The cellulosome: the exocellular organelle of Clostridium.

The cellulolytic enzyme complex of the anaerobic thermophile Clostridium thermocellum is reviewed. This complex, called the cellulosome, is cell associated and has a mass of from 2 x 10(6) to 6.5 x 10(6) Daltons. It consists of from 14 to 26 different polypeptides. Cellulosomes form larger complexes, polycellulosomes, with masses from 50 x 10(6) to 80 x 10(6) Daltons. The cellulosome efficiently hydrolyzes crystalline cellulose whereas individual polypeptides alone or in mixtures do not. Many of the polypeptides are catalytically active and can be characterized as endoglucanases, xylanases, and cellodextrinases. Several of the polypeptides have been sequenced including the largest subunit, CipA, that is a glycoprotein with a mass of 210 kDa. CipA has a cellulose-binding domain and nine internal repeated sequences postulated to bind eight catalytic subunits and a special peptide (ORF3p). The ORF3p anchors the CipA to the cell surface. CipA can be characterized as a scaffold holding the catalytic subunits that line up with the cellulose fiber. This arrangement allows a multiple cutting of the cellulose glucan chain. A similar system has been observed for other cellulosome-like complexes, notably Clostridium cellulovorans.