RNA-binding protein Miso/CG44249 is crucial for minor splicing during oogenesis in Drosophila

Pre-mRNA introns are removed by two distinct spliceosomes: the major (U2-type) spliceosome, which splices over 99.5% of introns, and the minor (U12-type) spliceosome, responsible for a rare class of introns known as minor introns. While the major spliceosome contains U1, U2, U4, U5, and U6 small nuclear RNAs (snRNAs) along with numerous associated proteins, the minor spliceosome comprises U11, U12, U4atac, U5, and U6atac snRNAs and includes specialized proteins. The function and regulation of the minor spliceosome are critical. Mutations in its specific component, RNA-binding protein RNPC3/65K, are linked to human diseases such as primary ovarian insufficiency. In this study, we identify RNA-binding protein Miso (CG44249), which shares 31% and 27% amino acid sequence identity with human RNPC3 and RBM41, respectively, as a key factor in minor splicing and oogenesis in Drosophila Miso associates with U11 and U12 snRNAs in ovaries. miso mutant females exhibit smaller ovaries, reduced germline stem cell numbers, disrupted oogenesis, reduced fecundity, and lower fertility. In miso mutant ovaries, significant minor intron retention is observed, accompanied by a reduction in spliced RNAs and protein products. Our findings establish Miso as a critical factor for minor intron splicing and underscore its essential role in Drosophila oogenesis.