Fats are carried in the bloodstream by lipoprotein particles. The delivery of these fats to the different tissues in the body is dependent upon the proteins which are attached to the surface of the lipoprotein particles. Abnormalities in the structure and concentrations of the apolipoproteins affect their efficiency in the transport of fat through the circulation. Most previous research has focused upon the primary amino acid sequence of these proteins. We have undertaken a series of basic research studies identifying, characterizing, and evaluating the physiologic relevance of several post-translational modifications of the apolipoproteins. We have previously determined that several apolipoprotein undergo intracellular phosphorylation, fatty acid acylation, and glycosylation. We have made the following observations: 1. O-linked glycosylation of apolipoprotein A-II markedly affects the binding of this protein to lipoprotein particles. 2. The glycosylated form of apoA-II does not associate with high density lipoproteins, providing a means of potentially modifying apolipoprotein physiologic function. 3. The tissue-specific secretion of nascent apolipoproteins to either the apical or the baso-lateral surface of cells is dependent on cell-specific mechanisms of intracellular protein-trafficking and is not solely dependent on structural information present in the apolipoprotein. 4. Methods of using isoelectric focusing for apolipoprotein purification have been developed which now permits the direct evaluation of apolipoprotein A-I by electrospray and plasma desorption mass spectrometry to discern the amino acid reside in pro-apoA-I which undergoes fatty acid acylation. Only the pro-isoform of apolipoprotein A-I undergoes fatty acid acylation. 5. Phosphorylation of apolipoprotein B in the hepatoma cell line Hep G2 leads to enhanced intracellular degradation of the protein. These combined results indicate that a variety of post-translational modifications of the apolipoprotein affect nascent lipoprotein particle synthesis and secretion.
