Nucleic acid-independent retrovirus assembly can be driven by dimerization.

The Gag protein of retroviruses alone can polymerize into regular virus-like particles (VLPs) both in vitro and in vivo. In most circumstances the capsid (CA) and nucleocapsid (NC) domains of Gag as well as some form of nucleic acid are required for this process. The mechanism by which NC-nucleic acid interaction promotes assembly has remained obscure. We show here that while deletion of the NC domain of Rous sarcoma virus Gag abolishes formation and budding of VLPs at the plasma membranes of baculovirus-infected insect cells, replacement of NC with a dimer-forming leucine zipper domain restores budding of spherical particles morphologically similar to wild-type VLPs. The positioning of the dimerization domain appears to be critical for proper assembly, as the insertion of a 5-amino-acid flexible linker upstream of the zipper domain leads to budding of tubular rather than spherical particles. Similar tubular particles are formed when the same linker is inserted upstream of NC. The tubes are morphologically distinct from tubes formed when the p10 domain upstream of CA is deleted. The fact that a foreign dimerization domain can functionally mimic NC suggests that the role of nucleic acid in retroviral assembly is not to serve as a scaffold but rather to promote the formation of Gag dimers, which are critical intermediates in the polymerization of the Gag shell.