The identification of telomerase subunits: catalysing telomere research

HuB and HuD repress telomerase activity by dissociating HuR from TERC

The ubiquitous RNA-binding protein HuR (ELAVL1) promotes telomerase activity by associating with the telomerase noncoding RNA TERC. However, the role of the neural-specific members HuB, HuC, and HuD (ELAVL2-4) in telomerase activity is unknown. Here, we report that HuB and HuD, but not HuC, repress telomerase activity in human neuroblastoma cells. By associating with AU-rich sequences in TERC, HuB and HuD repressed the assembly of the TERT-TERC core complex. Furthermore, HuB and HuD competed with HuR for binding to TERC and antagonized the function of HuR that was previously shown to enhance telomerase activity to promote cell growth. Our findings reveal a novel mechanism controlling telomerase activity in human neuroblastoma cells that involves a competition between HuR and the related, neural-specific proteins HuB and HuD.

HuR regulates telomerase activity through TERC methylation

Telomerase consists of the catalytic protein TERT and the RNA TERC. Mutations in TERC are linked to human diseases, but the underlying mechanisms are poorly understood. Here we report that the RNA-binding protein HuR associates with TERC and promotes the assembly of the TERC/TERT complex by facilitating TERC C106 methylation. Dyskeratosis congenita (DC)-related TERC U100A mutation impair the association of HuR with TERC, thereby reducing C106 methylation. Two other TERC mutations linked to aplastic anemia and autosomal dominant DC, G107U, and GC107/108AG, likewise disrupt methylation at C106. Loss-of-HuR binding and hence lower TERC methylation leads to decreased telomerase activity and telomere shortening. Furthermore, HuR deficiency or mutation of mTERC HuR binding or methylation sites impair the renewal of mouse hematopoietic stem cells, recapitulating the bone marrow failure seen in DC. Collectively, our findings reveal a novel function of HuR, linking HuR to telomerase function and TERC-associated DC.

Mutations in the RNA component TERC can cause telomerase dysfunction but the underlying mechanisms are largely unknown. Here, the authors show that RNA-binding protein HuR regulates telomerase function by enhancing the methylation of TERC, which is impaired by several disease-relevant TERC mutations.

Intrachromatid excision of telomeric DNA as a mechanism for telomere size control in Saccharomyces cerevisiae

We have previously identified a process in the yeast Saccharomyces cerevisiae that results in the contraction of elongated telomeres to wild-type length within a few generations. We have termed this process telomeric rapid deletion (TRD). In this study, we use a combination of physical and genetic assays to investigate the mechanism of TRD. First, to distinguish among several recombinational and nucleolytic pathways, we developed a novel physical assay in which HaeIII restriction sites are positioned within the telomeric tract. Specific telomeres were subsequently tested for HaeIII site movement between telomeres and for HaeIII site retention during TRD. Second, genetic analyses have demonstrated that mutations in RAD50 and MRE11 inhibit TRD. TRD, however, is independent of the Rap1p C-terminal domain, a central regulator of telomere size control. Our results provide evidence that TRD is an intrachromatid deletion process in which sequences near the extreme terminus invade end-distal sequences and excise the intervening sequences. We propose that the Mre11p-Rad50p-Xrs2p complex prepares the invading telomeric overhang for strand invasion, possibly through end processing or through alterations in chromatin structure.

DNA polymerase of the T4-related bacteriophages

The DNA polymerase of bacteriophage T4, product of phage gene 43 (gp43), has served as a model replicative DNA polymerase in nucleic acids research for nearly 40 years. The base-selection (polymerase, or Pol) and editing (3'-exonuclease, or Exo) functions of this multifunctional protein, which have counterparts in the replicative polymerases of other organisms, are primary determinants of the high fidelity of DNA synthesis in phage DNA replication. T4 gp43 is considered to be a member of the "B family" of DNA-dependent DNA polymerases (those resembling eukaryotic Pol alpha) because it exhibits striking similarities in primary structure to these enzymes. It has been extensively analyzed at the genetic, physiological, and biochemical levels; however, relationships between the in vivo properties of this enzyme and its physical structure have not always been easy to explain due to a paucity of structural data on the intact molecule. However, gp43 from phage RB69, a phylogenetic relative of T4, was crystallized and its structure solved in a complex with single-stranded DNA occupying the Exo site, as well as in the unliganded form. Analyses with these crystals, and crystals of a T4 gp43 proteolytic fragment harboring the Exo function, are opening new avenues to interpret existing biological and biochemical data on the intact T4 enzyme and are revealing new aspects of the microanatomy of gp43 that can now be explored further for functional significance. We summarize our current understanding of gp43 structure and review the physiological roles of this protein as an essential DNA-binding component of the multiprotein T4 DNA replication complex and as a nucleotide-sequence-specific RNA-binding translational repressor that controls its own biosynthesis and activity in vivo. We also contrast the properties of the T4 DNA replication complex to the functionally analogous complexes of other organisms, particularly Escherichia coli, and point out some of the unanswered questions about gp43 and T4 DNA replication.

Human telomerase contains evolutionarily conserved catalytic and structural subunits

We have cloned and characterized a human gene encoding TP2 (telomerase-associated protein 2), a protein with similarity to reverse transcriptases and the catalytic telomerase subunits from Saccharomyces cerevisiae and Euplotes aediculatus. Indirect immunofluorescence revealed that TP2 was localized to the nucleus. Using antibodies to endogenous and epitope-tagged TP2, we found that TP2 was associated specifically with human telomerase activity and the recently identified telomerase-associated protein TP1. Mutation of conserved residues within the reverse transcriptase domain of TP2 severely reduced associated telomerase activity. These results suggest that telomerase is an evolutionarily conserved multisubunit complex composed of both structural and catalytic subunits.