Eric Schulze

Membrane Localization and Dynamics of Geranylgeranylated Rab5 Hypervariable Region

The small GTPase Rab5 is a key regulator of endosomal trafficking processes and a marker for the early endosome. The C-terminal hypervariable region (HVR) of Rab5 is post-translationally modified at residues Cys212 and Cys213 to accommodate two geranylgeranyl anchors (C20 carbon chain length) in order to associate Rab5 with the membrane. The structural role of the HVR regarding protein-early endosome membrane recruitment is not resolved due to its high degree of flexibility and lack of crystallographic information. Here, full-atomistic and coarse-grained molecular dynamics simulations of the truncated Rab5 HVR206-215 in three model membranes of increasing complexity (pure phospholipid bilayer, ternary membrane with cholesterol, six-component early endosome) were performed. Specific electrostatic interactions between the HVR206-215 Arg209 residue and the phosphate group of the inositol ring of PI(3)P were detected. This shows that PI(3)P acts as a first contact site of protein recruitment to the early endosome. The free energy change of HVR206-215 extraction from the bilayer was largest for the physiological negatively charged membrane. 5μs coarse-grained simulations revealed an active recruitment of PI(3)P to the HVR206-215 supporting the formation of Rab5- and PI(3)P enriched signaling platforms.

Simulation of Mixed Self-Assembled Monolayers on Gold: Effect of Terminal Alkyl Anchor Chain and Monolayer Composition

Self-assembling monolayers provide a reproducible synthetic microenvironment for tethering lipid bilayers to incorporate proteins and lay the ground for numerous applications in nanotechnology and biomedical engineering. Although the structure of single-component monolayers is well investigated, there is far less insight into the molecular behavior at the interface of mixed monolayers at different mole fractions. Here, we present and apply a novel procedure to simulate and analyze multicomponent self-assemblies of alkanethiols over a wide range of mole concentrations of anchoring compounds. In particular, the structural features of monolayers consisting of a matrix compound and either a short (C8) or a long (C16) anchor compound on Au(111)-like surfaces were investigated first using coarse-grained and subsequently full-atomistic molecular dynamics simulations. Different scenarios of spatial distributions (random vs clustering) of anchoring molecules on flat surfaces were probed. The results of the simulations are in excellent agreement with the experimental data from ellipsometry and infrared reflection absorption spectroscopy. For short anchoring molecules, a random spatial distribution in the matrix is obtained. At low, experimentally relevant anchor compound mole fractions < 0.1, only for long-chain (C16)-terminal alkyls, phase segregation and self-association of the anchoring molecules can be observed, which are also seen in experiment.