Abstract
The genome of the human immunodeficiency virus type 1 (HIV-1) encodes the canonical retroviral proteins, as well as additional accessory proteins that enhance the expression of viral genes, the infectivity of the virus and the production of virions. The accessory Viral Protein U (Vpu), in particular, enhances viral particle production, while also promoting apoptosis of HIV-infected human T lymphocytes. Some Vpu effects rely on its interaction with the ubiquitin–proteasome protein degradation system, but the mechanisms responsible for its pro-apoptotic effects in vivo are complex and remain largely to be elucidated.We took advantage of the Drosophila model to study the effects of Vpu activity in vivo. Expression of Vpu in the developing Drosophila wing provoked tissue loss due to caspase-dependent apoptosis. Moreover, Vpu induced expression of the pro-apoptotic gene reaper, known to down-regulate Inhibitor of Apoptosis Proteins (IAPs) which are caspase-antagonizing E3 ubiquitin ligases. Indeed, Vpu also reduced accumulation of Drosophila IAP1 (DIAP1). Though our results demonstrate a physical interaction between Vpu and the proteasome-addressing SLIMB/β-TrCP protein, as in mammals, both SLIMB/βTrCP-dependent and -independent Vpu effects were observed in the Drosophila wing. Lastly, the pro-apoptotic effect of Vpu in this tissue was abrogated upon inactivation of the c-Jun N-terminal Kinase (JNK) pathway. Our results in the fly thus provide the first functional evidence linking Vpu pro-apoptotic effects to activation of the conserved JNK pathway.
Highlights
Viral invasion involves the expression of foreign genes that alter and constrain the host cellular machinery to propagate the life cycle of the virus
Expression of viral type I membrane phosphoprotein U (Vpu) with dpp-Gal4 led to loss of wing tissue, mainly in the anterior region, between longitudinal vein 2 (L2) and L3, including part of L3, as well as loss of the proximal cross-vein between veins L3 and L4 associated with tissue loss between L3–L4 (Figure 1A,C black and purple arrow, respectively)
In human Human immunodeficiency virus type 1 (HIV-1) infected T cells and in immortalized cell lines transfected with Vpu-expressing constructs, Vpu promotes p53-mediated apoptosis in a b-Transducine repeat Containing Protein (b-TrCP)-dependent manner [23]
Summary
Viral invasion involves the expression of foreign genes that alter and constrain the host cellular machinery to propagate the life cycle of the virus. Human immunodeficiency virus type 1 (HIV-1) expresses a unique set of accessory proteins (Vif, Nef, Vpr and Vpu) that interfere with various host cell functions thereby optimizing replicative efficiency and viral pathogenesis. A number of reports have shown the high complexity of the relationships between Vpu and cellular proteins of the host. They have highlighted the interaction between Vpu and the ubiquitylation/proteasome protein degradation system [2,5]. Cell-culture and in vitro experiments have demonstrated that Vpu can simultaneously bind CD4 and the b-Transducine repeat Containing Protein (b-TrCP), a F-box/WD40 substrate adaptor of the SCF (Skp-Cullin-F-box)/ CRL1 (Cullin1-RING ubiquitin ligase) E3 ubiquitin ligase complex leading to CD4 ubiquitylation and subsequent proteasomal degradation [10]. In cells arrested in early mitosis, the phosphorylation of another serine (Ser61) in Vpu may trigger its proteasomal degradation through an unknown E3-ubiquitin ligase, distinct from the SCF/ CRL1-b-TrCP complex [11]
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