The picornavirus family includes poliovirus (PV) (genus: enterovirus), human rhinoviruses (enterovirus) and foot-and-mouth disease virus (FMDV) (aphthovirus). These are responsible for important human and animal health concerns worldwide including poliomyelitis, the common cold and foot-and-mouth disease (FMD) respectively. In picornavirus particles, the positive-sense RNA genome (ca. 7–9 kb) is packaged within a protein shell (capsid) usually consisting of three surface exposed proteins, VP1, VP2 and VP3 plus the internal VP4, which are generated following cleavage of the capsid precursor by a virus-encoded protease. We have previously identified a motif near the C-terminus of FMDV VP1 that is required for capsid precursor processing. This motif is highly conserved among other picornaviruses, and is also likely to be important for their capsid precursor processing. We have now determined the plasticity of residues within this motif for virus infectivity and found an important interaction between FMDV residue VP1 R188 within this conserved motif and residue W129 in VP2 that is adjacent in the virus capsid. The FMDV (VP1 R188A) mutant virus has only been rescued with the secondary substitution VP2 W129R. This additional change compensates for the defect resulting from the VP1 R188A substitution and restored both capsid precursor processing and virus viability.