FlgD is known to be absolutely required for hook assembly, yet it has not been detected in the mature flagellum [1]. It appears to act as a hook-capping protein to enable assembly of hook protein subunits [1]. FlgD regulates the assembly of the hook cap structure to prevent leakage of hook monomers into the medium and hook monomer polymerisation and also plays a role in determination of the correct hook length, with the help of the FliK protein [2]. This family represents the N-terminal conserved region of FlgD. A recent crystal structure showed that this region was likely to be flexible and was cleaved off during crystallisation [3].

1: FlgD is a scaffolding protein needed for flagellar hook assembly in Salmonella typhimurium. Ohnishi K, Ohto Y, Aizawa S, Macnab RM, Iino T; J Bacteriol 1994;176:2272-2281. PMID:8157595

2: Cloning, purification, crystallization and preliminary X-ray studies of flagellar hook scaffolding protein FlgD from Pseudomonas aeruginosa PAO1. Luo M, Niu S, Yin Y, Huang A, Wang D; Acta Crystallogr Sect F Struct Biol Cryst Commun. 2009;65:795-797. PMID:19652342

3: Crystal structure of the C-terminal domain of a flagellar hook-capping protein from Xanthomonas campestris. Kuo WT, Chin KH, Lo WT, Wang AH, Chou SH; J Mol Biol. 2008;381:189-199. PMID:18599076