Organizing potential of sphingolipids
Sequenced genomes do not permit meaningful predictions on the dynamic properties of the supramolecular membrane systems of cells. A major challenge in cell biology is to understand how lipids influence proteins and how proteins influence lipids in cellular bilayers. Using a bioinformatics-based cloning strategy, we previously identified two families of sphingolipid biosynthetic enzymes that drive membrane maturation by converting fluid bilayers into rigid ones [Huitema et al., 2004; Vacaru et al., 2013]. By redirecting these enzymes from ‘late’ to ‘early’ secretory compartments, we seek to uncover novel molecular principles by which cells exploit the cross-talk between proteins and lipids to sustain their compartmental organization.
Candidate sensor for suicide lipid ceramide
Particularly exciting is our recent identification of a candidate ceramide sensor that acts as suppressor of mitochondrial apoptosis [Vacaru et al., 2009; Tafesse et al., 2014]. Ceramides are the precursors of all sphingolipids, but also act as potent mediators of programmed cell death. By closely monitoring ceramide levels in the ER, the candidate sensor prevents ceramides from ‘leaking’ into mitochondria where they trigger cell death. Our current efforts are aimed at: i) understanding how the sensor works; ii) evaluating its potential as target for modulating drug-induced apoptosis in tumors; iii) elucidating the mechanism by which ER ceramides reach mitochdondria to commit cells to death.
Hunt for novel sphingolipid effectors
We contribute to the development of novel tools to elucidate the cross-talk between proteins and lipids in cellular bilayers. For a global analysis of protein-lipid interactions in cells and organisms, we use bifunctional lipid analogues that contain a photoactivatable and clickable group [Haberkant et al., 2013]. Cellular proteins in contact with these analogues are first tagged by photoaffinity labeling and then derivatized with a reporter molecule (fluorophore, biotin) using click chemistry, allowing their visualization and purification. We currently use this approach to track down novel effectors of sphingolipids (ceramides, ceramide phosphoethanolamines) and thereby improve our understanding of how these unusually versatile biomolecules help execute key physiological and pathological processes.
Origin and impact of lipid asymmetry
Our group carried out pioneering studies on lipid pumps responsible for creating lipid asymmetry in ‘late’ secretory and endocytic compartments [Pomorski et al., 2003; Alder-Baerens et al., 2006]. We found evidence that these so-called flippases participate in endocytic vesicle formation at the plasma membrane, hence challenging the dogma that this process is primarily governed by the assembly of cytosolic coat proteins. We envision that flippases contribute to membrane vesiculation by creating an imbalance in surface area between the two leaflets of the bilayer. We are testing this model by cell-free translation of flippases into giant liposomes. Our recent studies indicate that flippases evolved from an ancient family of cation pumps through the acquisition of accessory subunits [Lenoir et al., 2009; Bryde et al., 2010]. Our ongoing efforts are aimed at elucidating the inner workings of flippases at the molecular level [Puts et al., 2012].