THE CELLULOSOME: A NOVEL MECHANISTIC CONCEPT IN MACROMOLECULAR AGGREGATES. Pedro M. Alzari, Institut Pasteur, Paris, France.
The cellulosome, a cellulolytic multi-enzyme complex secreted by clostridia and other bacteria, is a specialized exocellular structure that enables cells to obtain energy from cellulose and hemicellulose, two abundant but intrinsically intractable substrates. Unlike other well-characterized macromolecular agregates such as viral capsids, the ribosome or the pyruvate dehydrogenase complex in which protein-protein interactions stabilize a highly ordered quaternary structure, the cellulosome makes use of a flexible mechanism to enhance the synergistic action of its various enzymatic components.
The cellulosome of the anaerobic thermophile Clostridium thermocellum contains numerous functional subunits, most of which are glycosidases conveying distinct carbohydrate specifities. Their catalytic domains display a variety of protein folding topologies including immunoglobulin-like and lectin-like folds, ([[beta]]/[[alpha]])8-barrels, and ([[alpha]]/[[alpha]])6-barrels. Genetic and crystallographic studies of these enzymatic components have revealed both a conserved active site architecture which has evolved to acquire different substrate specificities and dissimilar protein frameworks which have converged towards the same functional specificity. Most cellulosomal enzymes share a highly conserved duplicated domain - the dockerin domain - that serves to anchor the individual enzymes to a non-catalytic cellulosomal subunit. This scaffolding protein, called CipA (Cellulosome-Integrating Protein), harbors the cellulose-binding function of the complex, serves to attach the cellulosome to the cell surface, and is responsible of organizing the various enzymatic components into the complex. In addition to a cellulose-binding domain and a cell membrane-binding domain, CipA contains a linear tandem of homologous subunits - the cohesin domains - that specifically bind to the dockerin domains of glycosyl hydrolases, giving rise to the "rows of equidistantly spaced polypeptides" observed in electron micrographs. This modular mode of macromolecular assembly provides a simple mechanism to achieve a complex multi-functional structure, and may offer a wide range of applications for the design of new biochemical entities.