Molecular function of CAP superfamily members.

Sterols, essential lipids of eukaryotic cells, are synthesized in the endoplasmic reticulum (ER) then rapidly trafficked to populate many organelles, mainly the plasma membrane (PM). However, the molecular mechanisms that control sterol transport are not well characterized. Our aim is to investigate the mode of sterol transfer by studying proteins characterized as lipid transfer proteins (LTPs). Under anaerobic or heme deficient conditions, Saccharomyces cerevisiae takes up sterols from the environment, incorporates them into the PM and transports them back to the ER, where they are converted into steryl esters. We have previously identified a sterol acetylation/deacetylation cycle, mediated by two ER enzymes: Atf2 and Say1. This cycle acts as lipid detoxification pathway in which some compounds are acetylated and secreted into the culture media whereas others are deacetylated and retained within the cell. Subsequently, we identified proteins called Pathogen Related in Yeast (Pry) as being required for sterol binding, solubilization and secretion. By using yeast cells lacking the capacity to synthesize Heme and deleted for Say1, Pry1 and Pry2, we studied the binding and export of sterols by different LTPs.