IDENTIFICATION AND CHARACTERIZATION OF 3 MEMBERS OF THE HUMAN SR FAMILY OF PRE-MESSENGER-RNA SPLICING FACTORS

SR proteins have a characteristic C-terminal Ser/ Arg-rich repeat (RS domain) of variable length and constitute a family of highly conserved nuclear phosphoproteins that can function as both essential and alter native pre-mRNA splicing factors. We have cloned a cDNA encoding a nov el human SR protein designated SRp30c, which has an unusually short RS domain, We also cloned cDNAs encoding the human homologues of Drosoph ila SRp55/B52 and rat SRp40/HRS. Recombinant proteins expressed from t hese cDNAs are active in constitutive splicing, as shown by their abil ity to complement a HeLa cell S100 extract deficient in SR proteins. A dditional cDNA clones reflect extensive alternative splicing of SRp40 and SRp55 premRNAs. The predicted protein isoforms lack the C-terminal RS domain and might be involved in feedback regulatory loops. The abi lity of human SRp30c, SRp40 and SRp55 to modulate alternative splicing in vivo was compared with that of other SR proteins using a transient cotransfection assay. The overexpression of individual SR proteins in HeLa cells affected the choice of alternative 5' splice sites of aden ovirus E1A and/or human beta-thalassemia reporters. The resulting spli cing patterns were characteristic for each SR protein. Consistent with the postulated importance of SR proteins in alternative splicing in v ivo, we demonstrate complex changes in the levels of mRNAs encoding th e above SR proteins upon T cell activation, concomitant with changes i n the expression of alternatively spliced isoforms of CD44 and CD45.