2A peptide

Translational recoding by 2A peptides.

Collaborative project with Martin Ryan, St. Andrews & Matt Sachs, Texas.

Positive-strand RNA viruses typically encode polyproteins that are cleaved by viral or host-encoded proteinases to produce mature, individual proteins. Alternatively proteins may be generated by translational effects such as ribosomal frame-shifting, read-through of "leaky" stop codons and coupled termination-reinitiation. Such programmed alterations of translation are not virus-specific but widespread (though rare) mechanisms of gene expression.

In foot-and-mouth disease virus (FMDV) and some other picornaviruses the oligopeptide (~20 amino acid) 2A region of the polyprotein mediates "cleavage" at its own C-terminus to release it from the 2B region. 2A is also active when placed between reporter proteins and in all eukaryotic systems tested - it acts as an autonomous element, making it an important tool for co-ordinated synthesis of multiple proteins from one open reading frame.

The 2A peptide ends in the conserved sequence -DVExNPGP-. The break in the polypeptide that results in translation through 2A is between the final glycine and proline codons of this conserved sequence. Our recent work (Doronina et al., 2008) has revealed that the 2A reaction is cotranslational, and indicates that translation terminating release factors play a critical role in the reaction, releasing the nascent chain from tRNA despite the presence of a sense (proline) codon in the A site of the ribosome. Based on this and other findings we have proposed a co-translational model for the 2A reaction.

In this model (Pathway A in the figure below) the conformation of 2A places strain on the peptidyltransferase centre of the ribosome, re-positioning the peptidyl(2A)-tRNA^Gly ester linkage. This steric effect prohibits nucleophilic attack by the incoming prolyl-tRNA^Pro amide nitrogen preventing formation of a new peptide bond. Instead, the N-terminal product is released from the ribosome by release factors, despite the lack of a stop codon. Following this, most ribosomes continue translation - with the proline encoded by the final proline codon of the 2A sequence becoming the first amino acid of the discrete downstream product. In some cases (pathway B) ribosomes read through the 2A site, the 2A peptide failing to interact poductively with the ribosome.

(Most of) the 2A collaborative group in 2008..... from left to right Martin Ryan, Lorraine Hughes, waiter(!), Pablo de Felipe, Jeremy Brown, interloper (Chris Blackwell) from the SRP project, Vicki Doronina, Matt Sachs.

For more information on 2A, which is also referred to as a cis-acting hydrolase element or CHYSEL please see Martin Ryan's web pages