Biophysical Society Thematic Meeting - October 13-15, 2015

Biophysics of Proteins at Surfaces: Assembly, Activation, Signaling Tuesday Speaker Abstracts

Interplay between Weak Nonspecific Electrostatics and Cation-π Interactions Governs the Peripheral Membrane Binding of a Bacterial Phospholipase Hanif M. Khan 1 , Cedric Grauffel 1 , Edvin Fuglebakk 1 , Boqian Yang 2 , Tao He 3 , Anne Gershenson 2 , Mary F. Roberts 3 , Nathalie Reuter 1 . 1 University of Bergen, Bergen, Norway, 2 University of Massachusetts, Amherst, MA, USA, 3 Boston College, Chestnut Hill, MA, USA. Bacillus thuringiensis phosphatidylinositol-specific phospholipase C (BtPI-PLC) is an amphitropic enzyme cleaving GPI-anchored proteins off the outer surface of eukaryotic plasma membranes. Amphitropic proteins bind specifically and transiently to the surface of cell membranes, and their functions are regulated upon binding. It is commonly acknowledged that non-specific electrostatic forces are responsible for their long- range interactions with membranes. Using continuum electrostatics calculations we show how, despite having an overall negative charge (-7e), the charge distribution of BtPI-PLC leads to favorable though quite low electrostatic binding free energy with anionic membranes. The in silico mutation of a single, key basic residue to alanine diminishes this long-range electrostatic contribution explaining the significant decrease in the experimentally measured Kd. Multiple 500 ns-long all-atom molecular dynamics simulations of BtPI-PLC docked to mixed bilayers with varying ratio of zwitterionic lipids show that, once close to the membrane surface, short range non-specific hydrophobic interactions and specific cation-pi interactions with the N(Me)3 groups of phosphatidylcholine (PC) lipids of the membrane come into play for BtPI-PLC binding to the membrane surface[1]. Comparing our simulation results with fluorescence correlation spectroscopy measurements of the membrane affinity of the wild-type enzymes and of various mutants, we conclude that the interplay between long-range electrostatics and short range, PC specific cation-pi interactions governs the specificity of BtPI-PLC for PC-rich membranes. Finally, we suggest that overlooked cation-pi interactions between membranes and aromatic amino acids of amphitropic proteins may play an important role not only in membrane binding but also in lipid specificity. [1] Cation-pi interactions as lipid-specific anchors for phosphatidylinositol-specific phospholipase-C. C.Grauffel, B.Yang, T.He, M.F. Roberts, A.Gershenson, N.Reuter*, Journal of the American Chemical Society (2013) 135(15):5740-50

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