This is a mannan-specific carbohydrate binding domain, previously known as the X4 module [1]. Unlike other carbohydrate binding modules, binding to substrate causes a conformational change [2].

1: X4 modules represent a new family of carbohydrate-binding modules that display novel properties. Bolam DN, Xie H, Pell G, Hogg D, Galbraith G, Henrissat B, Gilbert HJ; J Biol Chem. 2004;279:22953-22963. PMID:15004012

2: Structure of a mannan-specific family 35 carbohydrate-binding module: evidence for significant conformational changes upon ligand binding. Tunnicliffe RB, Bolam DN, Pell G, Gilbert HJ, Williamson MP; J Mol Biol. 2005;347:287-296. PMID:15740741