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Structural study of unconventional proteins, the membrane Hairpin Proteins, using DISCO light.
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National audience. Membrane proteins are proteins that interact with biological membranes. They are targets of over 50% of all modern medicinal drugs. Membrane proteins perform a variety of functions vital to the survival of organisms: signal reception and transduction, transport of molecules and ions across the membrane, enzymatic activities, etc. Various topologies are described such as, transmembrane proteins spanning the phospholipid bilayer or proteins attached to only one side of the membrane via hydrophobic loop, amphipathic helices or lipid anchor. However, one class of membrane protein is poorly described in the literature: membrane hairpin proteins. These proteins are present in all organisms and some of them are found associated with lipid droplets (LD). These observations suggest that membrane hairpin proteins share biological and biophysical properties. LD are lipid storage intracellular nanoparticles consisting mainly of a core of neutral lipids (triacylglycerols and/or steryl esters) enclosed in a monolayer of phospholipids. LD contain a number of proteins which vary considerably with the species. Some structural hydrophobic proteins stabilize the interface between lipid core and cellular aqueous environment. Seed LD are completely covered by oleosins, which belong to the membrane hairpin protein family. We developed a genetic approach using Saccharomyces cerevisiae to obtain heterologous expression of plant oleosins, but also mammalian stomatin and caveolin, and hepatitis C virus core protein. These transformed yeasts and purified LD were used for functional and structural studies. Yeast expressing oleosins overaccumulated LDs leading to a specific increase of storage lipids and these LDs harbor a high level of oleosins. Purified LDs were exposed to synchrotron light on DISCO beamline. We obtained structural data on whole cellular organelle and on oleosins inserted in LD using Sychrotron Radiation Circular Dichroism (SRCD).
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