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New nuclear functions for calmodulin
1Department of Cell Biology, Faculty of Medicine, University of Barcelona, Spain.
Abstract:
The data reported here summarize a series of results which reveal new functions for nuclear calmodulin (CaM). The addition of CaM inhibitors to cultures of proliferating NRK cells blocked the activity of the cyclin-dependent protein kinases 4 (cdk4) and 2 (cdk2), which are enzymes implicated in the progression of G1 and in the onset of DNA replication, respectively. CaM modulates the activity of cdk4 by regulating the nuclear location of both cdk4 and cyclin D, its associated regulatory subunit. By using CaM-affinity chromatography, we have recently identified two new nuclear CaM-binding proteins: (i) the protein La/SSB, which is an autoantigen implicated in several autoimmune diseases such as lupus erythematosus and Sjögren's syndrome (since La/SSB participates in the process of transcription mediated by RNA polymerase III, CaM could be involved in the regulation of this process); and (ii) the protein SAP145, a member of the spliceosome-associated proteins (SAPs) which is a subunit of the splicing factor SF3(b). This finding suggests the involvement of CaM in pre-mRNA splicing. Finally, a screening for new CaM-binding proteins in the fission yeast performed by using the phage display analysis, revealed that several nucleolar-ribosomal proteins associate to CaM, suggesting that CaM modulates ribosomal assembly and/or function.
Insights
Nuclear calmodulin (CaM) plays a crucial role in cell cycle progression by regulating cyclin-dependent kinases. This study identifies new CaM-binding proteins involved in transcription, splicing, and ribosomal assembly.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Nuclear calmodulin (CaM) is a calcium-binding protein with known roles in cellular signaling.
- The precise functions of nuclear CaM in cell proliferation and gene expression remain incompletely understood.
Purpose of the Study:
- To elucidate novel functions of nuclear calmodulin (CaM) in proliferating cells.
- To identify new nuclear CaM-binding proteins and their roles in cellular processes.
Main Methods:
- Utilized CaM inhibitors to assess effects on cell cycle kinases.
- Employed CaM-affinity chromatography to identify binding proteins.
- Performed phage display analysis in fission yeast to screen for CaM-binding proteins.
Main Results:
- CaM inhibition blocked cyclin-dependent protein kinases 4 (cdk4) and 2 (cdk2) activity, impacting G1 progression and DNA replication.
- Identified La/SSB and SAP145 as new nuclear CaM-binding proteins, suggesting CaM involvement in transcription and pre-mRNA splicing.
- Discovered nucleolar-ribosomal proteins binding to CaM in yeast, indicating a role in ribosomal assembly/function.
Conclusions:
- Nuclear CaM is a key regulator of cell cycle progression through modulation of cdk4 and cyclin D.
- CaM is implicated in diverse nuclear processes including transcription, pre-mRNA splicing, and ribosome biogenesis.
- These findings expand the known functional repertoire of nuclear calmodulin.