The Sumo-targeted ubiquitin ligase RNF4 regulates the localization and function of the HTLV-1 oncoprotein Tax

Abstract

The Really Interesting New Gene (RING) Finger Protein 4 (RNF4) represents a class of ubiquitin ligases that target Small Ubiquitin-like Modifier (SUMO)-modified proteins for ubiquitin modification. To date, the regulatory function of RNF4 appears to be ubiquitin-mediated degradation of sumoylated cellular proteins. In the present study, we show that the Human T-cell Leukemia Virus Type 1 (HTLV-1) oncoprotein Tax is a substrate for RNF4 both in vivo and in vitro. We mapped the RNF4-binding site to a region adjacent to the Tax ubiquitin/SUMO modification sites K280/K284. Interestingly, RNF4 modification of Tax IntroductionHuman T-cell Leukemia Virus Type 1 (HTLV-1) is the etiological agent for adult T-cell leukemia. 1 Immortalization and transformation of T lymphocytes can be attributed to the expression and activity of the viral oncoprotein Tax. 2 Although the exact mechanism of Tax-mediated transformation is unknown, studies indicate that Tax expression results in genomic instability via chronic disruption of the cellular DNA damage response. 3 It is generally accepted that cellular transformation is a by-product of the long period of genomic instability.Tax exhibits pleiotropic functionality, which is at least partly regulated by subcellular localization to nuclear or cytoplasmic compartments. 3,4 We demonstrated previously that Tax shuttles between the nucleus and cytoplasm. 5 The mechanism for the regulation of Tax localization is unknown, although localizationspecific structural elements have been uncovered. The Tax protein contains a nuclear localization signal, 6 and we recently identified a specific signal sequence that targets Tax to nuclear bodies. 7 We and others have described a nuclear export signal. 5,8 In addition, Lamsoul et al showed that ubiquitylation of Tax is correlated with its accumulation in the cytoplasm, whereas sumoylation of Tax is required for nuclear localization. 9 Therefore, the molecular switch for nuclear versus cytoplasmic localization of Tax at least in part depends on the protein modifiers Small Ubiquitin-like Modifier (SUMO) and ubiquitin, but the mechanism underlying this switch is unknown.Coincident sumoylation and ubiquitylation of a substrate protein is common, and in some cases, these modifications work cooperatively to regulate specific biologic processes (for review, see Ulrich 10 ). The recent discovery of a novel class of Really Interesting New Gene (RING)-domain proteins called SUMOtargeted ubiquitin ligases (STUbLs) has contributed to our understanding of how ubiquitylation of proteins is regulated. STUbL proteins contain SUMO-interacting motifs (SIMs) to interact with the SUMO or SUMO-like domains of their ubiquitylation targets. Therefore, STUbLs are well suited to play an important role in the cross-talk between SUMO and ubiquitin pathways. 11 The role of STUbLs was first realized by studying 2 RING domain-containing proteins, Slx5 and Slx8, in budding and fission yeast. [12][13][14][15] The Slx5/Slx8 complex was subsequently found to mediate quality control of...

Low nuclear body formation and tax SUMOylation do not prevent NF-kappaB promoter activation

“…Furthermore, we show here that fusion of SUMO-1 stabilizes both Tax and Tax-P79AQ81A in the cytosol but only significantly increases ubiquitination of Tax-P79AQ81A. This latter effect is reminiscent of the cytoplasmic relocalization and enhanced ubiquitination of the GFP-tagged Tax construct upon RNF4 overexpression [27]. Why RNF4 would only increase the ubiquitination of Tax mutants but not wt Tax remains to be elucidated.…”

HTLV Tax: A Fascinating Multifunctional Co-Regulator of Viral and Cellular Pathways

“…It has also been shown that the SUMO-dependent ubiquitin ligase, Really Interesting New Gene Finger Protein 4 (RNF4), binds to SUMOylated Tax next to the Lys280/Lys284 ubiquitin and SUMO modification sites and ubiquitinates the nuclear located protein. The ubiquitination of Tax by RNF4 leads to the cellular translocation of Tax to the cytoplasm and subsequently increases NF-κB activation while decreasing Tax activity associated with interaction with nuclear CREB (Fryrear et al, 2012). Ubiquitination of Tax at lysine residues 263, 280, and 284 is also required for Tax assisted localization of IKKγ to the Golgi (Harhaj et al, 2007).…”

RNF4-Dependent Oncogene Activation by Protein Stabilization

“…Moreover, STUbL-mediated ubiquitylation does not only target proteins for degradation. For example, STUbL-mediated ubiquitylation affects the affinity of protein-protein interactions, which, in turn, affects subcellular localization, as well as enhances transcriptional activation (Fryrear et al, 2012; Poukka et al, 2000; Hu et al, 2010). Previously, we showed that Dgrn, the sole Drosophila STUbL protein, inactivates the SUMOylated co-repressor Groucho/Transducing-like enhancer of split (Gro/TLE) and inhibits Gro-dependent gene repression in vivo (Abed et al, 2011).…”