Role for NIP45 in Telomere Recruitment to PML Bodies in ALT Cancer Cells
Farley, Demetra Dannielle
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Telomere length maintenance is critical for continued cell proliferation. The SMC5/6 complex, required for double-strand break (DSB) repair in both yeast and humans, has been implicated in the maintenance of telomere length in certain cancer cells. In the absence of active telomerase, SMC5/6 complex-dependent homologous recombination is utilized to maintain telomere length at PML bodies, a mechanism referred to as alternative lengthening of telomeres (ALT). Sumoylation of several telomere-binding proteins is required for the localization of telomeres to PML bodies in G2 phase cells (APBs). We demonstrate that NIP45, a SUMO-like domain (SLD) containing protein, also affects telomere targeting in ALT cells. Loss of endogenous NIP45 protein results in decreased localization of telomeres to PML bodies in a manner independent of the SMC5/6 complex. NIP45 stimulates telomere binding protein sumoylation, as knockdown of the NIP45 protein negatively affects their sumoylation. Importantly, the NIP45 C-terminal SUMO-like domain (SLD2) is sufficient to rescue both APB formation and telomere-binding-protein sumoylation. NIP45 localizes to PML bodies, but not telomeres, in log phase cells, yet interacts efficiently with TIN2, a sumoylatable telomere binding protein. Additionally, a fragment of NIP45 containing the functional SLD2 domain is sufficient to maintain TIN2 binding. We predict, then, that NIP45 might act to recruit telomeres to the PML bodies via its interaction with TIN2, ultimately allowing for SMC5/6 complex-dependent telomere maintenance in G2 phase cells. In keeping with this hypothesis, loss of endogenous TIN2 protein also negatively affects localization of telomeres to PML bodies, even in the presence of NIP45, supporting a requirement for the TIN2-NIP45 interaction in telomere localization to PML bodies. Through this work, we have defined a role for the NIP45 protein in ALT cancer cell telomere length maintenance, further detailing the mechanism by which telomerase-negative cancer subtypes achieve unlimited replicative potential.