128
Alternatively Spliced Genes
5
ss
3
ss
Exon 3
3
ss
Pre-mRNA splicing
Translation
Polypeptide
Pre-mRNA
Exon 1
5
ss
Exon 2
mRNAs
Exon 3
Exon 1
Exon 2
Exon 3
Exon 1
mRNA export
Fig. 1
The schematics for major posttranscriptional processes in
eukaryotic gene expression. Different splice isoforms of mRNAs can
be produced as a result of alternative selection of splice sites.
intron), the spliceosome contains U1, U2,
U4/6, and U5snRNPs. The splicing of the
minor class of introns (also called the U12-
type) occurs in the spliceosome containing
U11
and
U12,
in addition
to
U4atac,
U6atac, and U5snRNPs (Figs. 2 and 3).
This chapter will focus on the splicing of
introns in the major class. This is because
this class accounts for more than 99% of
known introns. Most of our knowledge
about pre-mRNA splicing has also come
from studies of the major class of introns.
Comparison of genes coding for com-
ponents of splicing machinery, including
both snRNAs and protein factors, reveals a
high degree of conservation through evo-
lution. Amazing similarity is found among
spliceosomal components from yeast, fruit
fly,
and
human.
A
recent
study
us-
ing nanoscale microcapillary liquid chro-
matography tandem mass spectrometry to
analyze partially puri±ed human spliceo-
somes assembled on model splicing sub-
strates revealed 145 distinct spliceosomal
proteins. This suggests that the spliceo-
some is one of the most complex macro-
molecular machines for mammalian gene
expression. Table 1 shows the currently
known human proteins identi±ed in func-
tional spliceosomes assembled on model
splicing substrates. The role of individual
proteins in splicesome assembly and in
splicing regulation will be discussed.
1.2
Splicing Signals
The sites for cleavage and ligation in splic-
ing reactions are de±ned by conserved
splicing signals in the pre-mRNA, includ-
ing the 5
0
splice site (5
0
ss), the branch
site, the polypyrimidine tract, and the
3
0
splice site (3
0
ss) consensus sequence.
These cis-elements are important sites
for RNA–RNA and protein–RNA inter-
actions. Systematic analyses of both yeast
and higher eukaryotic splicing signal se-
quences led to the classi±cation of U2 and
U12 types of introns based on the key Us-
nRNPs involved in the recognition of the
branch sites in the corresponding introns
(Fig. 2).
In general, the spliceosome for U2-type
introns contains U1, U2, and U4/U6,
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