STRUCTURAL STUDIES OF LIPID/ PROTEIN SYSTEMS

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X-ray crystallography will be used to study the molecular structure of lipid:protein complexes ranging from extracellular proteins such as lipovitellin and apo Al to small intracellular lipid binding proteins. All the studies play a role in understanding the stabilization of lipid in an aqueous environment along with associated health related problems which involve the transient flux of lipids. Lipovitellin from lamprey oocytes is one of the crystalline proteins. It contains 15% w/w of lipid. The present model, though incomplete, has been obtained by x-ray and neutron diffraction data. The current focus is the assignment of the amino acid sequence to the electron density map. A second study will involve crystallographic studies of both apo Al, a constituent of HDL, and reconstituted apo Al :lipid particles. Using an existing form of recombinant proapo Al, a new expression system will eliminate the "pro" piece and add five histidine residues to the carboxy-terminal. This will permit a simple and large scale purification protocol. Another aspect of the studies focusses on a family of intracellular lipid binding proteins which are responsible for the translocation of fatty acids and retinoids within the cell. The background studies have shown that the hydrophobic compound is contained within an internalized water filled cavity.. Site- directed mutagenesis and crystallography will be used to gate ligand binding and to alter the internalized structured water. A protein which binds retinoic acid in a manner analogous to the fatty acid and retinol binding proteins will be analyzed by x-ray crystallography. The fourth and last study involves the purification and crystallization of the ligand binding domain of the nuclear receptor protein, RXR. The receptors for vitamin A analogues (RXR and RAR) play a dominant role in the development of a body plan as shown by teratogenic effects in embryos. These hormonal properties are the result of effects on the transcriptional rates of special targeted genes. Although structural studies on the segment of the protein which binds to DNA hake been completed, no three dimensional data on the retinoic acid-binding domain is available. Studies of RXR are closely related to those of the intra- cellular retinoic acid binding protein mentioned above.