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The LIM domain protein LMO4 interacts with the cofactor CtIP and the tumor suppressor BRCA1 and inhibits BRCA1
Eleanor Y M Sum1, Benjamin Peng, Xin Yu
1Walter and Eliza Hall Institute of Medical Research and Bone Marrow Research Laboratories, Melbourne, Victoria 3050, Australia.
Abstract:
LMO4 belongs to the LIM-only (LMO) group of transcriptional regulators that appear to function as molecular adaptors for protein-protein interactions. Expression of the LMO4 gene is developmentally regulated in the mammary gland and is up-regulated in primary breast cancers. Using LMO4 in a yeast two-hybrid screen, we have identified the cofactor CtIP as an LMO4-binding protein. Interaction with CtIP appeared to be specific for the LMO subclass of LIM domain proteins and could be mediated by a single LIM motif of LMO4. We further identified the breast tumor suppressor BRCA1 as an LMO4-associated protein. The C-terminal BRCT domains of BRCA1, previously shown to bind CtIP, also mediated interaction with LMO4. Tumor-associated mutations within the BRCT repeats that abolish interaction between BRCA1 and CtIP had no effect on the association of BRCA1 with LMO4. A stable complex comprising LMO4, BRCA1, and CtIP was demonstrated in vivo. The LIM domain binding-protein Ldb1 also participated in this multiprotein complex. In functional assays, LMO4 was shown to repress BRCA1-mediated transcriptional activation in both yeast and mammalian cells. These findings reveal a novel complex between BRCA1, LMO4, and CtIP and indicate a role for LMO4 as a repressor of BRCA1 activity in breast tissue.
Insights
LIM-only protein 4 (LMO4) interacts with breast cancer suppressor BRCA1 and CtIP, forming a complex that represses BRCA1 activity in breast tissue.
Area of Science:
- Molecular Biology
- Cancer Biology
- Genetics
Background:
- LIM-only (LMO) proteins are transcriptional regulators acting as molecular adaptors.
- LMO4 expression is developmentally regulated in mammary glands and elevated in breast cancers.
Purpose of the Study:
- To identify LMO4-interacting proteins.
- To investigate the functional relationship between LMO4, BRCA1, and CtIP in breast tissue.
Main Methods:
- Yeast two-hybrid screening to identify binding partners.
- In vivo complex formation assays.
- Functional transcriptional activation assays in yeast and mammalian cells.
Main Results:
- CtIP and BRCA1 were identified as LMO4-binding proteins.
- A stable multiprotein complex of LMO4, BRCA1, CtIP, and Ldb1 was demonstrated in vivo.
- LMO4 repressed BRCA1-mediated transcriptional activation.
Conclusions:
- A novel LMO4-BRCA1-CtIP complex exists and is implicated in breast tissue.
- LMO4 acts as a repressor of BRCA1 activity, potentially contributing to breast cancer development.