Summary PICK1 is a neuronal scaffolding protein containing a PDZ domain and an autoinhibited BAR domain. BAR domains are membrane-sculpting protein modules generating membrane curvature and promoting membrane fission. Previous data suggest that BAR domains are organized in lattice-like arrangements when stabilizing membranes but little is known about structural organization of BAR domains in solution. Through a small-angle X-ray scattering (SAXS) analysis, we determine the structure of dimeric and tetrameric complexes of PICK1 in solution. SAXS and biochemical data reveal a strong propensity of PICK1 to form higher order structures and SAXS analysis suggest an offset, parallel mode of BAR-BAR oligomerization. Furthermore, unlike accessory domains in other BAR domain proteins, the positioning of the PDZ domains is flexible, enabling PICK1 to perform long-range, dynamic scaffolding of membrane-associated proteins. Together with functional data, these structural findings are compatible with a model where oligomerization governs auto-inhibition of BAR domain function.

externalization in cells undergoing apoptosis which is an important biomarker to be used in anti-cancer treatment. The in vitro fluorescence measurements on the PS bound aptamers primarily validate the applicability of the mentioned computational aptamer designing methodology. We have further studied the binding properties of the designed aptamers with phospholipid vesicles using a direct detection (non-fluorescence) methodology to measure concentrations of relative PS bound aptamers versus unbound ones. A novel technique has been proposed to analyze the phospholipid binding properties of aptamers without fluorescent tags attached to them. Our results suggest that the PS binding of aptamers is very much sequence specific and depends also on the number of nucleotides in the sequence with shorter aptamers showing better binding. Finally, we have been able to properly rank the aptamers considering the best binding properties with PS and thus have found a set of candidates for preclinical trials. This may open up the possibility of discovering aptamer based diagnostic tools to be used in treatments of many diseases including cancer. 490-Pos Board B259Binding of b-Amyloid (1-42) Peptide to Negatively Charged Phospholipid Membranes in the Liquid-Ordered State Plaques formed by aggregation of amyloid b peptide (Ab 42) are characteristic of Alzheimer's disease. The mechanism of plaque formation from Ab 42 monomers, oligomers and fibrils is not understood, although there are data indicating that negatively-charged membrane surfaces catalyze the process. It has also been suggested that the pathogenic peptides are originated in raft-like microdomains in the liquid-ordered state. In order to explore the initial stages of the process, we have studied the interaction of Ab 42 in the monomeric form with lipid monolayers and bilayers in the liquid-ordered state, composed of equimolar brain sphingomyelin and cholesterol, to which either 5 or 20 mol% phosphatidic acid (PA) were added. Molecular dynamics simulations of the system were performed, as well as experimental measurements of changes in surface pressure at the air-water interface, and isothermal calorimetry of peptide-vesicle interactions. In the absence of the negatively charged PA interaction is weak and it cannot be detected calorimetrically. However in the presence of PA interaction is detected by all three methods and in all cases interaction is strongest with the lipid composition containing 5 mol% PA. Molecular dynamics pictures are different for the low-and highcharge bilayers, in the former case the peptide is bound through many contact points to the bilayer, while for the bilayer containing 20 mol% PA only a small fragment of the peptide appears to be bound. Interestingly, the calorimetric data indicate decreased entropy upon binding only for the system predicted to contain highly bound peptide. Moreover the molecular dynamics methods indicate the development of a b-sheet structure by the peptide in bilayers containing 5% PA, while in the presence of 20 mol% PA t...