Targeting a nucleolar SUMO protease for degradation: A mechanism by which ARF induces SUMO conjugation

The p19^ARF p14^ARF in humans protein acts as a tumour suppressor through p53 dependent and independent mechanisms. A well-established role for ARF is to regulate the post-translational modification of substrate proteins with ubiquitin and ubiquitin-like molecules such as SUMO. It is now evident that induction of ARF causes a dramatic accumulation of SUMO conjugates and this has been related to the p53 independent functions of ARF. The majority of these conjugates appear to accumulate in the nucleolus where most of ARF is also found. An obvious function for ARF, which would result in increase of SUMOylation, is to act as an atypical SUMO E3-ligase. Indeed, initial studies suggested that ARF could directly interact with the SUMO E2-conjugating enzyme Ubc9 and therefore bringing the SUMO conjugation machinery in close proximity to its interacting substrates.¹ However, the highly basic charged nature of ARF makes biochemical analysis difficult and there is no clear demonstration that ARF can fulfill the criteria for an E3-ligase in vitro. Therefore, the mechanism(s) behind this phenomenon are not currently understood. As with ubiquitination, SUMO conjugation is a dynamic process controlled by E3-ligases and proteases that specifically remove SUMO from substrates. In this issue of Cell Cycle studies from the Sherr lab suggest that ARF can increase SUMO conjugation by controlling the stability of the nucleolar SUMO protease SENP3.² Recent studies have shown that SENP3 can deconjugate SUMO-2 and SUMO-3 from substrates including nucleophosmin (NPM). NPM is a nucleolar protein, which among other processes is involved in the processing of rRNA during ribosome biosynthesis. NPM interacts with ARF and this results in increased SUMOylation of NPM. SENP3 can counteract the effect of ARF by deconjugating SUMO from NPM and this appears to be critical for NPM function in rRNA processing.³ The new study now suggests that there is an opposing functional relationship between ARF and SENP3. ARF promotes phosphorylation dependent ubiquitination of SENP3, which results in SENP3 degradation and increase in NPM SUMO conjugation. In this process, NPM seems to act as a "platform" for ARF and SENP3, bringing in close proximity its two regulators. The new study suggests an interesting and complex mechanism by which ARF can control SUMOylation. It is now evident that post-translational modifications cooperate to control protein function. The new data suggest that ARF engages phosphorylation to promote ubiquitination and proteasomal degradation of a SUMO protease. This model would propose the existence of a kinase/phosphatase and an E3-ubiquitin ligase/de-ubiquitinating enzyme set which would cooperate their actions to control the stability of SENP3. Given that ARF has multiple binding partners, it would not be surprising that ARF would interact with components of the above enzymatic steps and control their activity. It would therefore be interesting to identify the role of ARF in this process. It is not clear whether degradation of SENP3 per se is sufficient to induce NPM SUMO conjugation and if this is the case which SUMO E3-ligases drive the forward reaction. Even if in this study an interaction of ARF with Ubc9 could not be demonstrated it may be the case that ARF mediates both the degradation of SENP3 and recruitment of the SUMO conjugation machinery, which will result in fast and efficient accumulation of SUMOylated NPM. Another possibility is the effect of ARF on NPM stability itself. Previous studies have shown that ARF can induce ubiquitin-mediated degradation of NPM.⁴ As NPM is important to prevent destabilisation of SENP3, ARF-mediated degradation of NPM could be part of SENP3 degradation. Another point that arises from this is the site of degradation for SENP3. Nucleoli have been suggested to be deficient for proteasomal activity, suggesting that ARF through the phosphorylation/ubiquitination events may alter the localisation/mobility of SENP3 making it susceptible to nucleoplasmic/cytoplasmic proteasomal degradation. The effect of ARF in controlling protein ubiquitination is now well established. Interaction of ARF with E3-ligases such as Mdm2 and ARF-BP1/Mule inhibits their function resulting in inhibition of p53 proteasomal degradation.^5,6 Therefore, the ability of ARF to induce ubiquitination and proteasomal degradation of SENP3 and NPM shows a complex and diverse role for ARF to control protein stability. Further experiments will show whether the ability of ARF to promote degradation of SENP3 or possibly other SUMO proteases is a general mechanism through which ARF induces SUMO conjugation of its binding partners or that the NPM/SENP3 system is a unique example.

References

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