Shelterin is a Dimeric Complex with Extensive Structural Heterogeneity
Human shelterin is a six-subunit complex - comprised of TRF1, TRF2, Rap1, TIN2, TPP1, and POT1 - that binds telomeres, protects them from the DNA-damage response, and regulates the maintenance of telomeric DNA. Although high-resolution structures have been generated of the individual structured domains within shelterin, the architecture and stoichiometry of the full complex are currently unknown. Here we report the purification of shelterin subcomplexes and reconstitution of the entire complex using full-length, recombinantly produced components. By combining negative-stain electron microscopy (EM), crosslinking mass spectrometry (XLMS), mass photometry, and native mass spectrometry (MS), we obtain stoichiometries as well as domain-scale architectures of shelterin subcomplexes and determine that they are extensively conformationally heterogenous. For POT1/TPP1 and POT1/TPP1/TIN2, we observe high variability in the positioning of the POT1 DNA-binding domain, the TPP1 OB fold, and the TIN2 TRFH domain with respect to the C-terminal domains of POT1. Truncation of unstructured linker regions in TIN2, TPP1, and POT1 did not reduce the conformational variability of the heterotrimer. Both shelterin and the TRF1/TIN2/TPP1/POT1 subcomplex primarily adopt fully dimeric complexes, even in the absence of DNA substrates. TRF1/TIN2/TPP1/POT1 and shelterin complex showed extensive conformational variability, regardless of the presence of DNA substrates. We conclude that shelterin adopts a multitude of conformations and argue that its unusual architectural variability is beneficial for its many functions at telomeres.