SUMO-l Modification of BRCAl Modulates Its Transcriptional Activity and Subnuclear Localization

다국어 초록 (Multilingual Abstract)

Carriers of mutations in the tumor suppressor gene BRCA1 are predisposed to familial breast and ovarian cancers. BRCA1 plays the diverse roles in transcription, cell cycle checkpoint and DNA repair. BRCA1 functions in transcription as an activator but also a suppressor, and in DNA repair by maintaining genetic integrity. Herein, we demonstrate that BRCA1 can interact with SUMO-1, and they are colocalized as foci in the nuclei. BRCA1 can be modified by SUMO-1 conjugation at seven K residues throughout its full length, in vivo as well as in vitro. Overall SUMO-1 modifications of BRCA1 can modulate its transcriptional activity, leading to repressing its transcriptional activity on Gadd45 gene expression. Mutations of these target residues(K to R) affect the transcriptional activity of BRCA1 in the diverse manners. SUMO-1 modification negative BRCA1s at N-terminus function as a super-active transcriptional activator for Gadd45 expression, suggesting that these three K residues are required for the transcriptional suppression of BRCA1. However, SUMO1-modification mutations at the C-terminus activate more strongly than the wild type, implying these residues are also involved in repression, while intermediary region mutations activate Gadd45 transcription as the wild type does. Together with these transcriptional activities, SUMO-1 modification of the ring domain in the N-terminal may mainly be engaged in suppression, whereas the BRCT domain at the C-terminal functions in both activation and suppression. Additionally, we studied subnuclear localization of the wild type and SUMO-1 modification mutants, to see whether SUMO-1 modification plays a critical role in subnuclear localization of BRCA1. The ability of BRCA1 to form foci upon DNA damaging, is alleviated in all SUMO-1 modification mutants, but some are profoundly unable. These residues belong to the domains interacting with Rad51, BRCA2 or Rb those are involved in maintaining genomic integrity, implying that SUMO-1 modification of BRCA1 can modulate genomic integrity. All these SUMO-1 modification mutations impair significantly foci of SUMO-1 or Rad51, resulting in dispersion of them. One SUMO-1 modification mutation alters the localization Rad51 or BRCA2, thus they are localized in the cytoplasm. SUMO-1 modification of BRCA1 is necessary for its proper subnuclear localization and compartmentalization, and furthermore may affect the proper localization of Rad51 and BRCA2 of BRCA1-associated complex. Overall, this study provides the first mechanistic insight into how SUMO-1 modification modulates the transcriptional function of BRCA1 in both opposing activation and repression, but also its subnuclear localization and colocalization with other proteins functioning in DNA damage response. Therefore, our study establishes, for the first time, functional interplay between SUMO-1 modification and a number of surrogate biological roles of BRCA1.