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). 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 (By similarity). Homodimer. Component of the shelterin complex (telosome). Interacts with terf2; the interaction is direct (By similarity). Belongs to the RAP1 family.