Lipoprotein biosynthesis: Examination of the lipidation process

Abstract

Lipoproteins, protein-lipid complexes that have a polar exterior and a non-polar interior, have been found in many vertebrate and insect species, and their basic structure and function appear to be conserved. They facilitate the intercellular transport of hydrophobic lipids through aqueous media. Their synthesis requires an unusual process referred to as lipidation, whereby lipids are added to the protein component of the lipoprotein. Lipidation is thought to occur during or immediately following translation of these proteins, but how this process occurs is unknown. Of particular interest is the extent to which the protein sequence of the apoproteins drives lipidation and the level of involvement of other proteins in this process. In this project, lipidation was studied by expressing the apolipoproteins from the tobacco hornworm, Manduca sexta, in two different expression systems. The first used budding yeast, Saccharomyces cerevisiae , to both determine the ability of unicellular organisms to produce lipoproteins and examine the role that the known secretory pathway for soluble proteins plays in lipoprotein biosynthesis. The second used Drosophila Schneider 2 cells to begin to examine the apolipoprotein sequence for regions that are crucial to lipoprotein biosynthesis. The yeast expression system revealed that unicellular organisms are capable of expressing, lipidating, and secreting M. sexta apolipoproteins. This is first demonstration of any apolipoprotein being expressed in a unicellular organism, and represents a major finding, as unicellular organisms have no need for a particle that functions in intercellular transport. A second major finding in this project is that lipophorin production occurs in the absence of the full apoLp-I sequence. This finding was true for both expression systems, and indicates that the lipidation code resides within the first 45% of the precursor protein sequence. Furthermore, deletion analysis has revealed that removal of any portion of the apoLp-II sequence prevents expression of the apolipoprotein. Taken together, these experiments indicate that all of the information required to make a lipoprotein is included in the apoLp-II sequence.