This is the C terminal domain of FliK. FliK controls the length of the flagellar hook by directly measuring the hook length as a molecular ruler [1]. This family also includes YscP of the Yersinia type III secretion system, and equivalent proteins in other pathogenic bacterial type III secretion systems.

1: FliK regulates flagellar hook length as an internal ruler. Shibata S, Takahashi N, Chevance FF, Karlinsey JE, Hughes KT, Aizawa S; Mol Microbiol. 2007;64:1404-1415. PMID:17542929