Fig. 7.

Working model for the fate of single Env trimers during HIV-1 assembly. a_i Env is synthesized in the endoplasmic reticulum and trimers traffic to the plasma membrane through the canonical secretory pathway. a_ii Gag is synthesized in the cytoplasm and targets the plasma membrane, then forming a protein lattice and budding from the plasma membrane. b Env trimers diffuse laterally on the plasma membrane and either become entrapped in assembling Gag lattices or are rapidly internalized through clathrin-mediated endocytosis. Truncation of the Env-CT (CTΔ144) impairs Env internalization, leading to increased levels of surface-exposed Env. c Endocytosed Env populations are sorted and trafficked to the recycling endosome. d In T cells, Env populations are predominantly retained intracellularly, while limited quantities of Env are recycled back to the plasma membrane. This model suggests that specific recycling is regulated by interactions between host cell factors and elements of the Env-CT, leading to a marked reduction in the levels of surface-exposed Env. e Upon recycling of Env back to the plasma membrane, Env trimers diffuse laterally on the plasma membrane and either become entrapped in assembling Gag lattices or are re-endocytosed. f Intracellular retention of Env leads to low levels of surface-exposed Env, increasing the probability that Env trimers will encounter pre-formed lattices rather than low-order Gag oligomers, resulting in the observed neck-biased distributions. Truncation of the Env-CT results in a defect in Env’s ability to interact with and become sterically trapped by Gag lattices. The d8 mutation disrupts complementary interactions between the Gag lattice and the Env-CT resulting in exclusion of d8-Env trimers from the lattice. This exclusion will result in re-endocytosis of Env trimers and potentiate intracellular accumulation. These disruptions to the HIV-1 assembly process ultimately manifest as bulk defects in Env trimer incorporation into released particles