This is the very C-terminal, chaperone, domain of the bacteriophage protein endosialidase. It releases itself, via the serine-lysine dyad at the N-terminus, from the remainder of the end-tail-spike. Cleavage occurs after the threonine which is the final residue of the End-tail-spike family, Pfam:PF12219. The endosialidase protein forms homotrimeric molecules in bacteriophages [1]. The catalytic dyad allows this portion of the molecule to be cleaved from the more N-terminal region such that the latter can fold and bind to polysialic acid in the bacterial outer envelope [1].

1: Characterization of a novel intramolecular chaperone domain conserved in endosialidases and other bacteriophage tail spike and fiber proteins. Schwarzer D, Stummeyer K, Gerardy-Schahn R, Muhlenhoff M; J Biol Chem. 2007;282:2821-2831. PMID:17158460