Alternative splicing determines the intracellular localization of the novel nuclear protein Nop30 and its interaction with the splicing factor SRp30c

We report on the molecular cloning of a novel human cDNA by its interaction with the splicing factor SRp30c in a yeast two-hybrid screen. This cDNA is predominantly expressed in muscle and encodes a protein that is present in the nucleoplasm and concentrated in nucleoli. It was therefore termed Nop3 0 (nucleolar protein of 30 kDa). We have also identified a related cDNA wit h a different carboxyl terminus. Sequencing of the NOP gene demonstrated th at both cDNAs are generated by alternative 5' splice site usage from a sing le gene that consists of four exons, spans at least 1800 nucleotides, and i s located on chromosome 16q21-q23. The alternative 5' splice site usage int roduces a frameshift creating two different carboxyl termini. The carboxyl terminus of Nop30 is rich in serines and arginines and has been found to ta rget the protein into the nucleus, whereas its isoform is characterized by proline/glutamic acid dipeptides in its carboxyl terminus and is predominan tly found in the cytosol. Interaction studies in yeast, in vitro protein in teraction assays, and co-immunoprecipitations demonstrated that Nop30 multi merizes and binds to the RS domain of SRp30c but not to other splicing fact ors tested. Overexpression of Nop30 changes alternative exon usage in prepr otachykinin and SRp20 reporter genes, suggesting that Nop30 influences alte rnative splice site selection in vivo.