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 lipoprotein particles lead to human disease. The structures of the different 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 investigations identifying, characterizing, and evaluating the physiologic relevance of several posttranslational modifications of the apolipoproteins. We have previously determined that several apolipoproteins 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 basolateral 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 structure. 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 animo acid residue in proapoA-I which undergoes fatty acid acylation. Only the pro-isoform of apolipoprotein A-I undergoes fatty acid acylation. These combined results indicate that a variety of posttranslational modifications of the apolipoproteins affect nascent lipoprotein particle synthesis and secretion. In addition, the study of apolipoprotein posttranslational modification may lead to novel approaches to prevent premature cardiovascular disease.
