4wa0
The structure of a possible adhesin C-terminal domain from Caldicellulosiruptor kronotskyensis
Structural highlights
FunctionPublication Abstract from PubMedA variety of catalytic and non-catalytic protein domains are deployed by select microorganisms to deconstruct lignocellulose. These extracellular proteins are used to attach to, modify, and hydrolyze the complex polysaccharides present in plant cell walls. Cellulolytic enzymes, often containing carbohydrate-binding modules (CBMs), are key to this process; however, these enzymes are not solely responsible for attachment. Few mechanisms of attachment have been discovered among bacteria that do not form large polypeptide structures, called cellulosomes, to deconstruct biomass. In this study, bioinformatics and proteomics analyses identified unique, discrete, hypothetical proteins ("tapirins", origin from Maori: to join), not directly associated with cellulases, that mediate attachment to cellulose by species in the non-cellulosomal, extremely thermophilic, bacterial genus Caldicellulosiruptor. Two tapirin genes are located directly downstream of a type IV pilus operon in strongly cellulolytic members of the genus, whereas homologs are absent from weakly cellulolytic Caldicellulosiruptor species. Based on their amino acid sequence, tapirins are specific to these extreme thermophiles. Tapirins are also unusual in that they share no detectable protein domain signatures with known polysaccharide-binding proteins. Adsorption isotherm and trans vivo analyses demonstrated the CBM-like affinity of the tapirins for cellulose. Crystallization of a cellulose-binding truncation from one tapirin indicated that these proteins form a long beta-helix core with a shielded hydrophobic face. Furthermore, they are structurally unique and define a new class of polysaccharide adhesins. Strongly cellulolytic Caldicellulosiruptor species employ tapirins to complement substrate-binding proteins from ABC-transporters and multi-domain extracellular and S-layer-associated glycoside hydrolases to process the carbohydrate content of lignocellulose. Discrete and structurally unique proteins (tapirins) mediate attachment of extremely thermophilic Caldicellulosiruptor species to cellulose.,Blumer-Schuette SE, Alahuhta M, Conway JM, Lee LL, Zurawski JV, Giannone RJ, Hettich RL, Lunin VV, Himmel ME, Kelly RM J Biol Chem. 2015 Feb 26. pii: jbc.M115.641480. PMID:25720489[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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