Co-translational, Intraribosomal Cleavage of Polypeptides by the Foot-and-mouth Disease Virus 2A Peptide*

During co-translational protein import into the endoplasmic reticulum ribosomes are docked onto the translocon. This prevents inappropriate exposure of nascent chains to the cytosol and, conversely, cytosolic factors from gaining access to the nascent chain. We exploited this property of co-translational translocation to examine the mechanism of polypeptide cleavage by the 2A peptide of the foot-and-mouth disease virus. We find that the scission reaction is unaffected by placing 2A into a co-translationally targeted protein. Moreover, the portion of the polypeptide C-terminal to the cleavage site remains in the cytosol unless it contains its own signal sequence. The pattern of cleavage is consistent with the proposal that the 2A-mediated cleavage reaction occurs within the ribosome itself. In addition, our data indicate that the ribosome-translocon complex detects the break in the nascent chain and prevents any downstream protein lacking a signal sequence from gaining access to the endoplasmic reticulum.