Genomes of extremely thermophilic <
em>
Caldicellulosiruptor<
/em>
species encode novel cellulose binding proteins, called $ t\overline{a}pirins\ $, located proximate to the type IV pilus locus. The C-terminal domain of <
named-content content-type='genus-species'>
Caldicellulosiruptor kronotskyensis<
/named-content>
$ t\overline{a}pirin\ $ 0844 (Calkro_0844) is structurally unique and has a cellulose binding affinity akin to that seen with family 3 carbohydrate binding modules (CBM3s). Here, full-length and C-terminal versions of $ t\overline{a}pirins\ $ from <
named-content content-type='genus-species'>
Caldicellulosiruptor bescii<
/named-content>
(Athe_1870), <
named-content content-type='genus-species'>
Caldicellulosiruptor hydrothermalis<
/named-content>
(Calhy_0908), <
named-content content-type='genus-species'>
Caldicellulosiruptor kristjanssonii<
/named-content>
(Calkr_0826), and <
named-content content-type='genus-species'>
Caldicellulosiruptor naganoensis<
/named-content>
(NA10_0869) were produced recombinantly in <
named-content content-type='genus-species'>
Escherichia coli<
/named-content>
and compared to Calkro_0844. All five $ t\overline{a}pirins\ $ bound to microcrystalline cellulose, switchgrass, poplar, and filter paper but not to xylan. Densitometry analysis of bound protein fractions visualized by SDS-PAGE revealed that Calhy_0908 and Calkr_0826 (from weakly cellulolytic species) associated with the cellulose substrates to a greater extent than Athe_1870, Calkro_0844, and NA10_0869 (from strongly cellulolytic species). Perhaps this relates to their specific needs to capture glucans released from lignocellulose by cellulases produced in <
em>
Caldicellulosiruptor<
/em>
communities. Calkro_0844 and NA10_0869 share a higher degree of amino acid sequence identity (>
80% identity) with each other than either does with Athe_1870 (~50%). The levels of amino acid sequence identity of Calhy_0908 and Calkr_0826 to Calkro_0844 were only 16% and 36%, respectively, although the three-dimensional structures of their C-terminal binding regions were closely related. Unlike the parent strain, <
named-content content-type='genus-species'>
C. bescii<
/named-content>
mutants lacking the $ t\overline{a}pirin\ $ genes did not bind to cellulose following short-term incubation, suggesting a role in cell association with plant biomass. Given the scarcity of carbohydrates in neutral terrestrial hot springs, $ t\overline{a}pirins\ $ likely help scavenge carbohydrates from lignocellulose to support growth and survival of <
em>
Caldicellulosiruptor<
/em>
species.The mechanisms by which microorganisms attach to and degrade lignocellulose are important to understand if effective approaches for conversion of plant biomass into fuels and chemicals are to be developed. <
em>
Caldicellulosiruptor<
/em>
species grow on carbohydrates from lignocellulose at elevated temperatures and have biotechnological significance for that reason. Novel cellulose binding proteins, called $ t\overline{a}pirins\ $, are involved in the way that <
em>
Caldicellulosiruptor<
/em>
species interact with microcrystalline cellulose, and additional information about the diversity of these proteins across the genus, including binding affinity and three-dimensional structural comparisons, is provided here.