A stochastic view of spliceosome assembly and recycling in the nucleus

Abstract

One of the steps in posttranscriptional modification of messenger RNA (mRNA) requires splicing of introns and ligation of exons forming functional mRNAs for translation. Splicing is catalyzed by the spliceosome, a large macromolecular structure made up of RNA-protein complexes called small nuclear ribonucleoproteins (snRNPs) as well as >100 different non-snRNPs. Spliceosomes are constantly formed during splicing and they appear to disassemble after each splicing event. The trigger that causes spliceosome formation and disassembly is, however, unknown, and the existence of this trigger is what the authors have investigated. Using imaging of photobleached structures in HeLa cells with or without splicing activity, the authors showed that splicing proteins are highly mobile and shuttle constantly between subnuclear compartments, namely the nuclear speckles and the nucleoplasm, where they form transient structures called Cajal bodies; further, this movement is independent of whether pre-mRNA synthesis is ongoing or absent. The imaging studies were combined with mathematical modeling using the quantitative kinetic data obtained from the photobleaching experiments of splicing activity inhibited by the drug 5,6-dichlorobenzimidazole riboside (DRB) or in mutants unable to carry out splicing. The model confirmed the authors' hypothesis that splicing proteins move about in a Brownian type of motion, constantly bumping into speckles to transiently remain in this subnuclear compartment or diffusing into the nucleus and randomly attaching to pre-mRNA as spliceosomes are assembled during splicing (Fig. 1). (Figure Presented) This research on the characteristics of spliceosome assembly and recycling is a step toward a greater understanding of the genomic program of organisms. Splicing is an integral process in RNA maturation. Indeed, it is now known that most pre-mRNAs are alternatively spliced (i.e., the pattern of intron excision is regulated so that a single pre-mRNA transcript can give rise to many different proteins). The finding in this study that the spliceosome is formed through random and transient collisions with the transcription products of genes adds a new layer of complexity to understanding the molecular basis for how a particular splicing pattern is selected. © 2007 Data Trace Publishing Company.