Proteins of the small ubiquitin-related modifier (SUMO) family are conjugated to proteins to regulate such cellular processes as nuclear transport, transcription, chromosome segregation and DNA repair. Recently, numerous insights into regulatory mechanisms of the SUMO modification pathway have emerged. Although SUMO-conjugating enzymes can discriminate between SUMO targets, many substrates possess characteristics that facilitate their modification. Other posttranslational modifications also regulate SUMO conjugation, suggesting that SUMO signaling is integrated with other signal transduction pathways. A better understanding of SUMO regulatory mechanisms will lead to improved approaches for analyzing the function of SUMO and substrate conjugation in distinct cellular pathways.

SUMMARYA key feature in Huntington disease (HD) is the accumulation of mutant Huntingtin (HTT) protein, which may be regulated by posttranslational modifications. Here, we define the primary sites of SUMO modification in the amino-terminal domain of HTT, show modification downstream of this domain, and demonstrate that HTT is modified by the stress-inducible SUMO-2. A systematic study of E3 SUMO ligases demonstrates that PIAS1 is an E3 SUMO ligase for both HTT SUMO-1 and SUMO-2 modification and that reduction of dPIAS in a mutant HTT Drosophila model is protective. SUMO-2 modification regulates accumulation of insoluble HTT in HeLa cells in a manner that mimics proteasome inhibition and can be modulated by overexpression and acute knockdown of PIAS1. Finally, the accumulation of SUMO-2-modified proteins in the insoluble fraction of HD postmortem striata implicates SUMO-2 modification in the age-related pathogenic accumulation of mutant HTT and other cellular proteins that occurs during HD progression.

Background: The RanBP2 internal repeat domain (IR1-M-IR2) catalyzes SUMO E3 ligase activity and binds SUMO1-RanGAP1/UBC9 at the nuclear pore complex.Results: Biochemistry and structures of RanBP2/SUMO-RanGAP1/UBC9 are presented.Conclusion: IR1 protects RanGAP1-SUMO1/UBC9 and functions as the primary E3 ligase of RanBP2, whereas IR2 interacts with SUMO1 to promote weaker SUMO1-specific E3 ligase activity.Significance: RanBP2/SUMO interactions provide insight to SUMO isoform specificity.

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