PV 2A pro is a protein composed of 149 amino acids that belongs to the cysteine protease group [54] . PV 2A pro is autocatalytically processed at its amino terminus between the capsid protein VP1 and 2A [64] (see Figure 1 (b)). The determinants of substrate specificity of picornaviral PV 2A pro have been investigated in detail by identification of cleavage sites by N-terminal Edman degradation, mutational analysis and using synthetic peptides as substrates [34, [65] [66] [67] . PV 2A pro can recognize a wide variety of amino acid residues at the P1 position. The determinants of substrate specificity for PV 2A pro lie at positions P4, P2, P1 , and P2 , which are preferentially occupied with Ile/Leu, Thr/Ser, Gly, and Pro, respectively. Moreover, the determinants of substrate specificity of HRV and coxsackievirus 2A pro are very similar to those found for PV 2A pro [65] [66] [67] . The yeast two-hybrid system has been used to identify the substrate sequence interacting with PV 2A pro . All the sequences identified contain the Leu-X-Thr-Z motif (X for any amino acid; Z for a hydrophobic residue) in positions from P4 to P1 suggesting the presence of a common interacting site on PV 2A pro substrates [68] .