title: Glycoside hydrolases as components of putative carbohydrate biosensor proteins in Clostridium thermocellum.
abstract: The composition of the cellulase system in the cellulosome-producing bacterium, Clostridium thermocellum, has been reported to change in response to growth on different carbon sources. Recently, an extensive carbohydrate-sensing mechanism, purported to regulate the activation of genes coding for polysaccharide-degrading enzymes, was suggested. In this system, CBM modules, comprising extracellular components of RsgI-like anti-σ factors, were proposed to function as carbohydrate sensors, through which a set of cellulose utilization genes are activated by the associated σ(I)-like factors. An extracellular module of one of these RsgI-like proteins (Cthe_2119) was annotated as a family 10 glycoside hydrolase, RsgI6-GH10, and a second putative anti-σ factor (Cthe_1471), related in sequence to Rsi24, was found to contain a module that resembles a family 5 glycoside hydrolase (termed herein Rsi24C-GH5). The present study examines the relevance of these two glycoside hydrolases as sensors in this signal-transmission system. The RsgI6-GH10 was found to bind xylan matrices but exhibited low enzymatic activity on this substrate. In addition, this glycoside hydrolase module was shown to interact with crystalline cellulose although no hydrolytic activity was detected on cellulosic substrates. Bioinformatic analysis of the Rsi24C-GH5 showed a glutamate-to-glutamine substitution that would presumably preclude catalytic activity. Indeed, the recombinant module was shown to bind to cellulose, but showed no hydrolytic activity. These observations suggest that these two glycoside hydrolases underwent an evolutionary adaptation to function as polysaccharide binding agents rather than enzymatic components and thus serve in the capacity of extracellular carbohydrate sensors.
authors.affiliation: Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Liat
authors.initials: L
authors.lastname: Bahari
authors.forename: Yuval
authors.initials: Y
authors.lastname: Gilad
authors.forename: Ilya
authors.initials: I
authors.lastname: Borovok
authors.forename: Hamutal
authors.initials: H
authors.lastname: Kahel-Raifer
authors.forename: Bareket
authors.initials: B
authors.lastname: Dassa
authors.forename: Yakir
authors.initials: Y
authors.lastname: Nataf
authors.forename: Yuval
authors.initials: Y
authors.lastname: Shoham
authors.forename: Raphael
authors.initials: R
authors.lastname: Lamed
authors.forename: Edward A
authors.initials: EA
authors.lastname: Bayer
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D050260:Carbohydrate Metabolism / Q000235:genetics; D002482:Cellulose / Q000378:metabolism; D044603:Cellulosomes / Q000378:metabolism; D048013:Acetivibrio thermocellus / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D017124:Conserved Sequence; D006026:Glycoside Hydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006868:Hydrolysis; D012808:Sigma Factor / Q000378:metabolism; D014990:Xylans / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D012808:Sigma Factor; D014990:Xylans; D002482:Cellulose; D006026:Glycoside Hydrolases
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