A CONSERVED EPITOPE ON A SUBSET OF SR PROTEINS DEFINES A LARGER FAMILY OF PRE-MESSENGER-RNA SPLICING FACTORS

The removal of introns from eukaryotic pre-mRNA occurs in a large ribo nucleoprotein complex called the spliceosome. We have generated a mono clonal antibody (mAb 16H3) against four of the family of six SR protei ns, known regulators of splice site selection and spliceosome assembly . In addition to the reactive SR proteins, SRp20, SRp40, SRp55, and SR p75, mAb 16H3 also binds similar to 20 distinct nuclear proteins in hu man, frog, and Drosophila extracts, whereas yeast do not detectably ex press the epitope. The antigens are shown to be nuclear, nonnucleolar, and concentrated at active sites of RNA polymerase II transcription w hich suggests their involvement in pre-mRNA processing. Indeed, most o f the reactive proteins observed in nuclear extract are detected in sp liceosomes (E and/or B complex) assembled in vitro, including the U1 7 0K component of the U1 small nuclear ribonucleoprotein particle and bo th subunits of U2AF Interestingly, the 16H3 epitope was mapped to a 40 -amino acid polypeptide composed almost exclusively of arginine altern ating with glutamate and aspartate. All of the identified antigens, in cluding the human homolog of yeast Prp22 (HRH1), contain a similar str uctural element characterized by arginine alternating with serine, glu tamate, and/or aspartate. These results indicate that many more splice osomal components contain such arginine-rich domains. Because it is co nserved among metazoans, we propose that the ''alternating arginine'' domain recognized by mAb 16H3 may represent a common functional elemen t of pre-mRNA splicing factors.