The impact of Lys → Arg surface mutations on the crystallization of the globular domain of RhoGDI

Acta Cryst. (2004). D60, 275–280 [doi:10.1107/S0907444903026271]

A stereo figure illustrating a crystal contact in crystals of a double mutant of RhoGDI in which two adjacent lysines (199 and 200) were replaced with arginines. Both the electron density and the model are shown. The arginines from adjacent molecules sequester two sulfate ions from the precipitant.

A crystal structure can only be determined if X-ray quality crystals are available. In the case of proteins, the preparation of such crystals is often difficult or even impossible. One possible way of circumventing the difficulties is to modify the protein’s surface by site directed mutagenesis. Previous work (Acta Cryst. D57, 679–688 and Acta Cryst. D58, 1983–1991) showed that introducing patches with low conformational entropy by mutating lysines and/or glutamates to alanines, promotes formation of crystal contacts mediated by these epitopes. However, such mutations reduce solubility. The present study showed how sometimes mutations of lysines to arginines can also be useful. In the particular case reported in the paper, two arginines introduced in place of lysines create a crystal contact by sequestering a sulfate ion.