The “Fifth” RNA Nucleotide: A Role for Ribosomal RNA Pseudouridylation in Control of Gene Expression at the Translational Level - Biophysical Approaches to Translational Control of Gene Expression - page 260

13.3.2.4 Nop10

Nop10 was first identified in yeast as an essential Gar1-interacting protein that
stabilizes H/ACA snoRNAs and is implicated in pseudouridylation of rRNA (Henras
et al. 1998 ). In vitro biochemical studies in human cells indicate that NOP10 directly
binds to dyskerin, and this interaction is required for the association of NHP2 with
the H/ACA snoRNP (Wang and Meier 2004 ) . These fi ndings suggest that NOP10
acts as a molecular bridge linking NHP2 to the snoRNP and reinforces the notion
that a hierarchy of component assembly must occur in order to generate a functional
H/ACA snoRNP complex. Crystal structure analyses of the archael H/ACA snoRNP
indicate that Nop10 interacts with a conserved region of Cbf5 in close proximity to
its catalytic center (Fig. 13.3 ) (Rashid et al. 2006 ). In archaea, it has been demon-
strated that Nop10 does not independently interact with RNA (Baker et al. 2005 ) ;
however in yeast, Nop10 forms two anti-parallel b-sheets, which interact weakly
with H/ACA snoRNAs by binding a region close to the 3¢ end of the H/ACA snoRNA
pseudouridine pocket (Khanna et al. 2006 ). It therefore seems likely that the asso-
ciation of Nop10 with Cbf5 provides a critical binding surface for the recruitment
of Nhp2, which, in turn, stabilizes the association of H/ACA snoRNAs with the
snoRNP complex, thus modulating target selection and the activity of the H/ACA
snoRNP. It remains to be determined if, in mammalian cells, the components of the
H/ACA snoRNP maintain similar functions in vivo.

13.3.3 Structural and Functional Organization of the H/ACA

SnoRNP Complex

A number of recent high resolution crystal structure studies in archaea have taken
advantage of the evolutionarily conserved structural similarities between archaea
and eukaryotes in order to provide functional insights into the eukaryotic H/ACA
snoRNP complex (Duan et al. 2009 ; Hamma et al. 2005 ; Li and Ye 2006 ; Rashid
et al. 2006 ). These studies reveal that the catalytic domain of Cbf5 is located in the
center of the complex, surrounded by Gar1, L7Ae (Nhp2 homologue), and its own
PUA domain (Fig. 13.3 ). As biochemically demonstrated (Sect. 13.3.2 ), X-ray crys-
tallography also provides evidence that the binding of Nop10 to Cbf5 results in a
conformational change in Nop10, which allows subsequent recruitment of Nhp2 to
the snoRNP complex (Rashid et al. 2006 ). Furthermore, an important thumb loop
domain close to the catalytic region of Cbf5 is predicted to help “lock” the substrate
RNA into the correct position, stabilizing the structure and favoring the conversion
of uridine to Y (Hamma et al. 2005 ; Rashid et al. 2006 ). The crystal structure of a
snoRNP bound to an H/ACA snoRNA has revealed that the ACA motif of the
snoRNA binds to the PUA domain of Cbf5, while the hairpin structure associates
with Cbf5, Nop10, and L7Ae (Li and Ye 2006 ) (Fig. 13.3 ), as previously demon-
strated by biochemical analyses (Sect. 13.3.2 ). Importantly, a complex containing