PURIFICATION AND CDNA CLONING OF HELA-CELL P54(NRB), A NUCLEAR-PROTEIN WITH 2 RNA RECOGNITION MOTIFS AND EXTENSIVE HOMOLOGY TO HUMAN SPLICING FACTOR PSF AND DROSOPHILA NONA BJ6/

While searching for a human homolog of the S.cerevisiae splicing facto r PRP18, we found a polypeptide that reacted strongly with antibodies against PRP18. We purified this polypeptide from HeLa cells using a We stern blot assay, and named it p54nrb (for nuclear RNA-binding protein , 54 kDa). cDNAs encoding p54nrb were cloned with probes derived from partial sequence of the purified protein. These cDNAs have identical c oding sequences but differ as a result of alternative splicing in the 5' untranslated region. The cDNAs encode a 471 aa polypeptide that con tains two RNA recognition motifs (RRMs). Human p54nrb has no homology to yeast PRP18, except for a common epitope, but is instead 71% identi cal 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 Gln re sidues outside the main homology region. The Drosophila puff-specific protein BJ6, one of three products encoded by the alternatively splice d no-on-transient A gene (nonA), which is required for normal vision a nd courtship song, is 42% identical to p54nrb in the same 320 aa regio n. The striking homology between p54nrb, PSF, and NONA/BJ6 defines a n ovel phylogenetically conserved protein segment, termed DBHS domain (f or Drosophila behavior, human splicing), which may be involved in regu lating diverse pathways at the level of pre-mRNA splicing.