The microbial degradation of cellulose is one of the most important processes on Earth, and it affects the human condition in many direct and indirect ways. If it did not occur, there would be an inexhaustible accumulation of plant cell refuse, and herbivorous life forms would largely vanish.The multienzyme cellulosome complex is a major mechanism by which some cellulolytic bacteria efficiently degrade cellulose and related plant cell wall polysaccharides (2,4,5,8,9,18,20,32,58,59). To date, cellulosomes have been found in several strains of anaerobic bacteria and fungi obtained from very different types of ecosystems. The first cellulosome was discovered in studies on the anaerobic thermophile Clostridium thermocellum (3,36,37). The cellulosome system of this organism consists of a variety of different enzymes bound to a noncatalytic scaffoldin subunit, which can, in turn, bind to one of several cell surface anchoring proteins. In this organism, both the attachment of the enzymes to the scaffoldin and the attachment of the scaffoldin to the anchoring proteins are accomplished by a special kind of protein-protein interaction, the cohesin-dockerin interaction. In this context, the enzyme subunits include a dockerin domain, and the scaffoldin contains multiple copies of cohesin modules for collective incorporation into the complex. The scaffoldin subunit itself harbors a single dockerin variant that interacts selectively with corresponding cohesin variants on the anchoring proteins. In C. thermocellum, the primary scaffoldin is trifunctional in that it also contains (in addition to the cohesins and dockerin) a substrate-targeting cellulose-binding domain (CBD). The anchoring proteins are bifunctional; in addition to the cohesins, they have another type of domain, the S-layer homology domain (SLH), which is known to bind strongly to the cell surface (12,40). Thus, the sequential cohesin-dockerin-mediated set of interactions among the enzymes, scaffoldin, and anchoring

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A cellulosomal scaffoldin gene, termed cipBc, was identified and sequenced from the mesophilic cellulolytic anaerobe Bacteroides cellulosolvens. The gene encodes a 2,292-residue polypeptide (excluding the signal sequence) with a calculated molecular weight of 242,437. CipBc contains an N-terminal signal peptide, 11 type II cohesin domains, an internal family III cellulose-binding domain (CBD), and a C-terminal dockerin domain. Its CBD belongs to family IIIb, like that of CipV from Acetivibrio cellulolyticus but unlike the family IIIa CBDs of other clostridial scaffoldins. In contrast to all other scaffoldins thus far described, CipBc lacks a hydrophilic domain or domain X of unknown function. The singularity of CipBc, however, lies in its numerous type II cohesin domains, all of which are very similar in sequence. One of the latter cohesin domains was expressed, and the expressed protein interacted selectively with cellulosomal enzymes, one of which was identified as a family 48 glycosyl hydrolase on the basis of partial sequence alignment. By definition, the dockerins, carried by the cellulosomal enzymes of this species, would be considered to be type II. This is the first example of authentic type II cohesins that are confirmed components of a cellulosomal scaffoldin subunit rather than a cell surface anchoring component. The results attest to the emerging diversity of cellulosomes and their component sequences in nature.