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. Over the past year we have made the following observations: 1. Using site-directed mutagenesis of the serine 201 residue that is phosphorylated and comparing mutagenized and native apolipoprotein A-I transfection methods, we have established that apolipoprotein A-I is secreted independent of phosphorylation. 2. We have determined that apolipoprotein B undergoes phosphorylation on serine residues by protein kinase C both in vitro and in vivo. 3. Phosphorylation of apolipoprotein B affects intracellular degradation of nascent protein. 4. The evaluation of apolipoprotein A-I by electrospray and plasma desorption mass spectrometry indicates that only the pro- isoform of apolipoprotein A-I undergoes fatty acid acylation. 5. O- glycosylation of apolipoprotein A-II directs the association of this apolipoprotein to HDL3, compared to the nonspecific association of nonglycosylated apoA-II with all HDL subspecies. 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.
