Assembly and trafficking of box C/D and H/ACA snoRNPs

Abstract

Box C/D and box H/ACA snoRNAs are abundant non-coding RNAs that localize in the nucleolus and mostly function as guides for nucleotide modifications. While a large pool of snoRNAs modifies rRNAs, an increasing number of snoRNAs could also potentially target mRNAs. ScaRNAs belong to a family of specific RNAs that localize in Cajal bodies and that are structurally similar to snoRNAs. Most scaRNAs are involved in snRNA modification, while telomerase RNA, which contains H/ACA motifs, functions in telomeric DNA synthesis. In this review, we describe how box C/D and H/ACA snoRNAs are processed and assembled with core proteins to form functional RNP particles. Their biogenesis involve several transport factors that first direct pre-snoRNPs to Cajal bodies, where some processing steps are believed to take place, and then to nucleoli. Assembly of core proteins involves the HSP90/R2TP chaperone-cochaperone system for both box C/D and H/ACA RNAs, but also several factors specific for each family. These assembly factors chaperone unassembled core proteins, regulate the formation and disassembly of pre-snoRNP intermediates, and control the activity of immature particles. The AAA+ ATPase RUVBL1 and RUVBL2 belong to the R2TP co-chaperones and play essential roles in snoRNP biogenesis, as well as in the formation of other macro-molecular complexes. Despite intensive research, their mechanisms of action are still incompletely understood.

Keywords: Hsp90/R2TP; RNP assembly; RNP trafficking; box C/D RNP; box H/ACA RNP; scaRNA; snoRNA.

Figure 1.

Structures of box C/D and H/ACA RNAs. (A) Secondary structure of a typical box C/D RNA. Consensus boxes called C, D, C’ and D’ pair to form box C/D and box C′/D′ which are folded into kink-turn and kink-loop respectively. The D and D’ boxes and their juxtaposed sequences are complementary to the target RNA, and the methylated nucleotide is the fifth nucleotide upstream of D or D’ boxes. (B) Secondary structure of a typical box H/ACA RNA. Each hairpin is composed of a lower stem, a bulge known as the pseudouridylation pocket, an upper stem and an apical loop. Hairpins are separated by the H box and the last hairpin contains an ACA box at its 3′end. The Ψ Pocket has sequence complementarity to the target RNA. The uridine to be isomerized is located under the upper stem. ScaRNAs specifically contain a CAB box which allows the RNA to be retained in CBs.

Figure 2.

Assembly and trafficking of box C/D snoRNPs. During the biogenesis of box C/D snoRNPs, a protein only complex containing SNU13 and NOP58 is pre-formed with the help of HSP90/R2TP complex, and is loaded on snoRNA in a co-splicing manner. Pre-snoRNP particles are then transported to CBs where final processing occurs. It is not clear if assembly factors leave pre-snoRNP before or after arriving in CBs. Catalytically active snoRNPs are targeted to nucleoli where they function in ribose methylation on rRNAs. The blue symbols at the extremities of the RNA represent exonucleolytic enzymes.

Figure 3.

Assembly and trafficking of box H/ACA RNPs. The first step of box H/ACA RNP assembly is the interaction of SHQ1 with DKC1 to prevent its illicit binding to non-specific RNAs. SHQ1 is released with the help of the R2TP complex, allowing the interaction of DKC1 to nascent H/ACA RNAs at their site of transcription. At this step, NHP2 and NOP10 are present in the pre-particle, as well as another assembly factor NAF1 that interacts with the CTD of RNA polymerase II and keeps the H/ACA RNP inactive. Finally NAF1 is replaced by GAR1 to produce mature and functional H/ACA RNPs. ScaRNPs are retained in the CBs, whereas box H/ACA snoRNPs are transported to the nucleoli.