ABSTRACTA sequence encoding a putative extracellular endoglucanase (sso1354) was identified in the complete genome sequence ofSulfolobus solfataricus. The encoded protein shares signature motifs with members of glycoside hydrolases family 12. After an unsuccessful first attempt at cloning the full-length coding sequences inEscherichia coli, an active but unstable recombinant enzyme lacking a 27-residue N-terminal sequence was generated. This 27-amino-acid sequence shows significant similarity with corresponding regions in the sugar binding proteins AraS, GlcS, and TreS ofS. solfataricusthat are responsible for anchoring them to the plasma membrane. A strategy based on an effective vector/host genetic system forSulfolobusand on expression control by the promoter of theS. solfataricusgene which encodes the glucose binding protein allowed production of the enzyme in sufficient quantities for study. In fact, the enzyme expressed inS. solfataricuswas stable and highly thermoresistant and showed optimal activity at low pH and high temperature. The protein was detected mainly in the plasma membrane fraction, confirming the structural similarity to the sugar binding proteins. The results of the protein expression in the two different hosts showed that the SSO1354 enzyme is endowed with an endo-β-1-4-glucanase activity and specifically hydrolyzes cellulose. Moreover, it also shows significant but distinguishable specificity toward several other sugar polymers, such as lichenan, xylan, debranched arabinan, pachyman, and curdlan.
Structural similarity between the DnaA-binding proteins HobA (HP1230) from Helicobacter pylori and DiaA from Escherichia coli