Insights into the mechanisms of splicing: more lessons from the ribosome

Maria M. Konarska; Charles C. Query

doi:10.1101/gad.1363105

Insights into the mechanisms of splicing: more lessons from the ribosome

Maria M. Konarska1,3 and
Charles C. Query2

¹The Rockefeller University, New York, New York 10021, USA; ²Department of Cell Biology, Albert Einstein College of Medicine, Bronx, New York 10461, USA

This extract was created in the absence of an abstract.

Detailed analyses of spliceosome complexes have revealed an impressive complexity of composition, with five snRNAs and several-hundred protein factors involved in their formation (e.g., Fromont-Racine et al. 2000; Makarov et al. 2002; Zhou et al. 2002; for review, see Burge et al. 1999; Jurica and Moore 2003). Not only is the spliceosome composed of a large number of factors, but formation of the catalytically competent complex requires multiple rearrangements of its five snRNP subunits, achieved through a series of assembly steps involving changes in both the composition of associated factors and their conformations within the complex (for review, see Nilsen 1998; Staley and Guthrie 1998). This changing structural complexity of the spliceosome reflects underlying mechanistic dynamics throughout the entire splicing process.

Indeed, although the spliceosome has long been recognized to be highly dynamic, this was generally regarded as a property of its assembly, and rarely considered in models of the catalytic phase of the reaction. Splicing catalysis proceeds through two consecutive transesterifications involving three sites of the intron; in the first reaction, the branch site nucleophilically attacks the 5′SS, producing a lariat intermediate and cleaved 5′ exon, and in the second reaction, the 5′ exon attacks the 3′SS, yielding spliced mRNA and lariat intron products. We have recently proposed a dynamic model of the catalytic spliceosome in which conformations of the complex supporting the two catalytic steps are in kinetic competition, with modulation of the stability of the first- and second-step conformations resulting in improvement of one of the catalytic steps to the detriment of the other (Query and Konarska 2004). In a recent issue of Genes & Development, Villa and Guthrie (2005) present new data in support of rearrangements of the spliceosome between the two catalytic steps.