PROTEIN OF THE SR FAMILY OF SPLICING FACTORS BINDS EXTENSIVELY TO EXONIC BALBIANI RING PRE-MESSENGER-RNA AND ACCOMPANIES THE RNA FROM THE GENE TO THE NUCLEAR-PORE

We report on the molecular cloning and intracellular localization of a heterogeneous nuclear ribonucleoprotein (hnRNP), Ct-hrp45, one of the major components of pre-mRNP particles in Chironomus tentans. It is s hown that hrp45 belongs to the SR family of splicing factors and exhib its high sequence similarity to Drosophila SRp55/B52 and human SF2/ASF . The distribution of hrp45 within the C. tentans salivary gland cells is studied by immunocytology. The hrp45 protein is found to be abunda nt in the nucleus, whereas it is undetectable in the cytoplasm. The fa te of hrp45 in specific pre-mRNP particles, the Balbiani ring (BR) gra nules, is revealed by immunoelectron microscopy. It is observed that h rp45 is associated with the growing BR pre-mRNP particles and is being added continuously concomitant with the growth of the transcript, ind icating that hrp45 is bound extensively to exon 4, which comprises 80- 90% of the primary transcript. furthermore, hrp45 remains bound to the BR RNP particles in the nucleoplasm and is not released until the par ticles translocate through the nuclear pore. Thus, hrp45 behaves as an hnRNP protein linked to exon RNA (and perhaps also to the introns) ra ther than as a spliceosome component connected to the assembly and dis assembly of spliceosomes. It seems that hrp45, and possibly also other SR family proteins, is playing an important role in the structural or ganization of pre-mRNP particles and is perhaps participating not only in splicing but also in other intranuclear events.