Cytoplasmic HIV-1 RNA is mainly transported by diffusion in the presence or absence of Gag protein

Full-length HIV-1 RNA plays a central role in viral replication by serving as the mRNA for essential viral proteins and as the genome packaged into infectious virions. Proper RNA trafficking is required for the functions of RNA and its encoded proteins; however, the mechanism by which HIV-1 RNA is transported within the cytoplasm remains undefined. Full-length HIV-1 RNA transport is further complicated when group-specific antigen (Gag) protein is expressed, because a significant portion of HIV-1 RNA may be transported as Gag-RNA complexes, whose properties could differ greatly from Gag-free RNA. In this report, we visualized HIV-1 RNA and monitored its movement in the cytoplasm by using single-molecule tracking. We observed that most of the HIV-1 RNA molecules move in a nondirectional, random-walk manner, which does not require an intact cytoskeletal structure, and that the mean-squared distance traveled by the RNA increases linearly with time, indicative of diffusive movement. We also observed that a single HIV-1 RNA molecule can move at various speeds when traveling through the cytoplasm, indicating that its movement is strongly affected by the immediate environment. To examine the effect of Gag protein on HIV-1 RNA transport, we analyzed the cytoplasmic HIV-1 RNA movement in the presence of sufficient Gag for virion assembly and found that HIV-1 RNA is still transported by diffusion with mobility similar to the mobility of RNAs unable to express functional Gag. These studies define a major mechanism of HIV-1 gene expression and resolve the long-standing question of how the RNA genome is transported to the assembly site.