Analysis of the aphthovirus 2A/2B polyprotein 'cleavage' mechanism indicates not a proteolytic reaction, but a novel translational effect: a putativeribosomal 'skip'

The 2A region of the aphthovirus foot-and-mouth disease virus (FMDV) polypr otein is only 18 aa long. A 'primary' intramolecular polyprotein processing event mediated by 2A occurs at its own C terminus. FMDV 2A activity was st udied in artificial polyproteins in which sequences encoding reporter prote ins flanked the 2A sequence such that a single, long, open reading frame wa s created. The self-processing properties of these artificial polyproteins were investigated and the co-translational 'cleavage' products quantified. The processing products from our artificial polyprotein systems showed a mo lar excess of 'cleavage' product N-terminal of 2A over the product C-termin al of 2A. A series of experiments was performed to characterize our in vitr o translation systems. These experiments eliminated the translational or tr anscriptional properties of the in vitro systems as an explanation for this imbalance. In addition, the processing products derived from a control con struct encoding the P1P2 region of the human rhinovirus polyprotein, known to be proteolytically processed, were quantified and found to be equimolar. Translation of a construct encoding green fluorescent protein (GFP), FMDV 2A and beta -glucuronidase, also in a single open reading frame, in the pre sence of puromycin, showed this antibiotic to be preferentially incorporate d into the [GFP2A] translation product. We conclude that the discrete trans lation products from our artificial polyproteins are not produced by proteo lysis. We propose that the FMDV 2A sequence, rather than representing a pro teolytic element, modifies the activity of the ribosome to promote hydrolys is of the peptidyl(2A)-tRNA(Gly) ester linkage, thereby releasing the polyp eptide from the translational complex, in a manner that allows the synthesi s of a discrete downstream translation product to proceed. This process pro duces a ribosomal 'skip' from one codon to the next without the formation o f a peptide bond.