The 2A/2B cleavage of aphtho- and cardiovirus 2A polyproteins is mediated by their 2A proteins ‘cleaving’ at their own C termini. We have analysed this activity using artificial reporter polyprotein systems comprising green fluorescent protein (GFP) linked via foot-and-mouth disease virus (FMDV) 2A to β-glucuronidase (GUS) – forming a single, long, open reading frame. Analysis of the distribution of radiolabel showed a high proportion of the in vitro translation products (∼90%) were in the form of the ‘cleavage’ products GUS and [GFP2A]. Alternative models have been proposed to account for the ‘cleavage’ activity: proteolysis by a host-cell proteinase, autoproteolysis or a translational effect. To investigate the mechanism of this cleavage event constructs encoding site-directed mutant and naturally occurring ‘2A-like’ sequences were used to program in vitro translation systems and the gel profiles analysed. Analysis of site-directed mutant 2A sequences showed that ‘cleavage’ occurred in constructs in which all the candidate nucleophilic residues were substituted – with the exception of aspartate-12. This residue is not, however, conserved amongst all functional ‘2A-like’ sequences. ‘2A-like’ sequences were identified within insect virus polyproteins, the NS34 protein of type C rotaviruses, repeated sequences in Trypanosoma spp. and a eubacterial α-glucosiduronasesequence(Thermatoga maritima aguA). All of the 2A-like sequences analysed were active (to various extents), other than the eubacterial α-glucosiduronase 2A-like sequence. This method of control of protein biogenesis may well not, therefore, be confined to members of the Picornaviridae. Taken together, these data provide additional evidence that neither FMDV 2A nor ‘2A-like’ sequences are autoproteolytic elements.
The ‘cleavage’ activities of foot-and-mouth disease virus 2A site-directed mutants and naturally occurring ‘2A-like’ sequences
