New nuclear functions for calmodulin.

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.