Investigation on the crystal structure of proteins and their complexes in solution from combined TEM techniques and solubility measurements: the case of endo‐1,3(4)‐β‐glucanase

Endo ‐1,3(4)‐β‐glucanase was crystallized from a crude extract of protein solution by addition of a salt A + , X − . Optical microscopy showed needle‐shaped polydisperse crystals with a mean size of 10 µm. Using transmission electron microscopy with specific settings allows one to characterize the ordered arrays of a thin protein crystal and to determine the parameters of the crystal lattice. The results suggest that the unit cell of a endo ‐1,3(4)‐β‐glucanase crystal may consist in the association of a given number of these proteins. Such oligomers have already been seen for other proteins. The solubility of endo ‐1,3(4)‐β‐glucanase in aqueous solution was determined for several X − concentrations using BCA assay (titration of the total protein in solution) and electrophoresis. The new data are in agreement with a new simple model based on equilibrium between the oligomer in solution and neutral and charged X − complexes of the protein in the monomer state. The model provides a new approach to the usual salting‐in/salting‐out description of protein solubility. Copyright © 2003 Society of Chemical Industry