title: Discrete and structurally unique proteins (tāpirins) mediate attachment of extremely thermophilic Caldicellulosiruptor species to cellulose.
abstract: A variety of catalytic and noncatalytic 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, 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 ("tāpirins," origin from Māori: to join), not directly associated with cellulases, that mediate attachment to cellulose by species in the noncellulosomal, extremely thermophilic bacterial genus Caldicellulosiruptor. Two tāpirin genes are located directly downstream of a type IV pilus operon in strongly cellulolytic members of the genus, whereas homologs are absent from the weakly cellulolytic Caldicellulosiruptor species. Based on their amino acid sequence, tāpirins are specific to these extreme thermophiles. Tāpirins 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 carbohydrate-binding module-like affinity of the tāpirins for cellulose. Crystallization of a cellulose-binding truncation from one tāpirin indicated that these proteins form a long β-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 tāpirins to complement substrate-binding proteins from the ATP-binding cassette transporters and multidomain extracellular and S-layer-associated glycoside hydrolases to process the carbohydrate content of lignocellulose.
authors.affiliation: From the Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905.
authors.forename: Sara E
authors.initials: SE
authors.lastname: Blumer-Schuette
authors.affiliation: the Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, and.
authors.forename: Markus
authors.initials: M
authors.lastname: Alahuhta
authors.affiliation: From the Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905.
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Conway
authors.affiliation: From the Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905.
authors.forename: Laura L
authors.initials: LL
authors.lastname: Lee
authors.affiliation: From the Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905.
authors.forename: Jeffrey V
authors.initials: JV
authors.lastname: Zurawski
authors.affiliation: the Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Giannone
authors.affiliation: the Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Hettich
authors.affiliation: the Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, and.
authors.forename: Vladimir V
authors.initials: VV
authors.lastname: Lunin
authors.affiliation: the Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, and.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Himmel
authors.affiliation: From the Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, rmkelly@ncsu.edu.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Kelly
mesh_terms: D000327:Adsorption; D001419:Bacteria / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D001422:Bacterial Adhesion / Q000502:physiology; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002482:Cellulose / Q000378:metabolism*; D010861:Fimbriae, Bacterial / Q000378:metabolism; D005798:Genes, Bacterial; D008958:Models, Molecular; D010802:Phylogeny; D010944:Plants / Q000382:microbiology; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D011994:Recombinant Proteins; D002482:Cellulose
keywords: Adhesin; Bacteria; Biodegradation; Caldicellulosiruptor; Cellulose; Cellulose-binding Protein; Extreme Thermophile; Protein Structure