Novel Noncatalytic Cellulase-Binding Proteins Identified in Caldicellulosiruptor
Lignocellulose-degrading microorganisms often produce cellulosomes, which are protein complexes containing cellulase enzymes and noncatalytic binding modules. However, the genus Caldicellulosiruptor does not encode for cellulosomes, indicating that this genus uses alternative attachment mechanisms. To look for cellulose-binding proteins in Caldicellulosiruptor kronotskyensis, researchers from the Department of Energy’s BioEnergy Science Center performed a proteomic screen to detect proteins enriched in a cellulose-bound fraction. A comparison of amino acid sequences from the cellulose-binding proteins to the C. kronotskyensis genomic sequence identified the likely encoding gene and a closely related gene. These genes, subsequently named tapirins, are unusual in that they share no detectable protein domain signatures with known polysaccharide-binding proteins. In addition, no genes homologous to these tapirin genes were found outside of the genus Caldicellulosiruptor. Heterologously expressed tapirin gene products demonstrated binding to insoluble substrates such as Avicel, switchgrass, and Populus biomass, with a high affinity and specificity. Crystallization of a cellulose-binding truncation from one tapirin indicated that these proteins form a long β-helix core with a shielded hydrophobic face and are structurally unique and define a new class of polysaccharide adhesins. Thus, the tapirins establish a new paradigm for how cellulolytic bacteria adhere to cellulose and may be used in engineering more efficient cellulase enzymes for more efficient lignocellulose deconstruction.
Blumer-Schuette, S. E., M. Alahuhta, J. M. Conway, L. L. Lee, J. V. Zurawski, R. J. Giannone, R. L. Hettich, V. V. Lunin, M. E. Himmel, and R. M. Kelly. 2015. “Discrete and Structurally Unique Proteins (Tapirins) Mediate Attachment of Extremely Thermophilic Caldicellulosiruptor Species to Cellulose,”The Journal of Biological Chemistry, DOI: 10.1074/jbc.M115.641480.