Human immunodeficiency virus type 1 virion density is not determined by nucleocapsid basic residues

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein is sufficie nt for assembly and release of virion-like particles from the plasma membra ne. To promote assembly, the Gag polyprotein must polymerize to form a shel l that lines the inner membrane of nascent virions. Several techniques have been used to functionally map the domain required for Gag polymerization ( the I domain). Among these methods, isopycnic centrifugation has been used under the assumption that changes in virion density reflect impairment in G ag-Gag interaction. If virion density is determined by efficient Gag-Gag in teraction, then mutation of basic residues in the nucleocapsid (NC) domain should disrupt virion density, since these residues constitute the I domain . However, we have previously shown that simultaneous disruption of up to 1 0 HIV-1 NC basic residues has no obvious effect on virion density. To rule out the possibility that HIV-1 NC basic residues other than those previousl y mutated might be important for virion density, mutations were introduced at the remaining sites and the ability of these mutations to affect Gag-Gag interaction and virion density was analyzed. Included in our analysis is a mutant in which all NC basic residues are replaced with alanine, Our resul ts show that disruption of HIV-1 NC basic residues has an enormous effect o n Gag-Gag interaction but only a minimal effect on the density of those vir ions that are still produced, Therefore, the determinants of the I domain a nd of virion density are genetically distinguishable.