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Materials Science Division |
Ursula Perez-SalasPrincipal Investigator
Materials Science Division; Bldg. 223 Phone: 630-252-8621 Research InterestsMy research interests are focused on investigating the physical properties of biological membranes and obtaining structure information on membrane proteins using solutions of lipid mixtures as model systems. Membranes in cells, from the cell's outer wall to the enclosing boundary of the different organelles in the cell, are highly selective barriers. Membranes are mainly impermeable lipid bilayers and their selectivity comes principally from the specific membrane proteins embedded in and spanning the lipid bilayer. Membrane proteins are responsible for the communication between intracellular compartments and between the cell and its environment. Membrane proteins, lipid composition and other associated membrane components, like cholesterol, give a membrane not only its functional specificity, but also leads to specific structural differences that enable their function. As can be seen in the figure below, one obvious structural difference, is that some are folded to form a stacked system while others remain locally extended. Further, these membranes are not structurally static. Local changes are needed, for example, to form or incorporate (fuse) vesicles which are small, spherical, membrane-enclosed compartments responsible for transporting compounds in and out of the cell as well as inside the cell (see figure). Although these structural differences and changes in the membranes of the cell are due to specific composition variations on the plane of the membrane, they can be broadly viewed as being driven by a "softening" process, where the elastic membrane energy is key. Understanding this softening process and how it relates to structure in the plane of the membrane - which ultimately enables its function - is one of my main research interests.
Model systems that will shed light on the structural differences found in biological membranes are mixed lipid solutions. These have rich phase diagrams, including bicontinuous regimes, from which the physical properties of the bilayer can be extracted by scattering experiments. Complementary techniques like rheology and optical microscopy add to the full characterization and basic understanding of the mixed lipid solutions physical properties. The compositions in these mixed lipid solutions will emulate the compositions found in biological membranes and thus the corresponding results will be extrapolated to the biological membranes. Another exciting aspect of lipid mixture solutions exhibiting the bicontinuous cubic phase, is that they have been recently found to be a promising template for crystallizing membrane proteins (see figure below). In spite of the fact that membrane proteins are encoded by a third of the genes in multi-cellular organisms, the number of proteins whose structure has been obtained lags behind soluble proteins by many orders of magnitude. A clear understanding of the phase behavior of mixed lipid solutions that exhibit the cubic phase will allow the in cubo technique to be used more generally. Control on lipid composition, temperature range and size of the cube unit will make the system very flexible to accommodate many membrane proteins, potentially allowing many more structures to be obtained. At present, the lack of membrane protein structural data obscures our understanding of the biological membrane and slows the development of biotechnologies that would benefit from this knowledge.
M. Caffrey, Journal of Structural Biology 142 (2003) 108 Education
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