The cleavage activities of aphthovirus and cardiovirus 2A proteins

The primary 2A/2B polyprotein cleavage of aphtho- and cardioviruses is mediated by their 2A proteins cleaving C-terminally. Whilst the aphthovirus 2A region is only 16 aa (possibly 18 aa) long, the cardiovirus 2A protein is some 150 aa. We have previously shown that foot-and-mouth disease virus (FMDV) 2A is able to mediate cleavage in an artificial (chloramphenicol acetyltransferase/FMDV 2A/beta-glucuronidase [CAT-2A-GUS]) polyprotein system devoid of any other FMDV sequences with high (similar to 85%), although not complete, cleavage. In this paper we show that insertion of upstream FMDV capsid protein 1D sequences increases the activity. In addition, we have demonstrated that the cardiovirus Theiler's murine encephalomyelitis virus (TME) 2A protein, when linked to GUS in a single ORF, is able to cleave at its own C terminus with high efficiency - if not completely. The C-terminal 19 aa of TME 2A, together with the N-terminal proline residue of protein 2B, were inserted into the CAT/GUS artificial polyprotein system (in a single ORF). This recombinant [CAT-Delta TME2A-GUS] polyprotein was able to mediate cleavage with high (similar to 85%) efficiency - directly comparable to the activity observed when FMDV 2A was inserted. A similar insertion into [CAT-GUS] of the C-terminal 19 aa of the cardiovirus encephalomyocarditis virus (EMC) 2A, together with the N-terminal proline residue of protein 2B, produced a [CAT-Delta EMC2A-GUS] polyprotein which also mediated cleavage at similar to 85%. Analysis of the products of expression of these artificial polyproteins in a prokaryotic translation system did not, apparently, reveal any GUS cleavage product.