Splicing of Pre‐mRNA

Yu‐Shiou Lu, Institute of Biomedical Sciences, Taipei, Taiwan
Woan‐Yuh Tarn, Institute of Biomedical Sciences, Taipei, Taiwan

Published online: September 2010

DOI: 10.1002/9780470015902.a0005037.pub2

Eukaryotic messenger ribonucleic acids (mRNAs) are initially transcribed as intron-containing precursor mRNAs (pre-mRNAs). Removal of introns from each pre-mRNA by splicing occurs concurrently with ligation of the two flanking exons to generate a mature mRNA. Pre-mRNA splicing is catalysed by the spliceosome, a large ribonucleoprotein complex containing five small nuclear RNAs and numerous protein factors. In higher eukaryotes, alternative splicing provides a mechanism to generate multiple mRNA isoforms from a single gene. Alternative splicing not only increases proteome diversity but also regulates the level of gene expression posttranscriptionally. Moreover, alternative splicing has a substantial impact on regulation of many physiological processes such as cell differentiation and development. Defects in pre-mRNA splicing underlie a considerable number of genetic diseases and cancers.

Key Concepts:

  • In most eukaryotic pre-mRNAs, the coding sequences (exons) are interrupted by stretches of noncoding sequences (introns).
  • The spliceosome uses a two-step transesterification reaction to remove introns and join exons of pre-mRNAs.
  • The snRNAs and spliceosomal protein Prp8 are implicated in the key catalytic steps of splicing.
  • Alternative splicing extends proteome diversity and regulates gene expression levels.
  • Alternative splicing is determined by interplays between trans-acting factors and cis-elements of pre-mRNAs.
  • Alteration of cis-elements and aberrant control of trans-acting factors lead to diseases.

Keywords: pre-mRNA splicing; spliceosome; alternative splicing; snRNA; RNA-binding protein

Figure 1. The mechanism of splicing. (a) The consensus sequences of a typical metazoan intron (R: purine and Y: pyrimidine). (b) The two red arrows indicate the two nucleophilic attacks of the splicing reaction. In the first step, the 2¢-hydroxyl of the branch point adenosine attacks the 5¢ splice site, and in the second step, the 3¢-hydroxyl of exon I attacks the 3¢ splice site, resulting the ligated exons and a lariat intron.
Figure 2. The stepwise assembly of the spliceosome. The current model suggests that the U1, U2, U4/U6 and U5 snRNPs, and other auxiliary protein factors (not shown) sequentially assemble onto the pre-mRNA and in turn, comprise different stages of the spliceosomes. Complex B* represents the activated spliceosome, which is competent for the catalytic reaction of splicing. Finally, the ligated exons and excised introns are generated.
Figure 3. Regulated CD44 v5 exon inclusion by cellular signalling. Upon extracellular stimuli, Sam68 is phosphorylated by ERK via the Ras-Raf-MEK-ERK pathway and displaces or inhibits hnRNP A1. SRm160 acts as a splicing coactivator for v5 inclusion. Brm is a component of the chromatin remodelling SWI/SNF complex. Brm induces accumulation of C-terminal domain-phosphorylated RNA polymerase II on the v5 exon and thus promotes v5 inclusion.
Figure 4. Both exonic and intronic mutations can cause splicing defects and diseases. (a) The exonic nucleotide substitutions in exon 7 of SMN2 and exon 31 of dystrophin lead to exon skipping. (b) The intronic nucleotide mutations of IKBKAP and WT-1 induce exon 20 skipping and alter 5¢ splice site usage in exon 9 respectively. In intron 8 of CFTR, the variable-length UG-repeats and poly U sequence influence the efficiency of exon 9 utilisation.
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    Newman AJ and Nagai K (2010) Structural studies of the spliceosome: blind men and an elephant. Current Opinion in Structural Biology 20(1): 82–89.
    Wahl MC, Will CL and Luhrmann R (2009) The spliceosome: design principles of a dynamic RNP machine. Cell 136: 701–718.
    Zhong XY, Wang P, Han J et al. (2009) SR proteins in vertical integration of gene expression from transcription to RNA processing to translation. Molecular Cell 35: 1–10.

Related Sub-Topics:

Basic Cell Biology, Protein, RNA and DNA | RNA Structure and Function | Posttranscriptional Modification | Nucleic Acids: Structure, Function, Metabolism
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Article Title: Splicing of Pre‐mRNA
 
How to Cite close
Lu, Yu‐Shiou, and Tarn, Woan‐Yuh(Sep 2010) Splicing of Pre‐mRNA. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005037.pub2]