The disclosure is directed to an isolated polynucleotide encoding a modified picornavirus 3C protease, wherein the modified picornavirus 3C protease includes an altered secondary structure and one or more amino acid substitution(s) located at one or more amino acid position(s) corresponding to positions 16-25, 99-100 and 115-130 of a wild-type Fool-and-Mouth Disease Virus (FMDV) 3C protease, wherein the isolated polynucleotide encoding the modified picornavirus 3C protease, when transformed into and co-expressed in a host cell, enhances transgene expression of a P1 precursor polypeptide in comparison to an amount of P1 precursor polypeptide transgene expression exhibited in a host cell transformed and co-expressed with a control picornavirus 3C protease, wherein the one or more corresponding amino acid position(s) is/are identified by an alignment of the modified picornavirus 3C protease with the one or more of the wild type FMDV 3C protease(s). Methods for processing a picornavirus P1 precursor polypeptide into picornavirus viral proteins and/or virus-like particles using the isolated polynucleotides are also provided.