This domain has a tudor domain-like beta barrel fold. It is found in the FlgD protein the flagellar hook capping protein. The structure for this domain shows that it contains a nested Ig-like domain within it [1]. However in some firmicute proteins this inserted domain is absent such as Q67K21.

1: 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