Clostridium thermocellum is an anaerobic, thermophilic, and cellulolytic bacterium. It produces an extracellular cellulose complex called the cellulosome. The cellulosome is a huge protein aggregate with a total molecular weight of more than two million. It contains more than 14 subunits. One major subunit, CipA, is an anchorage/scaffolding protein which organizes the catalytic subunits (the cellulose) and brings them to the cellulose surface for its catalytic function. CipA consists of nine repeated domains (the receptors) of about 20kD, each serving as a receptor for the catalytic subunit. A cellulose binding domain (CBD) lies in between the second and the third repeated domains. All catalytic subunits of the cellulose each carry a duplicated, conserved sequence of about 9 kD at their c-terminus ends, called the ligand. This ligand sequence has been shown to bind to a receptor domain of CipA. Although CipA also has a conserved, duplicated sequence at its C-terminus end, it does not bind to its receptors. The overall goal of this project is to determine the structure of the ligand and the receptor complex of the cellulose to understand how the bacterial cellulase can be organized by this universal set of ligand-receptor. It is also of interest to compare the structure of the CipA ligand with the ligands of the catalytic subunits. The study is important for understanding not only the structure-function relationship of this novel and extremely unusual cellulase complex, but also for the supramolecular structure in general.
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