Dynamic Light Scattering Investigation in Aqueous Solutions of bc1-Complex Membrane Protein

Formation of framework nacre polypeptide supramolecular assemblies that nucleate polymorphs

The formation of aragonite in the mollusk shell nacre layer is linked to the assembly of framework protein complexes that interact with β-chitin polysaccharide. What is not yet understood is how framework nacre proteins control crystal growth. Recently, a 30 AA intrinsically disordered nacre protein sequence (n16N) derived from the n16 framework nacre protein was found to form aragonite, vaterite, or ACC deposits when adsorbed onto β-chitin. Our present study now establishes that n16N assembles to form amorphous nonmineralized supramolecular complexes that nucleate calcium carbonate polymorphs in vitro. These complexes contain unfolded or disordered (54% random coil, 46% β structures) n16N polypeptide chains that self-assemble in response to alkaline pH shift. The pH-dependent assembly process involves two stages, and it is likely that side chain salt-bridging interactions are a major driving force in n16N self-association. Intriguingly, Ca(II) ions are not required for n16N assembly but do shift the assembly process to higher pH values, and it is likely that Ca(II) plays some role in stabilizing the monomeric form of n16N. Using preassembled fibril-spheroid n16N assemblies on Si wafers or polystyrene supports, we were able to preferentially nucleate vaterite at higher incidence compared to control scenarios, and it is clear that the n16N assemblies are in contact with the nucleating crystals. We conclude that the framework nacre protein sequence n16N assembles to form supramolecular complexes whose surfaces act as nucleation sites for crystal growth. This may represent a general mineralization mechanism employed by framework nacre proteins in general.

Rationalization of membrane protein crystallization with polyethylene glycol using a simple depletion model

Based on the importance of crystallizing membrane proteins in a rational way, cytochrome bc(1) complex (BC1) was crystallized using polyethylene glycol (PEG) as a sole crystallization agent. Interaction between protein-detergent complexes of BC1 was estimated by dynamic light scattering, and was compared with the numerical calculation using the Derjaguin-Landau-Verwey-Overbeek potential plus a depletion potential, without considering specific surface properties of the protein-detergent complexes. The experiments and calculation were found to be consistent and we obtained a relation between PEG molecular weight M and the range of depletion zone delta as delta approximately M(0.48+/-0.02). The stability of liquid phase of BC1 solutions was controlled by a ratio of (the range of depletion zone)/(the radius of a BC1 particle), which was consistent with recent theoretical predictions. The crystallization was most successful under a condition where the stability of the liquid phase changed from stable to unstable. The PEG molecular weight that fulfilled this condition coincided with the one used empirically to crystallize BC1 in the past by a number of groups. These results are compared to the fact that membrane proteins were often successfully crystallized close to the detergent cloud point.