A bird's-eye view of post-translational modifications in the spliceosome and their roles in spliceosome dynamics

Abstract

Pre-mRNA splicing, the removal of noncoding intron sequences from the pre-mRNA, is a critical reaction in eukaryotic gene expression. Pre-mRNA splicing is carried out by a remarkable macromolecular machine, the spliceosome, which undergoes dynamic rearrangements of its RNA and protein components to assemble its catalytic center. While significant progress has been made in describing the "moving parts" of this machine, the mechanisms by which spliceosomal proteins mediate the ordered rearrangements within the spliceosome remain elusive. Here we explore recent evidence from proteomics studies revealing extensive post-translational modification of splicing factors. While the functional significance of most of these modifications remains to be characterized, we describe recent studies in which the roles of specific post-translational modifications of splicing factors have been characterized. These examples illustrate the importance of post-translational modifications in spliceosome dynamics.

Fig. 1

Dynamic RNP rearrangements occur throughout the spliceosome assembly pathway. The snRNPs are depicted as colored balls. DExD/H proteins are shown at each step, and the proteins with which the DExD/H proteins interact (and which have opposing functions) are shown in italics. The GTPase Snu114, which is a ubiquitin conjugate and coordinates Brr2 activity, is indicated in green text. Prp8 is also included to illustrate how post-translational modifications fit into the pathway.

Fig. 2

Ubiquitination regulates Brr2’s role in formation of a catalytically active spliceosome. During the splicing reaction, formation of the catalytic spliceosome is facilitated by rearrangements resulting in tri-snRNP disassembly and U4 snRNP release. Ubiquitin negatively regulates spliceosome activation by suppressing Brr2-mediated U4/U6 unwinding. Once the ubiquitin moiety is removed or occluded, Brr2 activity is no longer suppressed and facilitates U4 snRNP dissociation.