Purification and cDNA cloning of HeLa cell p54nrb, a nuclear protein with two RNA recognition motifs and extensive homology to human splicing factor PSF andDrosophilaNONA/BJ6

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Abstract
While searching for a human homolog of the S.cerevlslae splicing factor PRP18, we found a polypeptide that reacted strongly with antibodies against PRP18. We purified this polypeptide from HeLa cells using a Western blot assay, and named it p54nrb (for nuclear RNA-blnding protein, 54 kDa). cDNAs encoding p54nrb were cloned with probes derived from partial sequence of the purified protein. These cDNAs have identical coding sequences but differ as a result of alternative splicing in the 5' untranslated region. The cDNAs encode a 471 aa polypeptide that contains two RNA recognition motifs (RRMs). Human p54nrb has no homology to yeast PRP18, except for a common epitope, but is instead 71% identical to human splicing factor PSF within a 320 aa region that includes both RRMs. In addition, both p54nrb and PSF are rich in Pro and Gin residues outside the main homology region. The Drosophila puff-specif ic protein BJ6, one of three products encoded by the alternatively spliced no-on translent A gene (nonA), which is required for normal vision and courtship song, Is 42% identical to p54nrb in the same 320 aa region. The striking homology between p54nrb, PSF, and NONA/BJ6 defines a novel phylogenetically conserved protein segment, termed DBHS domain (for Drosophila behavior, human splicing), which may be involved in regulating diverse pathways at the level of pre-mRNA splicing.