LIPID-PROTEIN INTERACTIONS IN BIOLOGICAL AND MODEL SYSTEMS

Without membranes, there are no living organisms. Membranes separate the cells from the outside world, but at the same time membranes embed those proteins, which sense the signals of the outside world and perform material transport through them. In the interiors of the cells, the generation of energy takes place in membranes, including the primary energy source for all forms of life on Earth, photosynthesis. After sequencing the complete genome of several organisms, it seems that about 30% of their sequences encode membrane proteins. In contrast, from the several tens of thousands of proteins whose 3D structure is known in atomic resolution, only a few dozens are membrane proteins. The reason for this discrepancy between abundance and knowledge is that membranes are very complex assemblies, where proteins and lipids together form functional units. The lipids supply the insulation capacity of the membranes, and their hydrophobic double layer provides the working conditions for membrane proteins. Isolated membrane proteins are therefore very difficult, and frequently practically impossible to study, and there is always the intriguing question: to what extent the obtained structures, features correlate with those existing under in-membrane conditions.

For the resolution of this problem, two interrelated lines of research can be pursued. On the one hand, whole, possibly intact biological membranes can be studied as they are, by applying methods selectively sensitive for different membrane properties. On the other hand, model systems can be developed which, by mimicking natural conditions as close as possible, permit the study of individual components or particular processes. Then, by compiling the data obtained by the two approaches, one may hope to obtain a comprehensive picture of the lipid-protein interactions in biological membranes.

According to this strategy, we apply and further develop the perfectly non-invasive Fourier transform infrared (FTIR) spectroscopy in biological membranes. This method has the advantage of having the lipid- and protein-related spectral regions well separated. Thus, both lipids and proteins can be studied individually and, furthermore, via the correlations between changes in the lipid- and protein-related regions the lipid-protein interaction in the membranes can also be addressed.

Figure 1: Schematic view of the attenuated total reflection (ATR) mode of the Fourier transform infrared (FTIR) measurement. PEI - a positively charged polyelectrolyte, which adheres very well to surfaces. PGA – Poly(glutamic acid) – a negatively charged poly amino acid; PLL – Poly(lysine) – a positively charged poly amino acid.

In the other direction, developing better model systems, we use a nano-technological approach. We build protein-like surfaces layer-by-layer from oppositely charged polyelectrolytes. Thus, we can obtain surfaces, which mimic the cytoskeleton of the cells. Then, biological and model membranes can be adsorbed onto these surfaces. In this way, transport processes and lipid-protein interactions can be studied on a stable support on both fully natural membranes and “hybrid” systems built from natural and synthetic components. Infrared spectroscopy also plays an important role in these studies, since FTIR spectroscopy when used in attenuated total reflection mode (ATR) (which means that we measure the adsorbed sample on the surface of an internal total reflection element) can follow the structural changes associated step-by-step with the buildup of the polyelectrolyte film and the adsorption of any further component.

Concerning the complexity of biological membranes and the need for developing new techniques for their study, at the beginning our research is purely basic research, and we are in this phase now. We keep in mind, however, that detailed knowledge and effective experimental techniques can have important bio-medical applications. Therefore, when selecting the systems for study, we consider biologically, medically important objects (selected lipid species, ion-channel proteins, etc.).