Shelterin complex

The shelterin complex is a six-protein assembly — TRF1, TRF2, TIN2, TPP1, POT1, and RAP1 — that constitutively coats the TTAGGG repeats at chromosome ends, preventing their recognition as double-strand DNA breaks. TRF2 drives t-loop formation (the 3′ single-stranded overhang invades the adjacent duplex repeat), shielding the terminus from ATM kinase-driven DNA-damage response (DDR); POT1, recruited via TIN2–TPP1, suppresses the parallel ATR pathway by occluding the overhang; TIN2 bridges both TRF proteins to the TPP1–POT1 module. During replicative aging, progressive telomere shortening reduces shelterin occupancy below the threshold needed to suppress DDR, triggering p53/p21 activation and irreversible cellular senescence — connecting shelterin status to the Hayflick limit. In aging endothelial cells, TRF1 mRNA declines ~33 % alongside a ~62 % reduction in telomere length (Mir et al. 2020). RAP1, the sole shelterin subunit that cannot contact telomeric DNA directly, also modulates NF-κB-dependent gene expression, linking telomere dysfunction to inflammaging. Small molecules targeting POT1 or TRF2 remain preclinical; causal evidence in humans that restoring shelterin stoichiometry extends healthspan is absent.