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Staufen is a host protein that is selectively incorporated into human immunodeficiency virus type 1 (HIV-1) particles in a poorly defined process that involves the selection of HIV-1 genomic RNA for encapsidation and the activity of its third double-stranded RNA-binding domain (dsRBD3). To better understand this, we characterized its interactions with pr55^Gag, the principal mediator of HIV-1 genomic RNA encapsidation. Chimeric proviruses harboring wild-type or mutant forms of Staufen were expressed in 293T cells. Cell fractionation analyses demonstrated that Staufen cosedimented with pr55^Gag within detergent-resistant, trypsin-sensitive complexes that excluded mature capsid and matrix proteins. Coimmunoprecipitation and bioluminescence resonance energy transfer assays demonstrated a specific and direct interaction between Staufen and the nucleocapsid domain of pr55^Gag in vitro and in live cells. This …