Gauging Gag assembly

Hendrix et al. use fluorescence fluctuation imaging to reveal that HIV-1 particles start assembling in the cytoplasm of infected cells.

The HIV-1 polyprotein Gag contains three different domains that promote viral assembly: the membrane-binding matrix domain, the RNA-binding nucleocapsid domain, and the capsid domain that mediates Gag oligomerization. Gag assembles into new viral particles at specialized regions of the plasma membrane, but whether the protein only oligomerizes after it is recruited to these sites, or whether it begins to assemble while still in the cytoplasm, remains unclear.

Hendrix et al. used a series of fluorescence fluctuation imagining techniques to identify two populations of Gag molecule diffusing in the cytosol: a faster-moving monomeric form and a slowly moving oligomeric species. Even the Gag monomers diffused more slowly than most proteins of a similar size, a property the researchers mainly attributed to the nucleocapsid domain’s transient interactions with cytoplasmic RNAs.

Gag oligomerization depended on the protein’s concentration in the cytosol and was promoted by both the capsid domain’s self-association and the nucleocapsid domain’s interaction with RNA. A small number of Gag molecules therefore assemble on viral RNAs in the cytoplasm. Senior author Don Lamb now wants to test the idea that these small oligomers then associate with the plasma membrane and nucleate the assembly of viral particles by recruiting additional Gag monomers.