22 Nov 2004

Defeating the yokozuna, the highest ranked professional sumo wrestlers, is almost impossible. Stopping sumoylation, the modification of proteins with small ubiquitin-like modifiers (SUMO) may be easier. In the November 19 Molecular Cell, Susanna Chiocca and colleagues from the European Institute of Oncology, Milan, Italy, and the University of St. Andrews, Scotland, report that Gam1, a protein from the lightweight adenovirus, inhibits the pathway.

While ubiquitination tags proteins for degradation, sumoylation is thought to have the opposite effect, rendering targeted proteins more stable. Experimental models have implicated the pathway in several neurodegenerative diseases, including Alzheimer disease, where SUMO-3 expression has been shown to attenuate Aβ production (see ARF related news story) and Huntington disease, where sumoylation of huntingtin has been shown to exacerbate pathology (see ARF related news story). Sumoylation has also been implicated in transcriptional regulation, sometimes to activate (as in heat shock proteins, see Goodson et al., 2001), and sometimes to inactivate (as with the transcription factor SP3; see Bies et al., 2002 ).

In fact, Gam1 is a transcriptional activator. Its role in host cells is to create a favorable environment for replication by turning on essential host genes. Chiocca and colleagues asked whether Gam1 may somehow achieve this by regulating sumoylation. To test this, joint first authors Roberto Boggio and Riccardo Colombo added the adenoviral protein to test tube sumoylation reactions, finding that it completely abolished modification of test substrates. In contrast, a previously described inactive mutant of Gam1 had no effect.

The authors then carried out a series of experiments to elucidate exactly how Gam1 inhibits the pathway. Sumoylation, like ubiquitination, proceeds in stepwise fashion. First, proteins dubbed E1 catalyze the activation of the SUMO peptides. Second, SUMO is conjugated to the enzyme UBC9. Third, SUMO is passed from UBC9 to substrate proteins via a protein ligase called E3. The authors, again working with their in vitro system, found that if the E1-SUMO complex forms, then Gam1 has no effect on the subsequent reactions, indicating that the adenoviral protein acts on the first step.

So exactly how does Gam1 function? Using biochemical assays, the authors found that it does not proteolytically cleave SUMO from E1, nor does it recruit other proteases to make this happen. Instead, when Boggio and Colombo expressed Gam1 in hepatocytes, they found that SAE1 and SAE2, subunits of the E1 heterodimer, disappeared. Pulse-chase experiments showed that the half-lives of these proteins were dramatically shortened by Gam1. Also, the authors showed that immunoprecipitates of SAE1/2 and Gam1, but not of SAE1/2 and the Gam1 mutant, were inactive, suggesting that the adenoviral protein both inhibits the SUMO activators and hastens their demise.

As for regulation of transcription, the authors found that Gam1 induces expression of luciferase from an SP3-driven promoter. This is in keeping with the knowledge that, when sumoylated, SP3 transcription activity dramatically declines.

These findings may lead to the development of novel pharmaceutical agents, claim the authors. For example, Chiocca and colleagues have also recently demonstrated that Gam1 binds to and inactivates histone deacetylases (Chiocca et al., 2002). Because histone deacetylases (see ARF related news story) and sumoylation have been shown to exacerbate pathology in animal models of Huntington disease, mimicking the action of Gam1 could, in theory, deliver a double blow in the fight against HD.—Tom Fagan

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References

News Citations

1. SUMO-Wrestling with APP? 15 Jan 2003
2. SUMO versus Ubiquitin: A Fight for Huntington’s Disease? 2 Apr 2004
3. Drugs Slow Neurodegeneration in Fly Model of Huntington's 22 Nov 2002

Paper Citations

1. Goodson ML, Hong Y, Rogers R, Matunis MJ, Park-Sarge OK, Sarge KD. Sumo-1 modification regulates the DNA binding activity of heat shock transcription factor 2, a promyelocytic leukemia nuclear body associated transcription factor. J Biol Chem. 2001 May 25;276(21):18513-8. PubMed.
2. Bies J, Markus J, Wolff L. Covalent attachment of the SUMO-1 protein to the negative regulatory domain of the c-Myb transcription factor modifies its stability and transactivation capacity. J Biol Chem. 2002 Mar 15;277(11):8999-9009. PubMed.
3. Chiocca S, Kurtev V, Colombo R, Boggio R, Sciurpi MT, Brosch G, Seiser C, Draetta GF, Cotten M. Histone deacetylase 1 inactivation by an adenovirus early gene product. Curr Biol. 2002 Apr 2;12(7):594-8. PubMed.

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