Minor spliceosome, major surprise: it's cytoplasmic

One of the great surprises of modern biology was the discovery of introns and the consequent understanding that gene transcripts are spliced to form mature messenger RNA (mRNA). A further surprise was the recent discovery that there are two kinds of splicing systems, the major and minor, which act on different types of introns. Now, Harald König, Ferenc Müller (Institute for Toxicology and Genetics, Karlsruhe, Germany), and colleagues uncover yet one more surprise: the minor system acts not in the nucleus, but in the cytoplasm.

The minor spliceosome, found primarily in plants and animals, edits less than 1% of all introns, which are characterized by unique sequences at the splice sites. All genes containing minor introns also contain major introns, which are processed by the major spliceosome in the nucleus. Despite their rarity, minor introns are evolutionarily conserved, suggesting they have some important properties. Genes with minor introns include the E2F transcription factors and genes of the MAP kinase pathway.

The authors used in situ hybridization in zebrafish and mammalian cells to show that snRNAs of the minor spliceosome are primarily cytoplasmic. mRNAs that had their major introns removed but still contained minor introns were transported to the cytoplasm. The minor spliceosome can be inhibited by an antisense morpholino that obstructs access to the minor introns. Attaching a nuclear export signal to the morpholino, so that it primarily localized to the cytoplasm, further inhibited the spliceosome.

The authors reason that its cytoplasmic location might allow the minor spliceosome to continue to function during mitosis, when the nucleus is in disarray. They found that transcripts containing major introns accumulated during mitosis as expected, but transcripts containing only minor introns did not, suggesting that minor splicing continues even while the nucleus is being reorganized.

Suppression of minor spliceosome activity suggested that the minor spliceosome might regulate cell proliferation. In zebrafish, its suppression halted development during the formation of muscle and vertebra precursors, apparently due to increased apoptosis and a block in cell cycle progression. In human cells, the suppression prevented cells from progressing beyond G1.

So why is the minor splicing system segregated to the cytoplasm? “The minor system is much slower,” says Koenig. “It could be that its cytoplasmic localization evolved to cope with that slower processing, by following partially spliced transcripts into the cytoplasm.” Its segregation and specialization may explain the evolutionary conservation of the minor spliceosome in the face of a far more efficient nuclear system.

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