Role of the Endosomal ESCRT Machinery in HIV-1 Vpu-Induced Down- Regulation of BST2/Tetherin

The cellular protein “Bone marrow stromal antigen 2” (BST2 also called Tetherin, CD317, HM1.24) was identified as a major mediator of the innate immune defense against the dissemination of enveloped viruses. BST2 was shown to physically trap the de novo formed viral particles at the surface of infected cells, thereby reducing viral release. Lentiviruses have evolved specific strategies to down-regulate the expression level of BST2 from the surface of the cells and as such promote viral egress. In Human Immunodeficiency Virus-1 (HIV-1), the accessory protein Vpu counters BST2 antiviral activity. However, the cellular and molecular mechanisms involved are not fully understood. Vpumediated antagonism of BST2 antiviral activity seems to involve complex interplay between the viral protein and host components regulating protein turnover and vesicular trafficking. This review focuses on the interplay between Vpu and the ubiquitin/endosomal pathway in countermeasures of HIV-1 to BST2 restriction, with a particular emphasis on the “Endosomal Sorting Complexes Required for Transport” (ESCRT) machinery.