Several 2A pro variants have been generated in the entire PV genome or in the isolated 2A gene. Generation of PV 2A pro mutants was initially used to identify the Cys 106 , His 18 , and Asp 35 as the residues that form part of the catalytic triad of 2A pro [69, 70] . These data have been confirmed in the structure of HRV2 2A pro [71] . The role of the conserved Cys and His residues in the structure-function relationship has also been studied by mutagenesis. The residues Cys 55 , Cys 57 , Cys 115 , and His 117 play a critical role in the cis and trans proteolytic activity by maintaining 2A pro structure [72] . The structure of HRV2 2A pro shows that the Zn 2+ ion is coordinated tetrahedrally by the side chains of these conserved Cys and His residues. This Zn 2+ ion is tightly bound near to the C-terminal domain and may be important for the stability of the 2A pro [71, 73, 74] .