The SIBYLS beamline has recently been awarded 50,000 hours on the NERSC
(National Energy Research Scientific Computing Center) to perform solution structure modeling using experimental SAXS data. Besides the usual ab-initio
reconstructions programs a new approach in rigid body modeling BILBOMD
has been parallelized on the NERSC supercomputer. It is commonly acknowledged that flexibility between domains of proteins is often critical for function. These motions, and proteins with large scale flexibility in general, are often not readily amenable to conventional structural analysis such as X-ray crystallography, NMR, or electron microscopy. We have developed an analysis tool using experimental SAXS measurements to identify flexibility and validate a constructed minimal ensemble of models which represent highly populated conformations in solution. The resolution is sufficient to address questions about the extent of the domain conformational sampling in solution? In our rigid body modeling strategy BILBOMD
, molecular dynamics (MD) simulations are used to explore conformational space. A typical experiment involves MD simulation on the domain connections at very high temperature, where the additional kinetic energy prevents the molecule from becoming trapped in a local minimum. The MD simulations provide an ensemble of molecular models from which a SAXS curve is calculated and compared to the experimental curve. A genetic algorithm is then used to identify the minimal ensemble (Minimal Ensemble Search, MES) required to best fit the experimental data. If you are interested in learning about and/or using this valuable SAXS analysis tool please contact Michal Hammel (MHammel at lbl dot gov).