User:Benbest/sandbox/Shelterin

Shelterin is a complex composed of proteins tat sit at the end of the telomeres that cap and protect the ends of chromosomes (chromatids) in most eucaryotes. Shelterin protects telomeres against inappropriate DNA repair when telomeres are not being lengthened, and certain shelterin proteins can facilitate lengthening of telomeres by telomerase when required to do so. Mammalian shelterin is composed of six proteins. The yeast species Schizosaccharomyces pombe (S. pombe, fission yeast) has six proteins in its shelterin complex. But the yeast species Saccharomyces cerevisiae (S. cerevisiae, budding yeast) has no shelterin, although proteins do bind to the end of S. cerevisiae telomeres.

Mammalian shelterin
The mammalian shelterin complex contains the six proteins TRF1, TRF2, RAP1, TIN2, POT1, and TPP1. These proteins can be described as follows:
 * TRF1 (Telomeric Repeat binding Factor 1) along with TRF2 normally prevents telomerase from adding more telomere units to telomeres. But when telomere llengthening is required, TRF1 recruits helicases to facilitate the process.
 * TRF2 (Telomeric Repeat binding Factor 2) prevents Ataxia telangiectasia mutated activation, which is a DNA repair response to DNA double-strand breaks. But when DNA repair of telomeres is required, TRF2 recruits DNA repair proteins. TRF1 and TRF2 bind to double-stranded telomeric DNA independently, but their actions are co-ordinated.
 * TIN2 (TRF1 Interacting Nuclear factor 2) links TRF1 with TRF2, and connects both to POT1. TIN2 is believed to facilitate recruitment of single-stranded telomere-binding proteins to telomeres.
 * POT1 (Protector Of Telomeres 1) prevents Ataxia telangiectasia and Rad3 related activation, which is a DNA repair response to DNA double-strand breaks.
 * RAP1 (Repressor Activator Protein 1) binds to TRF2, and facilitates TRF2 function. Unlike the other shelterin proteins, RAP1 has functions independent of its function within the shelterin complex: RAP1 regulates transcription and affects NF-κB signaling.
 * TPP1 (TINT1, PTOP, PIP1) interacts with POT1 and regulates its function. When telomeres are to be lengthened, TPP1 is a central factor in recruiting telomerase to telomeres.