Players in the Nonpolar Lipid Game: Proteins Involved in Nonpolar Lipid Metabolism in Yeast

Abstract

Synthesis, storage, and degradation of nonpolar lipids enable cells to continue cell metabolism when nutrients are no longer provided by the environment. Major nonpolar lipids occurring in yeast are triacylglycerols and steryl esters. These hydrophobic molecules are sequestered from the cytosolic environment in the core of special organelles termed lipid droplets (lipid particles). When nutrients are no longer provided by the environment, hydrolytic enzymes catalyze the degradation of triacylglycerols and steryl esters. The respective breakdown products serve as energy source and/or building blocks for membrane formation. Here, enzymes catalyzing nonpolar lipid synthesis and degradation in the budding yeast Saccharomyces cerevisiae are described with special emphasis to their localization and regulation. Furthermore, examples are presented showing that the formation of lipid droplets is not only disturbed in cells defective in polypeptides directly involved in nonpolar lipid synthesis and degradation, but also in a number of mutants defective in polypeptides and pathways which are not obviously linked to nonpolar lipid turnover. Although research over the past decade provided major insights into nonpolar lipid metabolism, many aspects of nonpolar lipid synthesis, storage, and degradation remain to be elucidated. Research needs for a better understanding of nonpolar lipid turnover are outlined at the end of this chapter.