Although telomere-binding proteins constitute an essential part of telomeres, in vivo data indicating the existence of a structure similar to mammalian shelterin complex in plants are limited. Partial characterization of a number of candidate proteins has not identified true components of plant shelterin or elucidated their functional mechanisms. Telomere repeat binding (TRB) proteins from Arabidopsis thaliana bind plant telomeric repeats through a Myb domain of the telobox type in vitro, and have been shown to interact with POT1b (Protection of telomeres 1). Here we demonstrate co-localization of TRB1 protein with telomeres in situ using fluorescence microscopy, as well as in vivo interaction using chromatin immunoprecipitation. Classification of the TRB1 protein as a component of plant telomeres is further confirmed by the observation of shortening of telomeres in knockout mutants of the trb1 gene. Moreover, TRB proteins physically interact with plant telomerase catalytic subunits. These findings integrate TRB proteins into the telomeric interactome of A. thaliana.

• Keywords
• Arabidopsis thaliana, plant shelterin, telomerase, telomere, telomere protein interaction, telomere repeat binding (TRB),
• MeSH
• Arabidopsis enzymology   genetics   MeSH
• Arabidopsis Proteins metabolism   MeSH
• Telomere-Binding Proteins metabolism   MeSH
• Telomerase metabolism   MeSH
• Telomere metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Arabidopsis Proteins MeSH
• Telomere-Binding Proteins MeSH
• Telomerase MeSH

Telomere repeats are added onto chromosome ends by telomerase, consisting of two main core components: a catalytic protein subunit (telomerase reverse trancriptase, TERT), and an RNA subunit (telomerase RNA, TR). Here, we report for the first time evidence that HMGB1 (a chromatin-associated protein in mammals, acting as a DNA chaperone in transcription, replication, recombination, and repair) can modulate cellular activity of mammalian telomerase. Knockout of the HMGB1 gene (HMGB1 KO) in mouse embryonic fibroblasts (MEFs) results in chromosomal abnormalities, enhanced colocalization of γ-H2AX foci at telomeres, and a moderate shortening of telomere lengths. HMGB1 KO MEFs also exhibit significantly (>5-fold) lower telomerase activity than the wild-type MEFs. Correspondingly, enhanced telomerase activity is observed upon overexpression of HMGB1 in MEFs. HMGB1 physically interacts with both TERT and TR, as well as with active telomerase complex in vitro. However, direct interaction of HMGB1 with telomerase is most likely not accountable for the observed higher telomerase activity in HMGB1-containing cells, as revealed from the inability of purified HMGB1 protein to stimulate telomerase activity in vitro. While no transcriptional silencing of TERT is observed in HMGB1 KO MEFs, levels of TR are diminished (~3-fold), providing possible explanation for the observed lower telomerase activity in HMGB1 KO cells. Interestingly, knockout of the HMGB2 gene elevates telomerase activity (~3-fold) in MEFs, suggesting that the two closely related proteins of the HMGB family, HMGB1 and HMGB2, have opposite effects on telomerase activity in the cell. The ability of HMGB1 to modulate cellular activity of telomerase and to maintain telomere integrity can help to understand some aspects of the protein involvement in chromosome stability and cancer.

• MeSH
• Cell Line MeSH
• Chromosome Aberrations MeSH
• Down-Regulation MeSH
• Fibroblasts cytology   metabolism   MeSH
• Microscopy, Fluorescence MeSH
• DNA Fragmentation MeSH
• Gene Knockout Techniques * MeSH
• Histones genetics   metabolism   MeSH
• In Situ Hybridization, Fluorescence MeSH
• Mice MeSH
• DNA Damage MeSH
• HMGB1 Protein genetics   metabolism   MeSH
• HMGB2 Protein genetics   metabolism   MeSH
• DNA Replication MeSH
• RNA genetics   metabolism   MeSH
• Telomerase genetics   metabolism   MeSH
• Telomere metabolism   pathology   MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• gamma-H2AX protein, mouse MeSH Browser
• Histones MeSH
• HMGB1 Protein MeSH
• HMGB2 Protein MeSH
• RNA MeSH
• Telomerase MeSH
• telomerase RNA MeSH Browser
• Tert protein, mouse MeSH Browser