BARD1 induces BRCA1 intranuclear foci formation by increasing RING-dependent BRCA1 nuclear import and inhibiting BRCA1 nuclear export.
BRCA1 is a tumor suppressor with several important nuclear functions. BRCA1 has no known cytoplasmic functions. We show here that the two previously identified nuclear localization signals (NLSs) are insufficient for nuclear localization of BRCA1 due to the opposing action of an NH2-terminal nuclear export signal. In transfected breast cancer cells, BRCA1 nuclear localization requires both the NLSs and NH2-terminal RING domain region; mutating either of these sequences shifts BRCA1 to the cytoplasm. The BRCA1 RING element mediates nuclear import via association with BARD1, and this is not affected by cancer-associated RING mutations. Moreover, BARD1 directly masks the BRCA1 nuclear export signal, and the resulting block to nuclear export is requisite for efficient import and nuclear localization of ectopic and endogenous BRCA1. Our results explain why BRCA1 exon 11 splice variants, which lack the NLSs but retain the RING domain, are frequently detected in the nucleus and in nuclear foci in vivo. In fact, co-expression of BARD1 promoted formation of DNA damage-induced nuclear foci comprising ectopic wild-type or NLS-deficient BRCA1, implicating BARD1 in nuclear targeting of BRCA1 for DNA repair. Our identification of BARD1 as a BRCA1 nuclear chaperone has regulatory implications for its reported effects on BRCA1 protein stability, ubiquitin ligase activity, and DNA repair.
Fabbro
Megan
M
Westmead Institute for Cancer Research, University of Sydney, Westmead Millennium Institute at Westmead Hospital, Westmead, 2145, New South Wales, Australia.
Rodriguez
Jose A
JA
Baer
Richard
R
Henderson
Beric R
BR
D021581:Active Transport, Cell Nucleus; D019313:BRCA1 Protein / Q000378:metabolism*; D015153:Blotting, Western; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism* / Q000502:physiology*; D002467:Cell Nucleus / Q000378:metabolism; D004249:DNA Damage; D004260:DNA Repair; D006801:Humans; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D008957:Models, Genetic; D008958:Models, Molecular; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D017434:Protein Structure, Tertiary; D013347:Subcellular Fractions; D014162:Transfection; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins; D044767:Ubiquitin-Protein Ligases
D019313:BRCA1 Protein; D002352:Carrier Proteins; D025521:Tumor Suppressor Proteins; C102449:BARD1 protein, human; D044767:Ubiquitin-Protein Ligases