Essential 170-kDa subunit for degradation of crystalline cellulose by Clostridium cellulovorans cellulase.

The cellulase complex from Clostridium cellulovorans has been purified and its subunit composition determined. The complex exhibits cellulase activity against crystalline cellulose as well as carboxymethylcellulase (CMCase) and cellobiohydrolase activities. Three major subunits are present with molecular masses of 170, 100, and 70 kDa. The 100-kDa subunit is the major CMCase, although at least four other, minor subunits show CMCase activity. The 170-kDa subunit has the highest affinity for cellulose, does not have detectable enzymatic activity, but is necessary for cellulase activity. Immunological studies indicate that the 170-kDa subunit is not required for binding of the catalytic subunits to cellulose and therefore does not function solely as an anchor protein. Thus this core subunit must have multiple functions. We propose a working hypothesis that the binding of the 170-kDa subunit converts the crystalline cellulose to a form that is capable of being hydrolyzed in a cooperative fashion by the associated catalytic subunits.