F. Fehrmann et al., INTRACISTERNAL A-TYPE PARTICLES EXPRESS THEIR PROTEINASE IN A SEPARATE READING FRAME BY TRANSLATIONAL FRAMESHIFTING, SIMILAR TO D-TYPE RETROVIRUSES, Virology, 235(2), 1997, pp. 352-359
Intracisternal A-type particles (IAP) are defective endogenous retrovi
ruses that accumulate in the endoplasmic reticulum of rodent cells. IA
P genomes share extensive sequence homologies with D-type retroviruses
, but were presumed to express the viral proteinase (PR) as part of th
e gag open reading frame (ORF) while D-type retroviruses express PR in
a separate ORF. Here we show that expression of the murine IAP elemen
t MIA14 yields three major translation products, corresponding to the
Gag, Gag-PR, and Gag-PR-Pol polyproteins. Sequence analysis revealed t
hat MIA14 PR is encoded in its own reading frame, separate from gag an
d pol. Frameshifting occurred with an efficiency of approximately 25%
between the gag and pro ORFs and 35% between pro and pol. The region c
ontaining the putative gag-pro frameshift signal consists of a hepta-n
ucleotide slippery sequence (A(6)C) and a stem-loop structure probably
forming a pseudoknot, Deletion of this structure element almost compl
etely 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 sim
ilar construct in tissue culture cells, on the other hand, led to effi
cient intracellular processing of the mutant polyproteins. (C) 1997 Ac
ademic Press.