The Leader protein of foot-and-mouth disease virus (FM DV) is the first component of the virus polyprotein. It is synthesized in two forms, Lab and Lb, both of which display the ability to cleave the L/P1 junction in trans and to induce the cleavage of the cap-binding complex component elF-4G (p220). The L protease has weak homology to the family of cysteine proteases, which have a catalytic dyad composed of a cysteine and a histidine. Mutations have been introduced into FMDV cDNA to modify each of the four cysteine residues and the three conserved histidine residues within the Lb species. The activities of the mutant L proteins have been determined. Modification of a single cysteine residue (residue 51) or of a single histidine residue (residue 148) abolished the abilities of L to cleave the L/P1 junction and to inhibit cap-dependent protein synthesis. In contrast, modification of each of the other cysteine residues and other conserved histidine residues had no apparent effect on these activities. (C) 1995 Academic Press, Inc.