Crystallization of foot-and-mouth disease virus 3C protease: surface mutagenesis and a novel crystal-optimization strategy.

Foot-and-mouth disease virus (FMDV) 3C protease (3C(pro)) plays a vital role in virus replication by performing most of the cleavages required to divide the viral polyprotein precursor into its functional component proteins. To date, no structural information has been available for FMDV 3C(pro), which is an attractive target for antiviral drugs. Targeted mutagenesis of surface amino acids identified two Cys residues that were detrimental to solubility and contributed to the time-dependent formation of a proteinaceous skin in samples of purified wild-type protein. Substitution of these amino acids, combined with trimming of the N- and C-termini, yielded a 3C(pro) construct that was amenable to crystallization. High-resolution diffraction (1.9 A) was only obtained following 'iterative screening' in which commercial crystal screening solutions were used as additives once initial crystallization conditions had been obtained.