Salt Dependence of Neurofilament Gel Phase Behavior - A Synchrotron X-ray Scattering Study

Abstract

Neurofilaments (NFs) are cytoskeletal proteins expressed in neuronal cells, and are believed to play a role in the determination and maintenance of the neuronal cell shape and mechanical integrity. NFs self-assemble as flexible cylinders from 3 protein subunits: NF-Low (NF-L), NF-Medium (NF-M), and NF-High (NF-H). The three subunits are structurally conserved with the exception of their “tail” domains, composed of amino acid strands of increasing length and charge respectively. Screening of tail charges is achieved by varying the salinity of the in vitro buffer. At high concentrations, the filaments interact amongst themselves through their unstructured tails that branch out from the filament core and form a viscous gel. Polarized microscopy was used to map out phase diagrams of the resultant neurofilament hydrogels, and thus characterize salt dependent phase behavior. Reassembled separate networks of NF-L + NF-M and NF-L + NF-H show phase transitions from isotropic to nematic gel phases at distinct salt concentrations, thus reflecting the differences in the governing sidearm interactions [1].We will describe synchrotron x-ray scattering experiments that have allowed us to quantitatively study the microscopic structure of the NF gels: shifts in average interfilament spacing demonstrate the phase boundary between the isotropic and anisotropic NF-liquid crystal gel phases and how they are shifted as a result of varying the in vitro buffer salt concentrations. Funding provided by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0503347.[1] J.B. Jones, C.R. Safinya, Biophys. J. 95, 823 (2008).