Abstract
Assembly of human immunodeficiency virus type 1 (HIV-1) is directed by the viral core protein Pr55gag. Depending on the cell type, Pr55gag accumulates either at the plasma membrane or on late endosomes/multivesicular bodies. Intracellular localization of Pr55gag determines the site of virus assembly, but molecular mechanisms that define cell surface or endosomal targeting of Pr55gag are poorly characterized. We have analyzed targeting of newly synthesized Pr55gag in HeLa H1 cells by pulse-chase studies and subcellular fractionations. Our results indicated that Pr55gag was inserted into the plasma membrane and, when coexpressed with the viral accessory protein Vpu, Pr55gag remained at the plasma membrane and virions assembled at this site. In contrast, Pr55gag expressed in the absence of Vpu was initially inserted into the plasma membrane, but subsequently endocytosed, and virus assembly was partially shifted to internal membranes. This endocytosis of Pr55gag required the host protein Tsg101. These results identified a previously unknown role for Vpu and Tsg101 as regulators for the endocytic uptake of Pr55gag and suggested that the site of HIV-1 assembly is determined by factors that regulate the endocytosis of Pr55gag.