Structure of CBM3b of the major cellulosomal scaffoldin subunit ScaA from Acetivibrio cellulolyticus

The carbohydrate‐binding module (CBM) of the major scaffoldin subunit ScaA of the cellulosome of Acetivibrio cellulolyticus is classified as a family 3b CBM and binds strongly to cellulose. The CBM3b was overexpressed, purified and crystallized, and its three‐dimensional structure was determined. The structure contained a nickel‐binding site located at the N‐terminal region in addition to a `classical' CBM3b calcium‐binding site. The structure was also determined independently by the SAD method using data collected at the Ni‐absorption wavelength of 1.48395 Å and even at a wavelength of 0.97625 Å in a favourable case. The new scaffoldin‐borne CBM3 structure reported here provides clear evidence for the proposition that a family 3b CBM may be accommodated in scaffoldin subunits and functions as the major substrate‐binding entity of the cellulosome assembly.