This is the N-terminal domain of cellulose-binding protein CttA present in Ruminococcus flavefaciens. CttA mediates attachment of the bacterial substrate via two carbohydrate-binding modules. The domain is known as the X-module and lacks a true hydrophobic core. Unlike the X-modules in other types of CohE-XDoc complexes it does not contribute to the binding surface. This X-module appears to serve as an extended spacer, which separates the cellulose-binding modules at the N terminus of CttA and the bacterial cell wall. The domain does not share structural similarity with other known X-modules from cellulolytic bacteria but does show similarity to G5-1 module of StrH from S. pneumoniae [1].

1: Atypical cohesin-dockerin complex responsible for cell surface attachment of cellulosomal components: binding fidelity, promiscuity, and structural buttresses. Salama-Alber O, Jobby MK, Chitayat S, Smith SP, White BA, Shimon LJ, Lamed R, Frolow F, Bayer EA; J Biol Chem. 2013;288:16827-16838. PMID:23580648