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Exploring the functional interaction between POSH and ALIX and the relevance to HIV-1 release

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Background

ALIX (ALG2 interacting protein X) is a multi-functional adaptor protein that plays a central role in the regulation of intracellular protein trafficking and apoptosis. As an ESCRT-associated regulator of protein trafficking, ALIX plays an essential role in retrovirus release, an activity that is dependent on the interaction between the central V-domain and the L-domain consensus sequence YPXnL in Gag [1, 2]. The trans-Golgi network RING finger protein POSH (Plenty of SH3) is a scaffold protein that acts as an E3 ligase and augments HIV-1 egress by facilitating the transport of Gag to the cell membrane [3]. Recently, it was reported, that POSH interacts with ALIX and thereby enhances ALIX mediated phenotypes in Drosophila [4].

Results

In this study we identified ALIX as a POSH ubiquiti-nation substrate in human cells: POSH induces polyubiquitination of ALIX that is modified on several lysine residues in vivo and in vitro. This ubiquitination does not destabilize ALIX, which suggests a regulatory function. Consistent with the well known activity of ALIX in virus release that rescues budding of L-domain mutant HIV-1 [2, 5], we demonstrated that wild type POSH, but not an ubiquitination inactive RING finger mutant (POSHV14A), enhances ALIX mediated release of HIV-1ΔPTAP variants. In further agreement with the idea of a cooperative function of POSH and ALIX, mutating the YPXnL-ALIX binding site in Gag completely abrogated augmentation of virus release by overexpression of POSH. However, the effect of the POSH-mediated ubiquitination appears to be auxiliary, but not necessary, as silencing of POSH by RNAi does not disturb ALIX mediated augmentation of virus release.

Conclusion

Thus, the cumulative results identified ALIX as an ubiquitination substrate of POSH and indicate that POSH and ALIX cooperate to facilitate efficient virus release. However, while ALIX is obligatory for the release of YPXnL-dependent HIV-1, POSH, albeit rate-limiting, may be functionally interchangeable.

References

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    Strack B, Calistri A, Craig S, Popova E, Gottlinger HG: AIP1/ALIX is a binding partner for HIV-1 p6 and EIAV p9 functioning in virus budding. Cell. 2003, 114: 689-699. 10.1016/S0092-8674(03)00653-6.

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    Fisher RD, Chung HY, Zhai Q, Robinson H, Sundquist WI, Hill CP: Structural and biochemical studies of ALIX/AIP1 and its role in retrovirus budding. Cell. 2007, 128: 841-852. 10.1016/j.cell.2007.01.035.

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    Alroy I, Tuvia S, Greener T, Gordon D, Barr HM, Taglicht D, Mandil-Levin R, Ben-Avraham D, Konforty D, Nir A, et al: The trans-Golgi network-associated human ubiquitin-protein ligase POSH is essential for HIV type 1 production. Proc Natl Acad Sci USA. 2005, 102: 1478-1483. 10.1073/pnas.0408717102.

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    Tsuda M, Seong KH, Aigaki T: POSH, a scaffold protein for JNK signaling, binds to ALG-2 and ALIX in Drosophila. FEBS Lett. 2006, 580: 3296-3300. 10.1016/j.febslet.2006.05.005.

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    Zhai Q, Fisher RD, Chung HY, Myszka DG, Sundquist WI, Hill CP: Structural and functional studies of ALIX interactions with YPX(n)L late domains of HIV-1 and EIAV. Nat Struct Mol Biol. 2008, 15: 43-49. 10.1038/nsmb1319.

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Author information

Correspondence to Jörg Votteler.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Keywords

  • Ring Finger
  • Scaffold Protein
  • Mediate Release
  • Protein Trafficking
  • Virus Release