Tsg101, a homologue of ubiquitin-conjugating (E2) enzymes, binds the L domain in HIV type 1 Pr55(Gag)

Ubiquitination appears to be involved in virus particle release from infect ed cells. Free ubiquitin (Ub), as well as Ub covalently bound to a small fr action of p6 Gag, is detected in mature HIV particles. Here we report: that the p6 region in the Pr55(Gag) structural precursor polyprotein binds to T sg101, a putative Ub regulator that is involved in trafficking of plasma me mbrane-associated proteins. Tsg101 was found to interact with Gag in (i) a yeast two-hybrid assay, (ii) in vitro coimmunoprecipitation by using purifi ed Pr55(Gag) and rabbit reticulocyte lysate-synthesized Tsg101, and (iii) i n vivo in the cytoplasm of COS cells transfected with gag. The PTAPP motif [or late (L) domain] within p6, which is required for release of mature vir us from the plasma membrane, was the determinant for binding pr55(Gag). The N-terminal region in Tsg101, which is homologous to the Ubc4 class of Ub-c onjugating (E2) enzymes, was the determinant of interaction with p6. Mutati on of Tyr-110 in Tsg101, present in place of the active-site Cys that binds Ub in E2 enzymes, and other residues unique to Tsg101, impaired p6 interac tion, indicating that features that distinguish Tsg101 from active E2 enzym es were important for binding the viral protein. The results link L-domain function in HIV to the Ub machinery and a specific component of the cellula r trafficking apparatus.