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HEIDELBERG UNIVERSITY

Britta Brügger 
Modulation of protein activities by interactions with lipids & Lipidomics

Groupleader: Britta Brügger

The traditional view of membrane lipids serving only as structural barriers between compartments and as solvents for membrane lipids has fundamentally changed over the past years. Our current perception of biological membranes is that these are dynamic assemblies of microdomains characterised by different protein and lipid compositions. Coexistence of such domains is thought to help organise important membrane activities in a reversible manner. Given the fact that membranes are composed of hundreds of different lipid species, it is believed that individual lipid species fulfill very specific functions, e.g. in generating specialised membrane domains or in specifically interacting with membrane proteins. While there are many examples of proteins interacting with lipid head groups, specific interactions of membrane proteins with lipids within the hydrophobic phase of the lipid bilayer are largely unexplored.

We have established novel in vivo and in vitro chemical biology tools to address the question if and how single lipid species interact specifically with proteins within the lipid bilayer. Capitalising on these tools we discovered a direct and highly specific interaction of one sphingomyelin SM species with the transmembrane domain TMD of p24, a protein involved in vesicular trafficking. This specific interaction is defined by a signature within the transmembrane domain of p24 that is responsible for lipid selection. Bioinformatic analyses that were performed in the laboratory of Gunnar von Heijne (Stockholm) using transmembrane proteins predict that this signature represents a conserved sphingolipid-binding motif in mammalian membrane proteins.

Our goal is to define structural determinants and functional consequences of lipid binding (covalent and non-covalent) to proteins. We are using multidisciplinary approach using chemical biology, biochemistry, cell biology and mass spectrometry to unequivocally identify lipids bound to motif-containing protein candidates and to understand functions of such protein-lipid interactions. Candidate proteins under investigation are cell surface receptors such as proteins of the family of metabotropic glutamate receptors. A potential function of non-annular lipids as cofactors for receptor activity, e.g. by facilitating dimerisation/assembly of (e.g. signalling-) active complexes makes these protein-lipid interactions a novel target for the development of small molecule-based drugs with a future perspective for translational medicine approaches.


Lipidomics

Lipidomics aims at providing qualitative and quantitative data on lipid profiles of a given sample being it as simple as a single lipid binding to a protein or as complex as subcellular organelles, tissues, up to multicellular organisms with the ultimate goal to understand biological functions of lipids in health and disease. Although this research field has emerged only recently, it is rapidly expanding. Lipids are increasingly recognised as important modulators of many intracellular processes, from regulation of protein function to modulation of cellular pathways. Mass spectrometric shotgun lipidomics approaches allow assessing the lipid composition of either total membranes or protein-lipid-assemblies directly from extracts of biological samples.
The Lipidomics facility builds on unique expertise in qualitative and quantitative lipid analysis by nano-mass spectrometry. Depending on the scientifc question, four complementary nano-platforms are available: a hybrid quadrupol Oribtrap mass spectrometer, a hybrid quadrupol time-of-flight mass spectrometer, a hybrid triple quadrupole linear ion trap mass spectrometer, and a triple quadrupole mass spectrometer. The triple qudrupole system operates in single injection mode, whereas the other three instruments are coupled to Nanomate devises. Sample analysis can be performed directly with an online nano-UPLC separation of lipid species prior to the MS experiment.

Over the last ten years we have continuously expanded our methods and tools towards a comprehensive and quantitative analysis of lipids.
Our ambition is to develop a technology platform that will cover analysis of this whole range of lipids within one laboratory. A long-term goal we aim at is analysing the lipid composition of single cells. First reports in the literature indicate that this goal can be achieved by optimizing novel nanostructured-surfaces. This approach would allow, for the first time, to analyse dynamic changes of lipids within a single cell rather than to measure a pool of cells in different states.

For details see also @ Facilities/Lipidomics 
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