Binding of TPP1 to TIN2 is required for POT1a,b-mediated telomere protection [DNA and Chromosomes]

July 23rd, 2014 by Frescas, D., de Lange, T.

The single-stranded DNA binding proteins in mouse shelterin, POT1a and POT1b, accumulate at telomeres as heterodimers with TPP1, which binds TIN2 and thus links the TPP1/POT1 dimers with TRF1 and TRF2/Rap1. When TPP1 is tethered to TIN2/TRF1/TRF2, POT1a is thought to block RPA binding to the ss telomeric DNA and prevent ATR kinase activation, Similarly, TPP1/POT1b tethered to TIN2 can control the formation of the correct single-stranded telomeric overhang. Consistent with this view, the telomeric phenotypes following deletion of POT1a/b or TPP1 are phenocopied in TIN2- deficient cells. However, the loading of TRF1 and TRF2/Rap1 are additionally compromised in TIN2 KO cells, leading to added phenotypes. Therefore, it was not excluded that in addition to TIN2, other components of shelterin contribute to the recruitment of TPP1/POT1a,b as suggested by prior reports. To test whether TIN2 is the sole link between TPP1/POT1a,b and telomeres, we defined the TPP1-interaction domain of TIN2 and generated a TIN2 allele that is unable to interact with TPP1 but retains its interaction with TRF1 and TRF2. We demonstrate that cells expressing TIN2ΔTPP1 instead of wild type TIN2 phenocopy the POT1a,b knockout setting without showing additional phenotypes. Thus, these results are consistent with TIN2 being the only mechanism by which TPP1/POT1 heterodimers bind to shelterin and function in telomere protection.