Responsible for initial absorption of the phage to the host bacterium. Degrades the alpha-2,8-linked polysialic acid K1 capsule by cleaving within the polymer chain of polysialic acid. The C-terminal chaperone protein mediates homotrimerization and proper folding of the catalytic endo-N trimer. Endohydrolysis of (2->8)-alpha-sialosyl linkages in oligo- or poly(sialic) acids. Homotrimer. Interacts with sialic acid. Tail spike. The N-terminus is blocked. Proteolytic cleavage and release of the chaperone stabilizes the folded protein. Belongs to the glycosyl hydrolase 58 family.