in lipids. We will also use novel methods to create lipid vesicles with asymmetric lipid compositions, to study the influence of asymmetry on folding. Understanding the origin of lipid control is crucial to predicting the properties of biological membranes that maintain the fold and function of membrane proteins, and thus mimicking these critical properties in vitro. We propose that bilayer asymmetry is a key factor. The different lipid compositions of the inner and outer leaflets of the bilayer will produce an asymmetric distribution of lateral forces that is likely to be vital to stabilising integral membrane proteins, which too have asymmetric structures.