Intracisternal A-type particles express their proteinase in a separate reading frame by translational frameshifting,...

Intracisternal A-type particles (IAP) are defective endogenous retroviruses that accumulate in the endoplasmic reticulum of rodent cells. IAP genomes share extensive sequence homologies with D-type retroviruses, but were presumed to express the viral proteinase (PR) as part of the gag open reading frame (ORF) while D-type retroviruses express PR in a separate ORF. Here we show that expression of the murine IAP element MIA14 yields three major translation products, corresponding to the Gag, Gag-PR, and Gag-PR-Pol polyproteins. Sequence analysis revealed that MIA14 PR is encoded in its own reading frame, separate from gag and pol. Frameshifting occurred with an efficiency of approximately 25% between the gag and pro ORFs and 35% between pro and pol. The region containing the putative gag-pro frameshift signal consists of a heptanucleotide slippery sequence (A6C) and a stem-loop structure probably forming a pseudoknot. Deletion of this structure element almost completely abolished frameshifting. Insertion of an additional base next to the frameshift signal placed gag and pro in the same ORF and resulted in predominant formation of Gag-PR and Gag-PR-Pol polyproteins which were not processed following in vitro translation. Expression of a similar construct in tissue culture cells, on the other hand, led to efficient intracellular processing of the mutant polyproteins.