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GCIP, a novel human grap2 and cyclin D interacting protein, regulates E2F-mediated transcriptional activity
C Xia1, Z Bao, F Tabassam
1Department of Medical Biochemistry and Genetics, Center for Cancer Biology and Nutrition, Institute of Biosciences and Technology, Texas A&M University System Health Science Center, Houston, Texas 77030, USA.
Abstract:
Regulation of mammalian cell growth and proliferation is governed through receptor-mediated signaling networks that ultimately converge on the cell cycle machinery. Adaptor proteins play essential roles in the formation of intracellular signaling complexes, relaying extracellular signals from the plasma membrane to the nucleus of a cell. The leukocyte-specific adaptor protein Grap2 is a central linker protein in immune cell signaling and activation. Using Grap2 as bait protein, we identified a novel human protein, GCIP (Grap2 cyclin-D interacting protein). We found that GCIP bound to Grap2 in both yeast two-hybrid assays and in mammalian cells through binding to the COOH-terminal unique domain and SH3 domain (designated QC domain) of Grap2. GCIP also associated with cyclin D both in vitro and in vivo. The expression of GCIP was found in all human tissues examined with the highest level of expression in the heart, muscle, peripheral blood leukocytes, and brain. Furthermore, phosphorylation of retinoblastoma protein by cyclin D-dependent protein kinase was reduced and E2F1-mediated transcription activity was inhibited in cells transfected with GCIP. High level expression of GCIP in terminally differentiated tissues and the inhibition of E2F1 transcription activation suggest that GCIP could play an important role in controlling cell differentiation and proliferation.
Insights
A novel protein, GCIP, interacts with Grap2 and cyclin D, impacting cell cycle regulation. GCIP
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Receptor-mediated signaling networks regulate mammalian cell growth and proliferation.
- Adaptor proteins are crucial for intracellular signaling complex formation.
- Leukocyte-specific adaptor protein Grap2 is key in immune cell signaling.
Purpose of the Study:
- To identify novel proteins interacting with Grap2.
- To characterize the function of a newly identified protein, GCIP (Grap2 cyclin-D interacting protein).
- To investigate GCIP's role in cell cycle regulation.
Main Methods:
- Yeast two-hybrid assays to identify protein interactions.
- Mammalian cell co-immunoprecipitation to confirm binding.
- In vitro and in vivo association studies with cyclin D.
- Analysis of GCIP expression across human tissues.
- Assessment of retinoblastoma protein phosphorylation and E2F1 transcription activity.
Main Results:
- GCIP was identified as a novel binding partner of Grap2.
- GCIP binds to Grap2 via its COOH-terminal unique and SH3 domains (QC domain).
- GCIP associates with cyclin D both in vitro and in vivo.
- GCIP expression is highest in heart, muscle, leukocytes, and brain.
- GCIP transfection reduced retinoblastoma protein phosphorylation and inhibited E2F1 transcription.
Conclusions:
- GCIP is a novel Grap2-interacting protein that also associates with cyclin D.
- GCIP's ability to inhibit cell cycle progression suggests a role in controlling cell differentiation and proliferation.
- The findings highlight GCIP's potential significance in terminally differentiated tissues.