Several viral families possess a self-cleaving 2A peptide. These peptides are 18 to 22 amino acid long and have a core sequence motif of DxExNPGP. They allow for the synthesis of two proteins from a single open reading frame (ORF).
The 2A peptide consists of a single C-terminal glycine, a proline N-terminal and a glycine-serine-glycine spacer (GSG) flexible linker sequence. The GSG sequence has been shown to improve the cleavage efficiency of the 2A peptide.
Self-cleaving 2A peptides are promising candidates for multicistronic vectors. However, there are a number of problems associated with their cleavage. Incomplete cleavage can lead to the loss of function of the proteins involved. Therefore, various approaches are needed to increase the cleavage efficiency of multicistronic vectors based on 2A peptides.
Two approaches to increasing cleavage efficiency are to use elongated 2A peptides and to add a furin cleavage site to the peptide sequence. Other approaches include codon optimization, which enhances the separation efficiency of the peptide sequence. These approaches increase the cleavage efficiency, but do not necessarily increase the yield of the final product.
The IRES sequence was initially discovered to translate viral proteins. Later, it was shown to also translate cellular genes. It also enables cap-independent protein synthesis. However, IRES-based multicistronic vectors have poor expression of the second protein.
The T2A peptide is derived from the Thosea asigna virus. The T2A peptide contains a self-cleaving sequence which leads to ribosomal skipping. This mechanism of ribosomal skipping is known as the StopGO.