Mechanism of the AAA+ ATPases pontin and reptin in the biogenesis of H/ACA RNPs

Abstract

The AAA+ ATPases pontin and reptin function in a staggering array of cellular processes including chromatin remodeling, transcriptional regulation, DNA damage repair, and assembly of macromolecular complexes, such as RNA polymerase II and small nucleolar (sno) RNPs. However, the molecular mechanism for all of these AAA+ ATPase associated activities is unknown. Here we document that, during the biogenesis of H/ACA RNPs (including telomerase), the assembly factor SHQ1 holds the pseudouridine synthase NAP57/dyskerin in a viselike grip, and that pontin and reptin (as components of the R2TP complex) are required to pry NAP57 from SHQ1. Significantly, the NAP57 domain captured by SHQ1 harbors most mutations underlying X-linked dyskeratosis congenita (X-DC) implicating the interface between the two proteins as a target of this bone marrow failure syndrome. Homing in on the essential first steps of H/ACA RNP biogenesis, our findings provide the first insight into the mechanism of action of pontin and reptin in the assembly of macromolecular complexes.

The R2TP chaperone complex: its involvement in snoRNP assembly and tumorigenesis

“…This modification is essential for the subsequent endonucleotic cleavage and processing of the pre-rRNA to produce mature rRNAs (21,27). The assembly of snoRNP is hierarchical and its proper assembly is essential for both the activity and stability of snoRNP (6,16,17,(28)(29)(30)(31). It has been proposed that the assembly of box C/D snoRNP starts with the interaction between box C/D snoRNA and Snu13/15.5k, followed by the binding of Nop58 and then Nop1/fibrillarin and Nop56 are recruited (28,32).…”

“…The Shq1 initially interacts with free form of Cbf5/dyskerin, which has yet to be co-transcriptionally assembled with box H/ACA snoRNA and other core proteins (36,38). In mammalian systems, R2TP subsequently interacts directly with Shq1-dyskerin complex, resulting in the latter's dissociation (17). Then, another assembly factor Naf1 interacts with dyskerin and the complex is brought to the transcription sites of box H/ACA snoRNA.…”

“…Next, Naf1 is replaced by Gar1 to form the mature snoRNP complex by assembling with other factors such as Nhp2, and Nop10 (39). Additionally, given that all core protein factors of box H/ACA snoRNP also assembles with TERC (telomerase RNA component), forming TERCcontaining RNP (34), its biogenesis could follow that of canonical box H/ACA snoRNPs by involving R2TP (17). In yeast, Rvbs are involved in box H/ACA snoRNP biogenesis (6,40), although the contribution of Pih1 and Tah1 to the process, if any, remains unknown.…”

“…For dyskerin, although downregulation of the expression has been observed in a subset of tumors (70), upregulated expression was determined in a number of human cancer cells and also its overexpression is correlated with aggressive proliferation of the tumor cells such as hepatocellular (71), colon (72), prostate (73,74), head and neck carcinomas (75). Given that the both Nop58 and dyskerin are unstable proteins and associated/stabilized by R2TP (16,17,76), the expression level of R2TP components also might be upregulated to maintain the target proteins level in above-mentioned cancers. In summary, it is shown how R2TP function is related to tumorigenesis through snoRNP biogenesis (Figure 3).…”

“…There are two major classes of snoRNAs -box C/D and box H/ACA snoRNAs -which function as guide RNA, targeting snoRNPs to specific nucleotides on pre-rRNA for enzymatic modifications, such as 2′-O-methylation and pseudouridylation, respectively ( Figure 2) (24,25). Recent studies have shown that the R2TP functions as a chaperone for the both box C/D and box H/ACA snoRNP assembly (16,17), and it regulates ribosome biogenesis and subsequently controls cell proliferation (16,26). Intriguingly, it has been reported that the expression levels of some components of R2TP, box C/D snoRNP, as well as box H/ACA snoRNP, are deregulated in various cancer cells (described in main section).…”