This domain was initially identified from proteins from C. hutchinsonii giving rise to the CHU name. This domain adopts an immunoglobulin like domain that is foundat the C-terminus of various bacterial cell surface proteins. This domain was shown to be essential for localisation of the CHU_3220 protein. It is likely that this domain target the proteins containing it to the type IX secretion system. This domain is cleaved off the protein which is then anchored to the outer membrane [2].

1: Identification and Characterization of a Large Protein Essential for Degradation of the Crystalline Region of Cellulose by Cytophaga hutchinsonii. Wang S, Zhao D, Bai X, Zhang W, Lu X; Appl Environ Microbiol. 2016; [Epub ahead of print] PMID:27742681

2: Type IX secretion: the generation of bacterial cell surface coatings involved in virulence, gliding motility and the degradation of complex biopolymers. Veith PD, Glew MD, Gorasia DG, Reynolds EC; Mol Microbiol. 2017;106:35-53. PMID:28714554