This is a neuraminidase-like domain, which is structurally homologous to neuraminidases. It can be found in TcA subunit in tripartite Tc toxin complexes of bacterial pathogens. Functional analysis suggest that the neuraminidase-like domain acts as an electrostatic lock that opens at high or low pH values [1].

1: Mechanism of Tc toxin action revealed in molecular detail. Meusch D, Gatsogiannis C, Efremov RG, Lang AE, Hofnagel O, Vetter IR, Aktories K, Raunser S; Nature. 2014;508:61-65. PMID:24572368