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

摘要

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.