(133a) Multiscale Modeling of Membrane Sculpting By the Protein exo70

A collection of membrane-binding proteins are responsible for sculpting cell membranes during cell processes, including endocytosis, exocytosis, and cell morphogenesis. The protein exo70 is a member of the exocyst complex, which mediates the secretion of matrix metalloproteinases (MMPs) at focal degradation sites during morphogenesis [1]. Overexpression of exo70 promotes tumor cell invadopodia formation, and in vitro experiments show that exo70 generates tubules on giant unilamellar vesicles (GUVs). We employ coarse-grained molecular dynamics simulations and continuum mechanics methods to quantify the membrane-bending action of exo70. Coarse-grained molecular dynamics simulations (CGMD) using the MARTINI force field [2] show that exo70 produces the strongest curvature when bound to a key lipid, phosphatidylinositol (4,5)bisphosphate (PIP2), and when bound to another monomer. To relate these simulations to experiments, we first quantify the deformation field generated by exo70 monomers and dimers using CGMD simulation, and then investigate the resulting cellular morphologies using a continuum mechanics model for membrane bending. In the first step, we characterize the deformation field by directly analyzing curved membrane conformations generated by CGMD simulations of mutant and wild-type exo70 on mixed lipid bilayers. Further analysis of the three-dimensional stress tensor [3] relates the observed membrane bending to changes in the balance of forces across the bilayer. In the second step, the calculated deformation field serves as the input for a continuum mechanics model for membrane bending which is based on the Helfrich Hamiltonian [4]. From the stand-point of methodological advance, we uncover new principles coupling the induced curvature field to membrane thermal undulations and use these results to perform system size scaling analysis. Monte Carlo simulation of this model yields membrane morphologies which we relate to cryo-transmission electron microscopy images of exo70 remodeling a GUV. This model agrees with experiments which show that PIP2 binding and dimerization are necessary for generating membrane tubules and protrusions. This multi-scale model allows us to relate the chemical details of exo70 membrane-binding and self-association to the induced deformation field and resulting cellular morphologies.

[1] Liu, J.; Yue, P.; Artym, V. V.; Mueller, S. C. & Guo, W. The Role of the Exocyst in Matrix Metalloproteinase Secretion and Actin Dynamics during Tumor Cell Invadopodia Formation, Molecular Biology of the Cell, 20,16,3763-3771.
[2] Marrink, S. J.; Risselada, H. J.; Yefimov, S.; Tieleman, D. P. & de Vries, A. H. The MARTINI Force Field: Coarse Grained Model for Biomolecular SimulationsJ. Phys. Chem. B, 2007, 111, 7812-7824.
[3] Ollila, O. H. S.; Risselada, H. J.; Louhivuori, M.; Lindahl, E.; Vattulainen, I. & Marrink, S. J., 3D Pressure Field in Lipid Membranes and Membrane-Protein Complexes, Phys. Rev. Lett., 2009, 102, 078101.
[4] Liu, J.; Tourdot, R.; Ramanan, V.; Agrawal, N. J. & Radhakrishanan, R. Mesoscale simulations of curvature-inducing protein partitioning on lipid bilayer membranes in the presence of mean curvature fields, Mol. Phys., 2012, 110, 11-12, 1127-1137.