The human immunodeficiency virus type 1 Gag polyprotein has nucleic acid chaperone activity: Possible role in dimerization of genomic RNA and placement of tRNA on the primer binding site

The formation of an infectious retrovirus particle requires several RNA-RNA interaction events. In particular, the genomic RNA molecules form a dimeri c structure, and a cellular tRNA molecule is annealed to an 18-base complem entary region (the primer binding site, or PBS) on the genomic RNA, where i t will serve as primer for reverse transcription. tRNAs normally possess a highly stable secondary and tertiary structure; it seems unlikely that anne aling of a tRNA molecule to the PBS, which involves unwinding of this struc ture, could occur efficiently at physiological temperatures without the ass istance of a cofactor, Many prior studies have shown that the viral nucleoc apsid (NC) protein can act as a nucleic acid chaperone (i.e., facilitate an nealing events between nucleic acids), and the assays used to demonstrate t his activity include its ability to catalyze dimerization of transcripts re presenting retroviral genomes and the annealing of tRNA to the PBS in vitro . However, mature NC is not required for these events in vivo, since protea se-deficient viral mutants, in which NC is not cleaved from the parental Ga g polyprotein, are known to contain dimeric RNAs with tRNA annealed to the PBS, In the present experiments, we have tested recombinant human immunodef iciency virus type 1 Gag polyprotein for nucleic acid chaperone activity. T he protein was positive by all of our assays, including the ability to stim ulate dimerization and to anneal tRNA to the PBS in vitro. In quantitative experiments, its activity was approximately equivalent on a molar basis to that of NC. Based on these results, we suggest that the Gag polyprotein (pr esumably by its NC domain) catalyzes the annealing of tRNA to the PBS durin g (or before) retrovirus assembly in vivo.