STRUCTURAL CONSEQUENCES OF REDUCTIVE METHYLATION OF LYSINE RESIDUES IN HEN EGG-WHITE LYSOZYME - AN X-RAY-ANALYSIS AT 1.8-ANGSTROM RESOLUTION

Chemical modification of proteins has been and continues to be an important biochemical tool for the study of protein structure and function. One such type of approach has been the reductive methylation of lysine residues. In order to address the consequences of such methylation on the crystallization and structural properties of a protein, the three-dimensional structure of hen egg white lysozyme in which all lysine residues have been alkylated has been determined and refined to a nominal resolution of 1.8 angstrom and a crystallographic R factor of 17.3%. Crystals used in the investigation were grown from 1.5-1.8 M MgSO4 and 50 mM Tris at pH 8.0 and belonged to, the space group P2(1)2(1)2, with unit cell dimensions of a = 30.6 angstrom, b = 56.3 angstrom, c = 73.2 angstrom, and one molecule per asymmetric unit. It was not possible to grow crystals of the modified lysozyme under the conditions normally employed for the hen egg white protein. Overall, the three-dimensional structures of the native lysozyme and the modified protein are very similar with only two surface loops differing to any significant extent. Specifically, the positions of the alpha-carbons for these two forms of the protein, excluding the surface loops, superimpose with a root-mean-square value of 0.40 angstrom. The magnitude of the structural changes observed between the modified and unmodified forms of lysozyme is similar to that seen when an identical protein structure is solved in two different crystalline lattices. Consequently, the methylation of lysine residues results in very little structural perturbations but can produce enormous effects on the crystallization properties of a protein. As described here, this technique was absolutely critical for obtaining X-ray quality crystals of myosin subfragment 1 and thus may prove to be valuable in the crystallization of other proteins that have so far resisted forming ordered arrays.