This domain family is found in bacteria and viruses, and is approximately 70 amino acids in length. This domain is found in the bacteriophage protein endosialidase. The two N-terminal domains (this domain and the beta propeller) assemble in the compact 'cap' whereas the C-terminal domain forms an extended tail-like structure. The very N-terminal part of the 'cap' region (residues 246 to 312) holds the only alpha-helix of the protein and is presumably the residual part of the deleted N-terminal head-binding domain. The endosialidase protein complexes to form homotrimeric molecules.

1: Crystal structure of the polysialic acid-degrading endosialidase of bacteriophage K1F. Stummeyer K, Dickmanns A, Muhlenhoff M, Gerardy-Schahn R, Ficner R; Nat Struct Mol Biol. 2005;12:90-96. PMID:15608653