Translation of picornavirus RNA is governed by the internal ribosome entry site (IRES) element, directing the synthesis of a single polyprotein. Processing of the polyprotein is performed by viral proteases that also recognize as substrates host factors. Among these substrates are translation initiation factors and RNA-binding proteins whose cleavage is responsible for inactivation of cellular gene expression. Foot-and-mouth disease virus (FMDV) encodes two proteases, L pro and 3C pro. Widespread definition of L pro targets suffers from the lack of a sufficient number of characterized substrates. Here, we report the proteolysis of the IRES-binding protein Gemin5 in FMDV-infected cells, but not in cells infected by other picornaviruses. Proteolysis was specifically associated with expression of L pro, yielding two stable products, p85 and p57. In silico search of putative L targets within Gemin5 identified two sequences whose potential recognition was in agreement with proteolysis products observed in infected cells. Mutational analysis revealed a novel L pro target sequence that included the RKAR motif. Confirming this result, the Fas-ligand Daxx, was proteolysed in FMDV-infected and L pro-expressing cells. This protein carries a RRLR motif whose substitution to EELR abrogated L pro recognition. Thus, the sequence (R)(R/K)(L/A)(R) defines a novel motif to identify putative targets of L pro in host factors.
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