FEN-like PIN domains of the 5'-3' exonucleases of DNA polymerase I, bacteriophage T4 RNase H and T5-5' nucleases, and homologs.
PIN (PilT N terminus) domains of the 5'-3' exonucleases (53EXO) of multi-domain DNA polymerase I and single domain protein homologs, as well as, the PIN domains of bacteriophage T5-5'nuclease (T5FEN or 5'-3'exonuclease), bacteriophage T4 RNase H (T4FEN), bacteriophage T3 (T3 phage exodeoxyribonuclease) and other similar nucleases are included in this family. The 53EXO of DNA polymerase I recognizes and endonucleolytically cleaves a structure-specific DNA substrate that has a bifurcated downstream duplex and an upstream template-primer duplex that overlaps the downstream duplex by 1 bp. The T5-5'nuclease is a 5'-3'exodeoxyribonuclease that also exhibits endonucleolytic activity on flap structures (branched duplex DNA containing a free single-stranded 5'end). T4 RNase H, which removes the RNA primers that initiate lagging strand fragments, has 5'- 3'exonuclease activity on DNA/DNA and RNA/DNA duplexes and has endonuclease activity on flap or forked DNA structures. These nucleases are members of the structure-specific, 5' nuclease family (FEN-like) that catalyzes hydrolysis of DNA duplex-containing nucleic acid structures during DNA replication, repair, and recombination. Canonical members of the FEN-like family possess a PIN domain with a two-helical structure insert (also known as the helical arch, helical clamp or I domain) of variable length (approximately 16 to 800 residues), and at the C-terminus of the PIN domain a H3TH (helix-3-turn-helix) domain, an atypical helix-hairpin-helix-2-like region. Both the H3TH domain (not included in this model) and the helical arch/clamp region are involved in DNA binding. The PIN domain belongs to a large nuclease superfamily. The structural properties of the PIN domain indicate its putative active center, consisting of invariant acidic amino acid residues (putative metal-binding residues), is geometrically similar in the active center of structure-specific 5' nucleases, PIN-domain ribonucleases of eukaryotic rRNA editing proteins, and bacterial toxins of toxin-antitoxin (TA) operons.
Comment:The active site includes a set of conserved catalytic residues that are essential for binding divalent metal ions required for nuclease activity. One or two of these residues are in the H3TH domain (not shown).
Structure:2IHN; Bacteriophage T4 RNase H binds fork DNA substrate, contacts at 5A A
Comment:The scissile phosphate, the first bond in the duplex DNA adjacent to the 5' arm, lies above a magnesium binding site.
Comment:Together with the helical arch and network of amino acids interacting with metal binding ions, the H3TH region (not included) defines a positively charged active-site DNA-binding groove in structure-specific 5' nucleases.
Jiyao W et al.(2023). "The conserved domain database in 2023.",