Dynamics of In Vitro Bacterial S-Layer Crystallization

Abstract

S-layer proteins form crystalline lattices on the outsides of certain bacteria. While the structures of many S-layers are known, the dynamics of their formation is poorly understood. In an effort to provide such understanding, the DeYoreo and Bertozzi groups at the Molecular Foundry have used atomic force microscopy to image in real time the deposition of a certain S-layer protein on a supported lipid bilayer. This protein forms a square crystal lattice whose dynamics of assembly are strikingly complex: proteins first aggregate into amorphous clusters on the membrane; clusters subsequently crystallize and grow via the addition of tetramers at the cluster edge.Similar ‘two-step’ crystallization mechanisms have been observed in computer simulations of globular proteins [1], polymer melts [2] and Lennard-Jones particles [3-5], and inferred experimentally from the observation, via dynamic light scattering, of dense liquid droplets present in solution prior to lysozyme crystallization [6]. Here we explore the origin of two-step crystallization in the S-layer system via a simple computer model of associating monomers on a substrate. Dynamical simulation reveals that phase separation induced by nonspecific monomer-monomer interactions facilitates phase ordering driven by directional binding. Our results suggest that the interplay of non-specific attractions and site-specific binding are crucial in driving crystallization in the S-layer system.[1] P. Wolde and D. Frenkel, Science 277, 1975 (1997).[2] R. Gee et al., Nature Materials 5, 39 (2005).[3] A. Fortini et al., Phys. Rev. E 78 (2008).[4] J. van Meel et al., J. Chem. Phys. 129, 204505 (2008).[5] B. Chen et al., J. Phys. Chem. B 112, 4067 (2008).[6] L. Filobelo et al., J. Chem. Phys 123, 014904 (2005).