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(CrPV) IRES assembles with the 40S and 60S ribosomal subunits (80S monosome)
on an initiation codon and starts translation without any canonical eIFs (Wilson
et al.
2000a,
b
). Importantly, translation of the CrPV IRES was severely impaired in
Dkc1
m
mouse cells, strongly indicating that defective IRES-dependent translation is
due to an intrinsic defect in ribosomes displaying a reduction in rRNA pseudouridy-
lation isolated from
Dkc1
m
mice (Yoon et al.
2006
) . Signi fi cantly, high-resolution
cryo-electron microscopy has resolved the structure of the CrPV IRES when it asso-
ciates with the ribosome, highlighting a direct role in accurate folding of ribosomal
subunits upon interaction with mRNAs during IRES-mediated translation initiation
(Costantino et al.
2008
). Thus, it seems likely that a reduction in Y modi fi cations in
rRNA may either impair the ability of ribosomal subunits to directly bind the IRES
elements or affect the conformational changes in ribosomal subunits upon binding.
In line with this hypothesis, a filter-binding assay was employed to demonstrate that
ribosomes isolated from
Dkc1
m
mouse cells are greatly impaired in their ability to
bind the CrPV IRES element. Importantly, it has also been demonstrated that rRNA
Y modifications have an evolutionarily conserved role in the recruitment of IRES
elements from yeast to human (Jack et al.
2011
) .
In contrast to the current understanding of viral IRES-mediated translation, the
molecular mechanisms underlying cellular IRES-dependent translation are still
poorly understood. IRES-dependent translation is a mechanism that modulates gene
expression at the translational level during specific cellular events such as mitosis,
quiescence, hypoxia, nutrient deprivation, apoptosis, and an accurate switch from
cap- to IRES-dependent translation has been shown to be critical in maintaining
cellular homeostasis during these circumstances (Barna et al.
2008
; Holcik and
Sonenberg
2005
; Krichevsky et al.
1999
; Lang et al.
2002
; Sherrill et al.
2004
) .
Therefore, a defect in Y modifications and IRES-mediated translation may contrib-
ute to specific pathological features present in X-DC (see Sect.
13.5.1
). However, it
is still not completely known how defects in rRNA pseudouridylation impinge on
translation of specific cellular mRNAs that have been identified with an unbiased
ribosomal profiling analysis (Yoon et al.
2006
) . Speci fi cally, downregulation of the
mammalian Y synthase dyskerin was found to affect the IRES-dependent transla-
tion of distinct IRES-containing mRNAs including the cell cycle regulators p27 and
p53, as well as anti-apoptotic factors such as XIAP and Bcl-xL (Bellodi et al.
2010a
;
Yoon et al.
2006
). While it is possible that impaired rRNA pseudouridylation due to
reduction of dyskerin activity may affect the direct engagement of the IRES ele-
ment, it is also plausible that the absence of an additional hydrogen bond donor site
upon loss of Y residues may disrupt critical RNA-protein interactions. For exam-
ple, in this regard several studies have highlighted the importance of RNA-binding
proteins known as
I
RES
trans
-
a
cting
f
actors (ITAFs) in modulating IRES-mediated
translation of several cellular mRNAs (Lewis and Holcik
2008
; Spriggs et al.
2005
) .
Therefore, it is possible that loss of Y modi fi cations may in fl uence the association
of ITAFs with the ribosome, thereby altering the expression of distinct mRNAs.
Overall, it is clear that rRNA pseudouridylation is important for the recruitment of
mRNAs containing IRES elements to the ribosome; however, the specific Y resi-
dues responsible for this recognition and the precise mechanism involved still
remain to be identified.
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