NUCLEATION AND CRYSTALLIZATION OF GLOBULAR-PROTEINS - WHAT WE KNOW AND WHAT IS MISSING

Recently, much progress has been made in understanding the nucleation and crystallization of globular proteins, including the formation of c ompositional and structural crystal defects. Insight into the interact ions of (screened) protein macro-ions in solution, obtained from light scattering, small angle X-ray scattering and osmotic pressure studies , can guide the search for crystallization conditions. These studies s how that the nucleation of globular proteins is governed by the same p rinciples as that of small molecules. However, failure to account for direct and indirect (hydrodynamic) protein interactions in the solutio ns results in unrealistic aggregation scenarios. Microscopic studies o f numerous proteins reveal that crystals grow by the attachment of gro wth units through the same layer-spreading mechanisms as inorganic cry stals. Investigations of the growth kinetics of hen-egg-white lysozyme (HEWL) reveal non-steady behavior under steady external conditions. L ong-term variations in growth rates are due to changes in step-origina ting dislocation groups. Fluctuations on a shorter timescale reflect t he non-linear dynamics of layer growth that results from the interplay between interfacial kinetics and bulk transport. Systematic gel elect rophoretic analyses suggest that most HEWL crystallization studies hav e been performed with material containing other proteins at percent le vels. Yet, sub-percent levels of protein impurities impede growth step propagation and play a role in the formation of structural/compositio nal inhomogeneities. In crystal growth from highly purified HEWL solut ions, however, such inhomogeneities are much weaker and form only in r esponse to unusually large changes in growth conditions. Equally impor tant for connecting growth conditions to crystal perfection and diffra ction resolution are recent advances in structural characterization th rough high-resolution Bragg reflection profiling and X-ray topography.