Acts both as a regulator of telomere function and as a transcription regulator. Involved in the regulation of telomere length and protection as a component of the shelterin complex (telosome). In contrast to other components of the shelterin complex, it is dispensible for telomere capping and does not participate in the protection of telomeres against non-homologous end-joining (NHEJ)-mediated repair. Instead, it is required to negatively regulate telomere recombination and is essential for repressing homology-directed repair (HDR), which can affect telomere length. Does not bind DNA directly: recruited to telomeric double-stranded 5'-TTAGGG-3' repeats via its interaction with TERF2. Independently of its function in telomeres, also acts as a transcription regulator: recruited to extratelomeric 5'-TTAGGG-3' sites via its association with TERF2 or other factors, and regulates gene expression. When cytoplasmic, associates with the I-kappa-B-kinase (IKK) complex and acts as a regulator of the NF-kappa-B signaling by promoting IKK-mediated phosphorylation of RELA/p65, leading to activate expression of NF-kappa-B target genes. Homodimer. Component of the shelterin complex (telosome) composed of TERF1, TERF2, TINF2, TERF2IP ACD and POT1. Binds to TERF2 (but not TERF1) with its C-terminus. Interacts with SLX4/BTBD12 (By similarity). Interacts with TERF2; the interaction is direct. Does not interact with TERF1. Associates with the I-kappa-B-kinase (IKK) core complex, composed of CHUK, IKBKB and IKBKG. Associates with chromosomes, both at telomeres and in extratelomeric sites (PubMed:20622869). Also exists as a cytoplasmic form, where it associates with the IKK complex (PubMed:20622870). Mice are viable and fertile. No major telomere dysfunction such as telomere fusions are observed. An increased telomere fragility and recombination due to defects in HDR are however present. Mice with conditional deletion in stratified epithelia display shorter telomeres and developed skin hyperpigmentation in adulthood. Shares a bidirectional promoter with KARS1 (PubMed:14659874). This shared promoter with an essential gene complicated the task when generating knockout mice; the problem was overcome by generating conditional knockout strategies (PubMed:20339076 and PubMed:20622869;). Belongs to the RAP1 family. Truncated N-terminus.