title: β-TrCP is dispensable for Vpu's ability to overcome the CD317/Tetherin-imposed restriction to HIV-1 release.
abstract: BACKGROUND
  The cellular transmembrane protein CD317/BST-2/HM1.24/Tetherin restricts HIV-1 infection by physically tethering mature virions to the surface of infected cells. HIV-1 counteracts this restriction by expressing the accessory protein Vpu, yet the mechanism of this antagonism is incompletely understood. β-TrCP is the substrate recognition domain of an E3 ubiquitin ligase complex that interacts with the di-serine motif S52/S56 in the cytoplasmic tail of Vpu to target the CD4 receptor for proteasomal degradation. Recently, it has been suggested that β-TrCP is also critically involved in Vpu's ability to overcome the CD317-mediated virion release block.
  
  
  RESULTS
  To test this model, we analyzed the consequences of several experimental strategies to interfere with the Vpu-β-TrCP protein-protein interaction. Under these conditions, we studied effects of Vpu on expression and localization of CD317 and CD4, as well as on its ability to promote HIV-1 release. Our results demonstrate a strict requirement for Vpu's di-serine motif for degradation of CD4 and also CD317, reduction of cell surface exposure of CD317, and HIV-1 release enhancement. We further show a critical role of β-TrCP2, but not of the structurally related β-TrCP1 isoform, for Vpu-mediated degradation of both receptors. Most importantly, Vpu remained active in downregulating CD317 from the cell surface and in overcoming the HIV-1 release restriction in β-TrCP-depleted cells.
  
  
  CONCLUSIONS
  These results demonstrate that β-TrCP is not strictly required for Vpu's ability to counteract the CD317-imposed virion release block and support the relevance of cell surface down-modulation of the restriction factor as a central mechanism of Vpu antagonism. Moreover, we propose the existence of a critical, yet to be identified cellular factor that interacts with Vpu via its di-serine motif to alter the trafficking of the restriction factor.
authors.affiliation: Department of Infectious Diseases, Virology, University of Heidelberg, Heidelberg, Germany.
authors.forename: Hanna-Mari
authors.initials: HM
authors.lastname: Tervo
authors.forename: Stefanie
authors.initials: S
authors.lastname: Homann
authors.forename: Ina
authors.initials: I
authors.lastname: Ambiel
authors.forename: Joëlle V
authors.initials: JV
authors.lastname: Fritz
authors.forename: Oliver T
authors.initials: OT
authors.lastname: Fackler
authors.forename: Oliver T
authors.initials: OT
authors.lastname: Keppler
mesh_terms: D015703:Antigens, CD / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism / Q000821:virology; D015536:Down-Regulation; D058851:GPI-Linked Proteins / Q000235:genetics / Q000378:metabolism; D015497:HIV-1 / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D006367:HeLa Cells; D054298:Human Immunodeficiency Virus Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D054334:Viral Regulatory and Accessory Proteins / Q000235:genetics / Q000378:metabolism*; D014771:Virion / Q000378:metabolism / Q000502:physiology; D057074:Virus Release / Q000502:physiology; D044844:beta-Transducin Repeat-Containing Proteins / Q000235:genetics / Q000378:metabolism*; D000074010:Bone Marrow Stromal Antigen 2; D000086082:Viroporin Proteins
chemical_list: D015703:Antigens, CD; D058851:GPI-Linked Proteins; D054298:Human Immunodeficiency Virus Proteins; D054334:Viral Regulatory and Accessory Proteins; D044844:beta-Transducin Repeat-Containing Proteins; C094572:BST2 protein, human; D000074010:Bone Marrow Stromal Antigen 2; C514859:vpu protein, Human immunodeficiency virus 1; D000086082:Viroporin Proteins