The journey to the end of the chromosome: delivering active telomerase to telomeres in plants

Abstract

Linear chromosomes offer many advantages over circular DNA for transcription and replication of large genomes, hence their prevalence in eukaryotes. But the linear arrangement of the DNA has a massive Achilles heel: the terminal ends, or telomeres, are unstable and prone to mutation. Moreover, DNA replication cannot proceed to the end of a linear DNA molecule because the synthesis of Okazaki fragments needs RNA primers to bind ahead of the lagging strand. Eukaryotes deal with both of these problems by adding repetitive DNA sequences to the telomeres that act as a disposable buffer, protecting terminal genes from being truncated during replication and from mutation. Because the telomere is shortened during each DNA replication, it is necessary to resynthesise telomere DNA using an enzyme, telomerase reverse transcriptase.Our understanding of telomere biology is dominated by research into human telomeres. This is understandable due to the links between telomere biology and cellular mortality, ageing and a range of diseases including cancer. However, telomere biology in plants shows some specific differences to humans which may be crucial in our understanding of telomere biology in general. For example, telomerase activity in plant cells is well balanced with the cellular proliferation rate. The reversible regulation of telomerase activity is thought to be important in this context: its activity is turned off in differentiated tissues and turned on during cell periods of active cell replication, for example, during regeneration of plant tissues. Understanding the mechanism for this reversible regulation of telomerase activity could be beneficial in biomedical applications of telomere biology in humans.But where to start? From protozoans and humans, it was known that telomerase was a ribonucleoprotein, carrying its own RNA molecules that are complementary to the telomere repeats and are used as a template for telomere elongation, catalysed by the reverse transcriptase activity of the enzyme. But, in addition, a number of accessory proteins are required to deliver functional telomerase to the telomeres, to regulate its activity and to protect the elongated telomere from DNA repair enzymes. These components assemble into two distinct complexes known as shelterin and CST. Functional homologues of the CST complex have been identified in plants, but the same is not true for the shelterin complex. In plants, not all of the homologues of the six core shelterin components exist, and only some of them seem to be associated with telomeres in vivo. The goal, therefore was to identify undiscovered telomerase accessory proteins in plants and to establish how active telomerase is formed and regulated.Ji r ı Fajkus and his research group at Masaryk University, have been working on plant telomeres for over 20 years. A key member of his team in the hunt for plant telomeraseassociated proteins has been Petra Proch azkov a Schrumpfov a, first as a Ph.D. student and then through several postdoc periods. Working in Arabidopsis, the group...

Composition and Function of Telomerase—A Polymerase Associated with the Origin of Eukaryotes

“…Similarly, we demonstrated that the plant RuvBL1 and RuvBL2a proteins interacted with TERT only indirectly in the nucleolus in vivo. In contrast to mammals, interactions between the TERT and RuvBL proteins in A. thaliana were not direct, but rather they were mediated by one of the Telomere Repeat Binding (TRB) proteins [134,177]. It was also shown that in A. thaliana, dyskerin directly interacted with NAF1.…”

Completing the TRB family: newly characterized members show ancient evolutionary origins and distinct localization, yet similar interactions

“…They interact directly with the catalytic protein subunit of telomerase (TERT) (Schrumpfová et al 2014) and mediate interactions between TERT and Recombination UV B -like (RUVBL) proteins (Schořová et al 2019), homologs of the essential mammalian telomerase assembly components Pontin and Reptin (Venteicher et al 2008). Nuclear and predominantly nucleolar localization of TRB1-3 interacting with TERT and RUVBLs, as well as with the plant ortholog of dyskerin, CBF5 (Lermontova et al 2007), was observed using Bimolecular uorescence complementation (BiFC) (Sweetlove and Gutierrez 2019). Moreover, the TRB1 protein interacts via its H1/5-like domain with Protection of telomeres 1 (POT1b) (Schrumpfová et al 2008), an A. thaliana homolog of the G-overhang binding protein Pot1, a core component of mammalian telomere cap complex, Shelterin ( Telomere shortening was observed in trb1 and trb3 knockout mutants, in the A. thaliana ecotype Columbia with otherwise-stable telomere lengths (Shakirov and Shippen 2004;Schrumpfová et al 2014;Zhou et al 2016).…”

Completing the TRB family: newly characterized members show ancient evolutionary origins and distinct localization, yet similar interactions

“…They interact directly with the catalytic protein subunit of telomerase (TERT) (Schrumpfová et al 2014) and mediate interactions between TERT and Recombination UV B - like (RUVBL) proteins (Schořová et al 2019), homologs of the essential mammalian telomerase assembly components Pontin and Reptin (Venteicher et al 2008). Nuclear and predominantly nucleolar localization of TRB1-3 interacting with TERT and RUVBLs, as well as with the plant ortholog of dyskerin, CBF5 (Lermontova et al 2007), was observed using Bimolecular fluorescence complementation (BiFC) (Sweetlove and Gutierrez 2019). Moreover, the TRB1 protein interacts via its H1/5-like domain with Protection of telomeres 1 (POT1b) (Schrumpfová et al 2008), an A. thaliana homolog of the G-overhang binding protein Pot1, a core component of mammalian telomere cap complex, Shelterin (Tani and Murata 2005; Surovtseva et al 2007).…”