pp1ab

pp1ab is a viral polyprotein encoded by coronaviruses, produced when a -1 ribosomal frameshift occurs at the boundary between ORF1a and ORF1b during translation of the viral genome. The result is a single longer polyprotein that combines the products of both ORFs and serves as the primary replicase driving RNA synthesis. In contrast, pp1a is translated from ORF1a alone without the frameshift and yields a shorter set of non-structural proteins.

The translation of pp1ab relies on a slippery sequence and a downstream RNA structure that promote frameshifting, allowing ribosomes to continue reading into ORF1b in a different reading frame. This mechanism ensures coordinated production of the complete replicase repertoire required for replication and transcription.

Once synthesized, pp1ab is cleaved by viral proteases into 16 non-structural proteins (nsps 1–16). The processing is mediated mainly by the papain-like protease (PLpro) in nsp3 and the 3C-like protease (3CLpro, also called the main protease) in nsp5. The resulting nsps form the replication-transcription complex, which orchestrates genome replication and subgenomic RNA transcription. Key nsps include the RNA-dependent RNA polymerase (nsp12), helicase (nsp13), proofreading exonuclease (nsp14), and various methyltransferases and processing enzymes (nsps 15–16).

pp1ab is central to coronavirus biology, providing the enzymatic machinery for RNA synthesis and processing within membrane-associated replication organelles. Its conserved structure across coronaviruses makes it a foundational element of their replication strategy and a common focus of study in virology.