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Ubc9 mediates nuclear localization and growth suppression of BRCA1 and BRCA1a proteins
Author(s) -
Qin Yunlong,
Xu Jingyao,
Aysola Kartik,
Begum Nurjahan,
Reddy Vaishali,
Chai Yuli,
Grizzle William E.,
Partridge Edward E.,
Reddy E. Shyam P.,
Rao Vee.
Publication year - 2011
Publication title -
journal of cellular physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.529
H-Index - 174
eISSN - 1097-4652
pISSN - 0021-9541
DOI - 10.1002/jcp.22695
Subject(s) - cytoplasm , subcellular localization , nuclear localization sequence , biology , mutant , microbiology and biotechnology , nuclear transport , ubiquitin ligase , gene knockdown , nuclear protein , nuclear export signal , cell nucleus , ubiquitin , gene , transcription factor , biochemistry
BRCA1 gene mutations are responsible for hereditary breast and ovarian cancers. In sporadic breast tumors, BRCA1 dysfunction or aberrant subcellular localization is thought to be common. BRCA1 is a nuclear–cytoplasm shuttling protein and the reason for cytoplasmic localization of BRCA1 in young breast cancer patients is not yet known. We have previously reported BRCA1 proteins unlike K109R and cancer‐predisposing mutant C61G to bind Ubc9 and modulate ER‐α turnover. In the present study, we have examined the consequences of altered Ubc9 binding and knockdown on the subcellular localization and growth inhibitory function of BRCA1 proteins. Our results using live imaging of YFP, GFP, RFP‐tagged BRCA1, BRCA1a and BRCA1b proteins show enhanced cytoplasmic localization of K109 R and C61G mutant BRCA1 proteins in normal and cancer cells. Furthermore, down‐regulation of Ubc9 in MCF‐7 cells using Ubc9 siRNA resulted in enhanced cytoplasmic localization of BRCA1 protein and exclusive cytoplasmic retention of BRCA1a and BRCA1b proteins. These mutant BRCA1 proteins were transforming and impaired in their capacity to inhibit growth of MCF‐7 and CAL51 breast cancer cells. Interestingly, cytoplasmic BRCA1a mutants showed more clonogenicity in soft agar and higher levels of expression of Ubc9 than parental MCF7 cells. This is the first report demonstrating the physiological link between cytoplasmic mislocalization of mutant BRCA1 proteins, loss of ER‐α repression, loss of ubiquitin ligase activity and loss of growth suppression of BRCA1 proteins. Thus, binding of BRCA1 proteins to nuclear chaperone Ubc9 provides a novel mechanism for nuclear import and control of tumor growth. J. Cell. Physiol. 226: 3355–3367, 2011. © 2011 Wiley Periodicals, Inc.