The long range goal of this program is the understanding of the role of proteolipid in the structure and function of myelin. The proteolipid is a hydrophobic membrane protein that is the major protein component of CNS myelin. The recent elucidation of its amino acid sequence has led to the development of a model for the orientation of the proteolipid within the membrane and this, in turn, permits us to address specific questions relevant to the chemical architecture of myelin. The key questions relate to the oxidation state of cysteine/cystine residues, the location of exposed domains and the site of covalently bound fatty acid. The overall approach is to label the protein with a suitable radioactive probe; the protein is then cleaved with proteolytic enzymes, labelled peptides are isolated, characterized and their location within the sequence and within the model determined.
The specific aims of this project are: 1) to determine the location of disulfide and sulfhydryl groups in the isolated proteolipid and in myelin membranes by labelling with radioactive sulfhydryl reagents: 2) to identify those segments of the protein that are accessible at the external surface, using impermeant chemical probes; 3) to determine if the orientation of the protein is the same in liposomes reconstituted with the apoprotein as it is in myelin; and 4) to determine the site of fatty acid esterification after in vivo acylation of the protein. Our approach is specifically relevant to the elucidation of factors that maintain the compact multilamellar structure characteristic of myelin. The approach also permits identification of sites normally exposed to immune surveillance and those which may become exposed upon myelin breakdown. The studies are part of an overall interest in lipid-protein interactions and are directed to understanding the control of myelin stability and the alterations in demyelinating disorders such as multiple sclerosis and the leukodystrophies.
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