On Thursday 11 December 2008, Santarsiero, Bernard D. wrote: > In parallel with the discussion around this off-CCP4-topic, are they any > good examples of the opposite case, where the protein is a monomer in > solution (as evident from light scattering, MW determination through > centrifugation, EPR, etc.) but crystallizes as a dimer or higher multimer?
I don't think such a question is entirely well-defined, for two reasons. 1) The monomer/dimer equilibrium in solution may well depend on the specific conditions (pH, concentration, presence of ligands, temperature, etc). Unless these conditions are replicated in your crystallization medium, it is uncertain to what extent the solution measurement is relevant. 2) How extensive an interface is required in order for it to be considered a dimer/multimer interaction? In the limiting case of very small interfaces, the entire crystal might be consider a single oligomer, with each lattice-packing contact constituting a monomer:monomer interaction. That's not a very useful place to set the threshold, but where do you set it - 100 A^2 ? 500 A^2 ? 1000 A^2? Some definition other than surface area? That said, I have some interest in the question as a practical matter. We have a new structure that is "obviously", but totally unexpectedly, a tetramer in the crystal. In this case the monomer:monomer interaction surface is >1500 A^2. But exactly what criteria would I use to argue that this is a real tetramer? What criteria would I use to argue that it is a crystal artifact? Yes, of course ideally one would go back to the lab and survey for solution measurements that are consistent with tetramerization, but that is not always practical, and may lead right back to your original question. -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle 98195-7742
