Research Focus

Lipid transporters and membrane asymmetry

Our knowledge on the composition and architecture of biological membranes has grown substantially in the past decades. However, we are just beginning to functionally and structurally chara​​cterize the molecular identities that underlie the complex organization of bilayers. One important feature of biomembranes is the asymmetric distribution of specific lipids between the two leaflets of the bilayer. This distribution is also subject to dynamic changes and serves important functions. For instance, exposure of phosphatidylserine to the extracellular leaflet– normally confined to the intracellular leaflet of the plasma membrane in the resting state – is a crucial event in processes as diverse as apoptosis, blood clotting, bone mineralization, myoblast fusion and in cell to cell spreading of viruses (apoptotic mimicry).

Crystal structure of the TMEM16 lipid scramblase from Nectria haematococca. The lipids traverse a hydrophilic membrane-facing spiral groove (arrows) in each subunit (yellow and cyan) upon activation by Ca2+. The hydrophilic surface lowers the energy barrier for the lipid headgroups to pass the hydrophobic interior of the bilayer which enables the rapid exchange of lipids between the leaflets.

We are interested in the origin and significance of membrane lipid asymmetry in bilayers throughout the cell and its concerted breakdown as signaling cue. In our research activities we focus on membrane proteins that impact the organization of the lipid bilayer, in particular lipid transporters that contribute to membrane lipid asymmetry and scramblases that randomize the distribution of lipids between the leaflets. We investigate how certain lipid species reach their destination in a distinct leaflet of the bilayer, why lipid asymmetry is important in membranes and which functions are elicited through the concerted breakdown of this asymmetry by scramblases. To address such questions, we structurally describe lipid transporters to reveal their ‘inner workings’ and regulation. The subcellular localization of these transporters in order to understand the cellular function as well as the network of interacting proteins is another emphasis. Our efforts aim to contribute to a deeper understanding of the involved proteins in lipid trafficking and signaling in healthy and pathological conditions.

Ion channels 
In the second line of research we investigate ion channels, particularly of intracellular membranes like the lysosome. The lysosome is the cellular recycling machinery and is increasingly recognized as sensor for the metabolic state of the cell. It is the destination membrane for fusion with autophagosomes, large vesicles that form in the cytosol to enclose solutes and proteins in bulk for subsequent degradation in this recycling organelle. This interplay makes lysosomes thus also pivotal for the removal of protein aggregates from the cytosol and connects them to neurological diseases like Parkinson disease. Ionic gradients across the lysosomal membrane are crucial for the function of the organelle making ion channels important regulators of lysosomal transport and fusion as well as the degradation of lysosomal contents. Consequently, modulation of ion channel activity in these organelles holds great prospects for medical applications. In our lab, we strive to understand how lysosomal ion channels achieve selectivity and how they are gated to open or close. We further investigate the significance of ion gradients across the lysosomal membrane for the turnover of autophagosomes and maintenance of the luminal pH. 


 

Electron density map of the TMEM175 channel (viewed from top) with a central K+ ion (red) and surrounding water molecules (green) visible.

Method development
Besides our basic research activities, we develop methodologies to greatly reduce the required biomass and sample consumption for the determination of membrane protein structures by cryo-EM. 

Crystal structure of the non-canonical TMEM175 K+ channel of Marivirga tractuosa with bound macrobodies (cartoon). The macrobodies, made of nanobodies fused to MBP act as a crystallization chaperone and enabled a high resolution structure of this channel