Biophysical Society Thematic Meeting - October 13-15, 2015

Biophysics of Proteins at Surfaces: Assembly, Activation, Signaling

Poster Abstracts

33-POS Board 33 Posttranslationally-modified Rab5 Protein Binding to Different Model Membranes Eileen Edler. Matthias Stein . Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany. Rab5 is a small GTPase and molecular switch that controls events in early endosome fusion by shuttling between its active GTP-bound and inactive GDP-bound state. After posttranslational modification, the protein is anchored to the plasma membrane via two C20 geranylgeranyl (GG) moieties close to its C-terminus. This CXC cysteine prenylation is required for Rab5 proteins to associate with cellular membranes. In the absence of this prenyl modification, the protein remains cytosolic and is unable to function properly. Membrane-associated Rab5 is able to recruit further effector proteins to the membrane and form local protein-protein complexes. The hypervariable N- and C-terminal regions could not be resolved by protein X-ray crystallography. We here present structural and dynamic properties of membrane-bound full-length Rab5 in the active and inactive states obtained from initial loop refinement and subsequent long-time all- atom Molecular Dynamics simulations. Simulations of Rab5 in different model membranes were performed and revealed a highly dynamic behavior of the membrane-anchored protein. Independent from the GTPase activation state, the G-domain rapidly changes orientation from a rather perpendicular position towards a position close to the membrane surface with two alpha-helices almost parallel to the bilayer. In addition to the residues in the lipid-exposed helices certain protein-membrane contacts are established by basic and polar residues in the flexible N- and C-terminal regions. The two geranylgeranyl anchors show a high degree of torsional flexibility; however, their motions are strongly correlated. Furthermore, the overall anchor insertion depth shows a dependence on the membrane phopsholipid composition, i.e. is slightly increasing with membrane thickness. The structural properties of the lipid bilayer seem to be rather unaffected by the protein anchor insertion.

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