C-terminal domain of cellulose binding protein tapirin
This family contains the C-terminal domain of tapirin, a unique cellulose binding protein that is present only in the extremely thermophilic bacterial species Caldicellulosiruptor that grow on carbohydrates from lignocellulose at elevated temperatures. Tapirins appear to be specifically attached to cellulose, having similar binding affinities to cellulose as family 3 carbohydrate binding modules (CBM3). Structures of the C-terminal region indicate that aromatic and hydrophobic residues are responsible for cellulose binding, while a flexible peptide loop may protect and control access to this region. The basis for the genomic localization of the tapirins is unknown; however, these proteins are located next to type IV pili in the Caldicellulosiruptor genomes and therefore may be exposed on the cell membrane beside or as part of pili proteins. Caldicellulosiruptor hydrothermalis, which has less capability to deconstruct lignocellulose itself, may use tapirin as one of the mechanisms for its survival in extreme environments by anchoring itself to biomass that is hydrolyzed by enzymes from other species. Understanding mechanisms by which these microorganisms attach to and degrade lignocellulose may be important in finding effective approaches for conversion of plant biomass into fuels and chemicals.