RESUMEN
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 post-translational modifications also regulate SUMO conjugation, suggesting that SUMO signalling is integrated with other signal transduction pathways. A better understanding of SUMO regulatory mechanisms will lead to improved approaches for analysing the function of SUMO and substrate conjugation in distinct cellular pathways.
Asunto(s)
Conformación Proteica , Transducción de Señal , Proteínas Modificadoras Pequeñas Relacionadas con Ubiquitina/metabolismo , Sumoilación/fisiología , Secuencia de Aminoácidos , Animales , Humanos , Modelos Biológicos , Modelos Moleculares , Dominios y Motivos de Interacción de Proteínas/fisiología , Proteína SUMO-1/química , Proteína SUMO-1/metabolismo , Transducción de Señal/fisiología , Proteínas Modificadoras Pequeñas Relacionadas con Ubiquitina/química , Especificidad por SustratoRESUMEN
The RanBP2 nucleoporin contains an internal repeat domain (IR1-M-IR2) that catalyzes E3 ligase activity and forms a stable complex with SUMO-modified RanGAP1 and UBC9 at the nuclear pore complex. RanBP2 exhibits specificity for SUMO1 as RanGAP1-SUMO1/UBC9 forms a more stable complex with RanBP2 compared with RanGAP1-SUMO2 that results in greater protection of RanGAP-SUMO1 from proteases. The IR1-M-IR2 SUMO E3 ligase activity also shows a similar preference for SUMO1. We utilized deletions and domain swap constructs in protease protection assays and automodification assays to define RanBP2 domains responsible for RanGAP1-SUMO1 protection and SUMO1-specific E3 ligase activity. Our data suggest that elements in both IR1 and IR2 exhibit specificity for SUMO1. IR1 protects RanGAP1-SUMO1/UBC9 and functions as the primary E3 ligase of RanBP2, whereas IR2 retains the ability to interact with SUMO1 to promote SUMO1-specific E3 ligase activity. To determine the structural basis for SUMO1 specificity, a hybrid IR1 construct and IR1 were used to determine three new structures for complexes containing UBC9 with RanGAP1-SUMO1/2. These structures show more extensive contacts among SUMO, UBC9, and RanBP2 in complexes containing SUMO1 compared with SUMO2 and suggest that differences in SUMO specificity may be achieved through these subtle conformational differences.
Asunto(s)
Proteínas Activadoras de GTPasa/química , Chaperonas Moleculares/química , Proteínas de Complejo Poro Nuclear/química , Proteína SUMO-1/química , Ubiquitina-Proteína Ligasas/química , Secuencia de Aminoácidos , Cristalografía por Rayos X , Proteínas Activadoras de GTPasa/genética , Proteínas Activadoras de GTPasa/metabolismo , Humanos , Chaperonas Moleculares/genética , Chaperonas Moleculares/metabolismo , Proteínas de Complejo Poro Nuclear/genética , Proteínas de Complejo Poro Nuclear/metabolismo , Unión Proteica , Estructura Cuaternaria de Proteína , Estructura Terciaria de Proteína , Proteína SUMO-1/genética , Proteína SUMO-1/metabolismo , Eliminación de Secuencia , Sumoilación/fisiología , Ubiquitina-Proteína Ligasas/genética , Ubiquitina-Proteína Ligasas/metabolismoRESUMEN
A 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.