TIN2 Stability Is Regulated by the E3 Ligase Siah2

Telomeres are coated by shelterin, a six-subunit complex that is required for protection and replication of chromosome ends. The central subunit TIN2, with binding sites to three subunits (TRF1, TRF2, and TPP1), is essential for stability and function of the complex. Here we show that TIN2 stability is regulated by the E3 ligase Siah2. We demonstrate that TIN2 binds to Siah2 and is ubiquitylated in vivo. We show using purified proteins that Siah2 acts as an E3 ligase to directly ubiquitylate TIN2 in vitro. Depletion of Siah2 led to stabilization of TIN2 protein, indicating that Siah2 regulates TIN2 protein levels in vivo. Overexpression of Siah2 in human cells led to loss of TIN2 at telomeres that was dependent on the presence of the catalytic RING domain of Siah2. In contrast to RNAi-mediated depletion of TIN2 that led to loss of TRF1 and TRF2 at telomeres, Siah2-mediated depletion of TIN2 allowed TRF1 and TRF2 to remain on telomeres, indicating a different fate for shelterin subunits when TIN2 is depleted posttranslationally. TPP1 was lost from telomeres, although its protein level was not reduced. We speculate that Siah2-mediated removal of TIN2 may allow dynamic remodeling of the shelterin complex and its associated factors during the cell cycle.