The Role of snRNAs in Spliceosomal Catalysis - Progress in Molecular Biology and Translational Science - page 208

reaction was shown to be magnesium ion-dependent, and terbium(III) ion
cleavage studies indicated magnesium ion binding by the internal bulge of
U6 ISL and the ACAGAGA sequence. 79,81
While there are clear similarities between this reaction and spliceosomal
catalysis both in the chemistry of the reaction and the RNA sequences
involved, the mechanism of the first step of splicing is distinct in the two
systems. Unlike the branching reaction observed in the spliceosome, the
cleavage of the 5 0 splice site occurs through hydrolysis in this minimal,
RNA-only system. This can be explained by the absence of a branch
site-like substrate, which precludes a branching reaction. It is possible that
with the careful addition of a third substrate to the reaction, the U6/U2
complex can also catalyze a branching reaction. Alternatively, the presence
of an additional spliceosomal factor might be necessary for enabling the
snRNAs to perform branching. Interestingly, many group II introns catalyze
the first step of splicing through hydrolytic cleavage of the 5 0 splice site both
in vitro and in vivo . 24,36,82 Thus, hydrolysis is a physiological alternative to
branching at least in group II introns. In addition, as mentioned above, it
has been recently shown that the spliceosomal active site is also capable of
performing hydrolytic cleavage in a reaction similar to the SER described
for group II introns, 24 providing a precedent for hydrolytic reactions cata-
lyzed by the spliceosomal active site. 50 While this snRNA-catalyzed two-
step splicing reaction is over 10-fold more efficient than the previously
observed reactions catalyzed by the U6/U2 complex, it is still significantly
less efficient than spliceosomal splicing, which suggests that other
spliceosomal factors, perhaps including spliceosomal proteins, are necessary
for the optimal catalytic function of the snRNAs.
8. THE snRNAs IN THE CONTEXT OF THE SPLICEOSOMAL
ACTIVE SITE
Based on the above data, the snRNAs seem to be fully competent to
form the majority if not the entirety of the spliceosomal active site and to
perform catalysis, similar to the self-splicing group II introns. In the activated
spliceosomes, however, several proteins interact with the snRNAs and the
pre-mRNA throughout the spliceosomal cycle ( Fig. 6.7 , also see below).
While the exact role played by the proteins in the spliceosomal catalytic core
is mostly unknown, their possible roles could include assisting the snRNAs
in assuming their functional structure, assisting or independently coordinat-
ing critical metal ions, participating in the positioning of the substrates,