Interactions and synergism between the recombinant CelD, an endoglucanase, and the cellulosome-integrating protein (CipA) of Clostridium thermocellum

The Clostridium thermocellum cellulosome Is a multicomponent cellulase complex. Its largest subunit, CipA, contains nine repeated domains (R's), each of which binds to a cellulosomal catalytic subunit, and a cellulose binding domain (CBD). We have previously reported that the activity of CelS, an exoglucanase subunit, is enhanced by the anchorage function of CipA. In this work, we examined the effect of anchorage on the activity of CelD, an endoglucanase subunit, using recombinant CelD (rCelD) and the CipA functional domains, R3 (a repeat next to CBD) and CBD/R3, expressed in Escherichia coli. rCelD formed a stable complex with CBD/R3 as analyzed by a gel-shift assay on a nondenaturing polyacrylamide gradient gel. Binding of rCelD to crystalline cellulose, as its activity toward both phosphoric acid-swollen and crystalline cellulose, was dependent on CBD/R3. These results indicate that the activity of an endoglucanase subunit of the cellulosome as that of the exoglucanase subunit, is enhanced by the anchorage function of CipA, Such anchorage function of CipA may thus augment the potential endo-exo synergism in the cellulosome.