This domain family is found in bacteria and viruses, and is approximately 160 amino acids in length. There are two conserved sequence motifs: VSR and YGA. This domain is the C terminal domain of the bacteriophage protein endosialidase. The endosialidase protein forms homotrimeric molecules and this domain complexes into a tail-spike stalk. The stalk region folds in a triple beta-helix that is interrupted by a small triple beta-prism domain. The tail-spike is a multifunctional protein device used by the phage to fulfill the following functions: (i) to adsorb to the bacterial polySia capsule (ii) to de-polymerise the capsule to gain access to the outer bacterial membrane, and finally (iii) to mediate tight adhesion to the membrane, a prerequisite for the initiation of the infection cycle.

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