Dynamics of Human Telomerase Holoenzyme Assembly and Subunit Exchange across the Cell Cycle

Abstract

Background: Telomerase elongation of chromosome ends is restricted within the cell cycle. Results: A human telomerase holoenzyme subunit mediating Cajal body and telomere localization disassembles reversibly from the RNP catalytic core in each cell cycle. Conclusion: Telomerase holoenzyme assembly and disassembly are favored at distinct stages of the cell cycle. Significance: Human telomerase is relicensed for telomere synthesis in G 1 .

Telomerase Mechanism of Telomere Synthesis

“…A fraction of hTR binds TERT to form catalytically active RNP, and the hTR 3′ stem loop CAB-box motif (Figure 2 c ) binds Telomerase and Cajal body protein 1 (TCAB1). Because active RNPs appear to assemble irreversibly (113) and have a long cellular half-life (114), only ~100 active RNP assembly events may occur in each cell cycle. The location within the nucleus where TERT assembles with hTR is unknown.…”

“…This control of telomerase action does not derive from regulated assembly of the RNP catalytic core, because telomerase activity is present in cell extracts at all stages of the cell cycle in all examined model systems. Holoenzyme assembly is regulated to some extent: S. cerevisiae assembles, disassembles, and degrades holoenzyme subunits with some cell cycle regulation (107, 149), and human telomerase dissociates TCAB1 in the cell cycle M phase (113). However, subunit overexpression studies indicate that these regulations do not account for S phase restriction of telomerase action.…”

Control of telomerase action at human telomeres

“…In comparison, HeLa-cell telomeres are slightly elongated by overexpression of TERT, but dramatic telomere elongation requires co-overexpression of TERT and hTR 36,40 . Once assembled, hTR and TERT remain stably associated throughout the cell cycle, as demonstrated by enzyme activity assays and subunit cross-linking in vivo 41,42 .…”

“…TCAB1 assembly with telomerase RNP increases in G1 (ref. 42), thus accounting for the G1 increase in the CB concentration of hTR 45 . CBs could provide important accessory factors for telomerase-telomere interaction, or they could indirectly promote telomerase association with telomeres by clustering active RNPs for multivalent association with shelterin.…”

Specific genomic cues regulate Cajal body assembly

“…Indeed, these mitotic CBs contain spliceosomal snRNPs but target gene transcription and splicing is abolished during this stage of the cell cycle. Accordingly, the composition of these mitotic CBs is different from interphase CBs, as the essential CB component TCAB1/WRAP53 is absent from the domain 51 but a thorough characterization of mitotic CB composition is necessary. Interestingly, as TCAB1/WRAP53 has been proposed to be the key element that targets sn/snoRNAs and scaRNAs to the CB, 52 this may coincide with a redistribution of these small RNAs through the nucleoplasm.…”