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Biochemical Analyses of the Interactions between Human Immunodeficiency Virus Type 1 Vpr and p6Gag

ABSTRACT The nonstructural human immunodeficiency virus type 1 Vpr protein is packaged into progeny virions at significant levels (∼200 copies/virion). Genetic analyses have demonstrated that efficient Vpr packaging is dependent upon a leucine-X-X-leucine-phenylalanine (LXXLF) motif located in the p6Gag domain of the structural Gag polyprotein. Recombinant proteins spanning full-length Vpr (Vpr1–97) or the amino-terminal 71 amino acids (Vpr1–71) formed specific complexes with recombinant p6 proteins in vitro. Complex formation required an intact LXXLF motif and exhibited an intrinsic dissociation constant of ∼75 μM. Gel filtration and cross-linking analyses further revealed that Vpr1–71 self-associated in solution. Our experiments demonstrate that Vpr can bind directly and specifically to p6 and suggest that oligomerization of both Vpr and Gag may serve to increase the avidity and longevity of Vpr-Gag complexes, thereby ensuring efficient Vpr packaging.