Spliceosomal Prp8 intein at the crossroads of protein and RNA splicing

Abstract

The spliceosome is a large ribonucleoprotein complex that removes introns from pre-mRNAs. At its functional core lies the essential Prp8 protein. Across diverse eukaryotes, this protein cofactor of RNA catalysis harbors a self-splicing element, called an intein. Inteins in Prp8 are extremely pervasive and are found at seven different sites in various species. Here, we focus on the Prp8 intein from Cryptococcus neoformans, a human fungal pathogen. We solved the crystal structure of this intein, revealing structural homology among self-splicing sequences in eukaryotes, including Hedgehog protein. Working with the C. neoformans Prp8 intein in a reporter assay, we find that the biologically relevant divalent metals copper and zinc inhibit intein splicing, albeit by two different mechanisms. Copper likely stimulates reversible modifications on a catalytically important cysteine, whereas zinc binds via the same critical cysteine with a Kd of ~1 nM. An intein-containing Prp8 precursor model is presented, suggesting that metal-induced protein splicing inhibition would disturb function of both Prp8 and the spliceosome. These results indicate that Prp8 protein splicing can be modulated, and that this could alter spliceosome function and RNA splicing under specific conditions.