title: Ubiquitination and proteasomal degradation of the BRCA1 tumor suppressor is regulated during cell cycle progression.
abstract: The BRCA1 tumor suppressor and the BARD1 protein form a stable heterodimeric complex that can catalyze the formation of polyubiquitin chains. Expression of BRCA1 fluctuates in a cell cycle-dependent manner, such that low steady-state levels of BRCA1 gene products are found in resting cells and early G1 cycling cells and high levels in S and G2 phase cells. Although transcriptional activation of the BRCA1 gene can account for induction of BRCA1 expression at the G1/S transition, the mechanisms by which BRCA1 is down-regulated during cell cycle progression have not been addressed. Here we show that the steady-state levels of BRCA1 protein remain elevated throughout mitosis but begin to decline at the M/G1 transition. This decline in BRCA1 levels coincides with the appearance of proteasome-sensitive ubiquitin conjugates of BRCA1 at the onset of G1. Formation of these conjugates occurs throughout G1 and S, but not in cells arrested in prometaphase by nocodazole. The proteasome-sensitive ubiquitin conjugates of BRCA1 appear to be distinct from BRCA1 autoubiquitination products and are probably catalyzed by the action of other cellular E3 ligases. Interestingly, co-expression of BARD1 inhibits the formation of these conjugates, suggesting that BARD1 serves to stabilize BRCA1 expression in part by reducing proteasome-sensitive ubiquitination of BRCA1 polypeptides. In summary, these data indicate that the cell cycle-dependent pattern of BRCA1 expression is determined in part by ubiquitin-dependent proteasomal degradation.
authors.affiliation: Institute for Cancer Genetics, Department of Pathology, Columbia University, New York, New York 10032, USA.
authors.forename: Atish D
authors.initials: AD
authors.lastname: Choudhury
authors.forename: Hong
authors.initials: H
authors.lastname: Xu
authors.forename: Richard
authors.initials: R
authors.lastname: Baer
mesh_terms: D019313:BRCA1 Protein / Q000032:analysis / Q000235:genetics* / Q000378:metabolism*; D002453:Cell Cycle / Q000502:physiology*; D002460:Cell Line; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D016193:G1 Phase; D016195:G2 Phase; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D007700:Kinetics; D008938:Mitosis; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015739:Nocodazole / Q000494:pharmacology; D011480:Protease Inhibitors / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex; D012333:RNA, Messenger / Q000032:analysis; D011993:Recombinant Fusion Proteins; D016196:S Phase; D014162:Transfection; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D025801:Ubiquitin / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D001749:Urinary Bladder Neoplasms
chemical_list: D019313:BRCA1 Protein; D009097:Multienzyme Complexes; D011480:Protease Inhibitors; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D025521:Tumor Suppressor Proteins; D025801:Ubiquitin; C102449:BARD1 protein, human; D044767:Ubiquitin-Protein Ligases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D015739:Nocodazole