Dr. Toon de Kroon

STUDENT PROJECTS are available in the research lines listed below

Membrane Lipid Homeostasis

Cells and cell organelles are confined by membranes, approximately 6 nm thick boundaries that consist of lipids organized in a bilayer with proteins embedded and associated. Membrane lipid composition determines the physical properties of biological membranes that are crucial for maintaining the membrane barrier and for the functioning of membrane proteins. Research in the group is aimed at understanding the principles that govern bulk membrane lipid composition and at identifying the underlying sensor-effector modules. These topics are investigated in the model eukaryote S. cerevisiae (baker’s yeast) using biochemical, molecular biological, and genetic approaches.

Regulation of the acyl chain length of membrane lipids

Proper function of membranes and membrane proteins critically depends on the regulation of the fluidity/viscosity of the membrane’s lipid matrix. This applies particularly to poikilothermic organisms including the model eukaryote baker’s yeast, that adapt their membrane lipid composition in response to changes in ambient temperature. Yeast can adjust the fluidity of its membranes by tuning the level of unsaturation and the average length of the lipid acyl chain moieties. Whereas the sense-and-response mechanism regulating acyl chain unsaturation has been elucidated, the mechanism controlling acyl chain length remains elusive.

Recent data suggest that yeast adapts the ratio between 16- and 18-carbon atom-long acyl chains in its membrane lipids by adjusting the activities of the enzymes acetyl-CoA carboxylase (Acc1) and fatty acid synthase (FAS). Acc1 synthesizes malonyl-CoA, the activated two-carbon donor that is used as substrate by FAS in an iterative process producing C16 or C18 fatty acyl-CoA. 
We hypothesize that average acyl chain length is determined by a tug of war between Acc1 and FAS, i.e. increased activity of Acc1 vs. FAS favors a rise in C18 over C16, whereas increased activity of FAS vs. Acc1 favors a drop in C18 over C16.

Our current research aims to prove the interdependence of Acc1 and FAS activities in shaping acyl chain length, and to resolve the molecular mechanism(s) regulating Acc1-FAS.

CoA biosynthesis and membrane lipids

Coenzyme A (CoA) is an essential cofactor in numerous metabolic reactions including fatty acid synthesis. In collaboration with the Sibon-lab (UMCG-Groningen), dr. Pavlo Stehantsev is investigating the interplay between CoA/phosphopantetheine metabolism and membrane lipid homeostasis. The project addresses the effects of manipulating CoA biosynthesis on fatty acid synthesis and membrane lipid composition.

Interplay between membrane lipid class and acyl chain composition

We have previously demonstrated that shortening of average acyl chain length is sufficient to render the major membrane lipid phosphatidyl-choline (PC) redundant. The loss of PC is compensated for by a rise in the non-bilayer preferring lipid phosphatidylethanolamine (PE). Mass spec analysis revealed that PC depletion is accompanied by dramatic changes in the acyl chain composition of PE that reduce its non-bilayer propensity and increase membrane fluidity. Current research takes advantage of the yeast PC biosynthetic mutants to identify gene products affecting acyl chain length.