A highly acid-stable and thermostable endo-β-glucanase from the thermoacidophilic archaeon Sulfoloblus solfataricus

The thermoacidophilic archaeon Sulfolobus solfataricus P2 encodes three hypothetic endo-beta-glucanases, SSO1354, SSO1949 and SSO2534. We cloned and expressed the gene sso1949 encoding the 334 amino acids containing protein SSO1949, which can be classified as a member of glycoside hydrolase family 12. The purified recombinant enzyme hydrolyses carboxymethylcellulose as well as cello-oligomers, with cellobiose and cellotriose as main reaction products. By following the hydrolysis of a fluorescently labelled cellohexaoside under a wide variety of conditions, we show that SSO1949 is a unique extremophilic enzyme. This archaeal enzyme has a pH optimum of approx. pH 1.8 and a temperature optimum of approx. 80degreesC. Furthermore, the enzyme is thermostable, with a half-life of approx. 8 h at 80degreesC and pH 1.8. The thermostability is strongly pH-dependent. At neutral pH, the thermal inactivation rate is nearly two orders of magnitude higher than at pH 1.8. Homology modelling suggests that the catalytic domain of SSO1949 has a similar fold to other mesophilic, acidophilic and neutral cellulases. The presence of a signal peptide indicates that SSO1949 is a secreted protein, which enables S. solfataricus to use cellulose as an external carbon source. It appears that SSO1949 is perfectly adapted to the extreme environment in solfataric pools. A cellulolytic enzyme with such a combination of stability and activity at high temperatures and low pH has not been described so far and could be a valuable tool for the large-scale hydrolysis of cellulose under acidic conditions.