Catalysis of pre-mRNA Splicing
Over three decades ago, the small nuclear RNA (snRNA) components of the spliceosome were proposed to mediate catalysis within the spliceosome. This hypothesis was motivated by the discovery of the first catalytic RNAs and bolstered by the identification of a particular class of self-splicing, intronic RNAs (group II introns) that, like nuclear pre-mRNA introns, spliced through a characteristic lariat intermediate. Similarities between the most conserved domains of the snRNAs and group II introns strengthened the hypothesis.
Nevertheless, definitive proof that the snRNAs mediate catalysis has been lacking. Further, a conserved protein component of the spliceosome has recently been suggested to contribute to catalysis. While it has been unclear what component of the spliceosome mediates catalysis, work from the Piccirilli lab, also at the University of Chicago, has established that the spliceosome is a metalloenzyme, such that divalent metals stabilize the leaving groups during both of the two chemical steps in splicing.
To identify the spliceosomal ligands for these metals, we teamed up with Piccirilli, combining his expertise in chemical biology and our expertise in spliceosome function, especially our understanding of proofreading mechanisms that operate at the catalytic stage. Together, we showed that the catalytic metals are positioned by U6 snRNA and thereby provided definitive evidence that the spliceosome utilizes RNA to catalyze splicing.
Further, our work revealed that the spliceosome and self-splicing group II RNAs share common catalytic mechanisms and likely evolutionary origins. These data support a hypothesis that key RNA-based machines in modern life, including the spliceosome and the ribosome, derived ultimately from a primordial “RNA world” (Fica et al., 2013, Nature). This work has raised new and fundamental questions regarding the mechanism of catalysis.