This article talks about the structural basis for Tetrahymena telomerase processivity factor Teb1 to single-stranded telomeric-repeat DNA. Telomerase is an enzyme that adds nucleotides to telomeres, especially in cancer cells. When DNA is synthesized, it loses a part of it's telomere each time. These are protective compound structures located at the ends of chromosomes. Telomerase duplicates its own internal RNA template to synthesize telomeric DNA repeats. Unlike other polymerases, telomerase can retain its single-stranded product through many rounds of template dissociation and repositioning to accomplish repeat addition processivity (RAP). Tetrahymena telomerase holoenzyme RAP is dependent on a subunit, Teb1, with independent DNA-binding activity. Sequence homology and domain modeling tell us that Teb1 is a paralog of RPA70C, the largest subunit of the single-stranded DNA-binding factor replication protein (RPA), but unlike RPA, Teb1 binds DNA with high specificity for telomeric repeats.
To begin to understand the structural basis and significance of telomeric-repeat DNA recognition by Teb1, researchers had to solve crystal structures of three proposed Teb1 DNA-binding domains and defined amino acids of each domain that had contributed to DNA interaction. Their studies indicate that two central Teb1 DNA-binding oligonucleotide/oligosaccharide-binding-fold domains, Teb1A and Teb1B, achieve high affinity and selectivity of telomeric-repeat recognition by principles similar to the telomere end-capping protein POT1 (protection of telomeres 1). An additional C-terminal Teb1 oligonucleotide/oligosaccharide-binding-fold domain, Teb1C, has similar characteristics as the RPA70 C-terminal domain including a presumed direct DNA-binding surface that is very important for high-RAP activity of reconstituted holoenzyme (which is a biochemically active compound formed by the combination of an enzyme with a coenzyme). The Teb1C zinc ribbon motif does not contribute to DNA binding but is nonetheless required for high-RAP activity, perhaps contributing to Teb1 physical association with the remainder of the holoenzyme. This research team's results suggest the biological model that high-affinity DNA binding by Teb1AB raises holoenzymes to telomeres and subsequent Teb1C-DNA association traps the product in a sliding-clamp-like manner that does not require high-affinity DNA binding in order to achieve high stability of enzyme-product association.
This is an animated picture of a Tetrahymena Telomerase molecule.
To begin to understand the structural basis and significance of telomeric-repeat DNA recognition by Teb1, researchers had to solve crystal structures of three proposed Teb1 DNA-binding domains and defined amino acids of each domain that had contributed to DNA interaction. Their studies indicate that two central Teb1 DNA-binding oligonucleotide/oligosaccharide-binding-fold domains, Teb1A and Teb1B, achieve high affinity and selectivity of telomeric-repeat recognition by principles similar to the telomere end-capping protein POT1 (protection of telomeres 1). An additional C-terminal Teb1 oligonucleotide/oligosaccharide-binding-fold domain, Teb1C, has similar characteristics as the RPA70 C-terminal domain including a presumed direct DNA-binding surface that is very important for high-RAP activity of reconstituted holoenzyme (which is a biochemically active compound formed by the combination of an enzyme with a coenzyme). The Teb1C zinc ribbon motif does not contribute to DNA binding but is nonetheless required for high-RAP activity, perhaps contributing to Teb1 physical association with the remainder of the holoenzyme. This research team's results suggest the biological model that high-affinity DNA binding by Teb1AB raises holoenzymes to telomeres and subsequent Teb1C-DNA association traps the product in a sliding-clamp-like manner that does not require high-affinity DNA binding in order to achieve high stability of enzyme-product association.
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