The Drosophila melanogaster genes Hrb87F and Hrb98DE encode the fly proteins HRB87F and HRB98DE (also known as hrp36 and hrp38, respectively) that are most similar in sequence and function to mammalian A/B-type hnRNP proteins. Using overexpression and deletion mutants of Hrb87F, we have tested the hypothesis that the ratio of A/B hnRNP proteins to SR family proteins modulates certain types of alternative splice-site selection. In flies in which HRB87F/hrp36 had been overexpressed 10- to 15-fold above normal levels, aberrant internal exon skipping was induced in at least one endogenous transcript, the dopa decarboxylase (Ddc) pre-mRNA, which previously had been shown to be similarly affected by excess HRB98DE/hrp38. In a second endogenous pre-mRNA, excess HRB87F/hrp36 had no effect on alternative 3' splice-site selection, as expected from mammalian hnRNP studies. Immunolocalization of the excess hnRNP protein showed that it localized correctly to the nucleus, specifically to sites on or near chromosomes, and that the peak of exon-skipping activity in Ddc RNA correlated with the peak of chromosomally associated hnRNP protein. The chromosomal association and level of the SR family of proteins were not significantly affected by the large increase in hnRNP proteins during this time period. Although these results are consistent with a possible role for hnRNP proteins in alternative splicing, the more interesting finding was the failure to detect significant adverse effects on flies with a greatly distorted ratio of hnRNPs to SR proteins. Electron microscopic visualization of the general population of active genes in flies overexpressing hnRNP proteins also indicated that the great majority of genes seemed normal in terms of cotranscriptional RNA processing events, although there were a few abnormalities consistent with rare exon-skipping events. Furthermore, in a Hrb87F null mutant, which is viable, the normal pattern of Ddc alternative splicing was observed, indicating that HRB87F/hrp36 is not required for Ddc splicing regulation. Thus, although splice-site selection can be affected in at least a few genes by gross overexpression of this hnRNP protein, the combined evidence suggests that if it plays a general role in alternative splicing in vivo, the role can be provided by other proteins with redundant functions, and the role is independent of its concentration relative to SR proteins.

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are abundant RNA-binding proteins that are implicated in splicing regulation. Here we investigate the role of a Drosophila hnRNP in splicing regulation in living animals. We find that overexpression of the Drosophila hnRNP HRB98DE leads to skipping of all internal exons in the Drosophila dopa decarboxylase (Ddc) pre-mRNA in vivo. These results indicate that HRB98DE has a splicing activity that promotes use of terminal splice sites. The effect of excess HRB98DE on Ddc splicing is transient, even though high levels of HRB98DE persist for at least 24 hr. This suggests that Drosophila larvae can induce a compensating mechanism to counteract the effects of excess HRB98DE.Nascent pre-mRNAs associate with a specific group of proteins to form heterogeneous nuclear ribonucleoprotein (hnRNP) complexes (1). The major protein components of hnRNP complexes from HeLa cells are the Al, A2, Bi, B2, Cl, and C2 polypeptides (2, 3). Several lines of indirect evidence suggest that hnRNPs might be involved in pre-mRNA splicing. Four hnRNPs (Al, C, D, and I/PBT) have been found to bind specifically to the conserved polypyrimidine tract located upstream of most 3' splice sites (4-6). The hnRNPs Al and I/PBT can also associate with 5' splice sites (7-9). The hnRNP Al protein has RNA-annealing activity (10-13) and can bind to the U2 small nuclear ribonucleoprotein (snRNP) (10). Efficient UV crosslinking of hnRNP Al to pre-mRNA requires both Ul and U2 snRNPs (14).Recent in vitro studies provide direct evidence that hnRNP Al might be actively involved in regulating alternative splicing. An excess of hnRNP Al promotes the use of distal 5' splice sites in model pre-mRNAs containing duplicated 5' splice sites (15). This preferential use of distal 5' splice sites can be counteracted by members of the SR protein family (16), such as SF2/ASF and SC35 (15,17,18 In this report, we investigate the role of the Drosophila hnRNP HRB98DE in splicing regulation in living larvae. We tested whether overexpression of HRB98DE affects splice site choice of the endogenous dopa decarboxylase (Ddc) premRNA (28). The Ddc pre-mRNA is expressed primarily in the central nervous system (CNS) and the hypoderm, and it is alternatively spliced in these two tissues (29). The Ddc CNSspecific mRNA contains all four exons, whereas the hypodermal mRNA contains only three exons, skipping the second exon B (see Fig. 2B) (29). Our previous results suggest that the Ddc hypodermal-specific splicing is negatively regulated by a blockage mechanism preventing splicing to exon B and indicate that the sequences of Ddc exon B and the adjacent upstream intron are sufficient for this negative regulation (30,31). Here, we show that in vivo overexpression of a Drosophila hnRNP, HRB98DE, causes profound changes in Ddc splicing. MATERIALS AND METHODSPlasmid Construction. The cDNAs of Hrb98DE 1A and Hrb98DE 1B, ovl2 and L3 (25), were cloned first into plasmid pBHS (32) and then into the PW8 P-element transformation vector that contai...

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