Both ubiquitin and SUMO are used by cells to modify proteins post-translationally. Through independent mechanisms, both molecules can be covalently attached to the side chains of lysine amino acids. Ubiquitination, which targets proteins for proteasome-mediated degradation, has been shown to attenuate the toxic effects of polyglutamine-expanded huntingtin, the mutated protein that causes HD. Sumoylation, on the other hand, has been shown to protect some proteins from degradation. It has also been found in neural tissue of Huntington’s patients (see Ueda et al., 2002) and has been linked to polyglutamine disease pathology in fruit flies (see Chan et al., 2002). So what is its role, if any, in the pathology of HD?

To answer this question, first author Joan Steffan and colleagues, including Elena Cattaneo at the University of Milan, Italy, first looked to see where SUMO-1 is located in the cell. They found that the molecule colocalizes with a truncated form of a pathogenic huntingtin fragment when the two are expressed in striatal neurons, and that Htt is sumoylated. As this variant of huntingtin has only three lysines (at positions six, nine, and fifteen), the authors mutated each in turn to determine which ones may be potential sites for addition of SUMO. Lysines six and nine, it turns out, are predominantly modified.

To determine how sumoylation affects a protein’s fate, Steffan made a fusion hybrid that has a SUMO moiety permanently attached to the N-terminal of polyQ huntingtin. Because the N-terminus and lysine-6 of huntingtin are relatively close, this fusion may behave similarly to naturally sumoylated Htt. When this hybrid was expressed in striatal cells, Steffan and colleagues found it was more stable than just a regular huntingtin fragment, and that it showed a reduced tendency to aggregate; in fact, most of it was soluble. What’s more, the scientists found that the fusion hybrid powerfully suppressed transcription. When the authors coexpressed the hybrid with a luciferase reporter driven by the multidrug resistance gene promoter, luciferase activity was less than one-fifth of that seen in cells without the SUMO hybrid.

Is SUMO there to prevent ubiquitination? There’s more to it than that, suggest the authors. They show that mutating all the Htt lysines substantially reduces neurodegeneration in a fly model of disease. Yet results from several labs have shown that without ubiquitination the pathology worsens. This doesn’t happen when ubiquitination sites are removed, suggest the authors, because the sumoylation sites are also removed and sumoylation results in a more aggressive pathology, most likely by increasing solubility and transcriptional suppressor activity. Thus, “inhibiting SUMO-1 ligases, or increasing isopeptidase activity to remove SUMO-1 could reduce the level of sumoylated Htt in neurons and suppress HD pathogenesis,” they write.

In support of the transcriptional suppression theory, Michael White and colleagues from the University of Texas Southwestern Medical Center, Dallas, report in the March 11 Journal of Biological Chemistry, in press, that they have identified 21 candidate proteins that are sumoylated in HEK293 cells. Over half of these may be linked to transcription. Seven are subunits of RNA polymerase, while six are associated with chromatin.—Tom Fagan.

References:
Steffan JS, Agrawal N, Pallos J, Rockabrand E, Trotman LC, Slepko N, Illes K, Lukacsovich T, Zhu Y-Z, Cattaneo E, Pandolfi PP, Thompson LM, Marsh JL. SUMO Modification of Huntingtin and Huntington’s Disease Pathology. Science 2004 April 2;304:100-104. Abstract

Zhao Y, Kwon SW, Anselmo A, Kaur K, White MA. Broad-spectrum identification of cellular SUMO substrate proteins. J Biol Chem. 2004 Mar 11 [Epub ahead of print]. Abstract