The Cellulosome System of Acetivibrio cellulolyticus Includes a Novel Type of Adaptor Protein and a Cell Surface Anchoring Protein

Abstract

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

Cellulase, Clostridia, and Ethanol

“…Important in this relationship are (i) cohesin domains on scaffoldin, (ii) dockerin domains on the enzymes, and (iii) a CBD on the scaffoldin, binding the complex to cellulose. Cellulosomes and scaffoldin have been found in many bacteria, such as Clostridium cellulovorans (306), Clostridium cellulolyticum (258), Clostridium josui (149), Clostridium acetobutylicum (254,279), Acetovibrio cellulolyticus (360), Bacteroides cellulosolvens, R. albus, Ruminococcus flavefaciens (273), Vibrio sp., and the anaerobic fungal genera Neocallimastix, Piromyces, and Orpinomyces (305). At least eight scaffoldin genes from cellulolytic bacteria have been sequenced (74).…”

Cell-surface Attachment of Bacterial Multienzyme Complexes Involves Highly Dynamic Protein-Protein Anchors

“…ScaC, in turn, acts as an anchoring scaffoldin by virtue of its C-terminal SLH module ( Fig. 1) (9). The recent sequencing of the A. cellulolyticus CD2 genome identified numerous additional cellulosomal components, gene regulatory elements, and cell anchoring modules (identified by the presence of dockerins or cohesins), suggestive of a much more elaborate and sophisticated cellulosome system than originally observed (13).…”

“…In the type I ScaB A. cellulolyticus dockerin, the specificity residues at positions 11 and 12 of the two duplicated segments consist of a conserved Ile-Asn motif. Thus, a lack of cross-specificity between the type I-Doc interactions that modulate the binding of ScaB into ScaC or the cellulosomal enzymes into ScaA (9,19) may be related to differences at these key residues.…”

“…Initial sequencing of an A. cellulolyticus gene cluster identified four tandem scaffoldin genes (scaA, scaB, scaC, and scaD (9,10)). The primary scaffoldin (where the enzymes of the cellulosome are recruited), ScaA, contains an internal carbohydrate-binding module, bordered by seven type I cohesin modules and a single X module that provides structural stability to the neighboring C-terminal type II dockerin domain.…”

The Clostridium cellulolyticum Dockerin Displays a Dual Binding Mode for Its Cohesin Partner

“…The integration of the plant cell wall hydrolases into the cellulosome has been proposed to potentiate the synergistic interactions between the enzymes and contributes to substrate targeting through the cellulose binding capacity of most Cip molecules (6,7). The Cip molecules of Clostridium cellulolyticum and Clostridium thermocellum (designated CipC and CipA, respectively) can bind eight and nine enzymes, respectively (8), although the cellulosomes of other anaerobic bacteria deploy multiple Cip and adapter molecules in assembling as many as 96 catalytic subunits into a single complex (9). In Clostridia the Cip contains multiple type I cohesin modules that bind tightly to the type I dockerins present on the catalytic subunits and thus assemble these enzymes into the cellulosome (10).…”