Where in the cell is the minor spliceosome?

The nucleus, a hallmark of eukaryotes, segregates the genome and the cellular machineries that transcribe and process mRNAs from the translation apparatus in the cytoplasm. Compartmentalization prevents primary gene transcripts (pre-mRNAs) from engaging in translation before they are fully matured. Thus, major mRNA processing events—pre-mRNA splicing and 3′ polyadenylation—take place in the nucleus. A recent paper by König et al. (1) challenged this view, claiming that the splicing of hundreds of genes occurs in the cytoplasm, after export of unspliced pre-mRNAs from the nucleus. This paper sparked intense controversy. Because the central issue of whether mRNA formation and mRNA translation occur in separate compartments is so fundamental to eukaryotic cell biology, we examine here the experimental bases for this claim and contrast them with the findings of Pessa et al. (2), in this issue of PNAS, which provide contradictory evidence. The vast majority of introns are spliced by the major spliceosome: a complex RNA–protein machine containing the U1, U2, U4/U6, and U5 small nuclear ribonucleoproteins (snRNPs). Splicing can occur cotranscriptionally and is generally thought to be completed in the nucleus before mRNA export. Although the biogenesis of four of the major spliceosomal snRNPs (U1, U2, U4, and U5) involves a cytoplasmic phase, the mature snRNPs and associated protein splicing factors ultimately concentrate in the nucleus. A minor class of introns is removed by a closely related machinery termed the “minor spliceosome.” The U11 and U12 snRNPs were described in 1988 as low-abundance particles similar to the well characterized snRNPs of the major spliceosome (3). In 1993, these particles were localized, like their U1 and U2 counterparts, by in situ hybridization to the nucleoplasm of mammalian cells (4). In 1996, they were assigned functions in the splicing of minor-class introns (5, 6), and the U4atac and U6atac snRNAs of the …