Characterization of a cellulosome dockerin domain from the anaerobic fungus Piromyces equi

The recycling of photosynthetically fixed carbon in plant cell walls is a k ey microbial process. In anaerobes, the degradation is carried out by a hig h molecular weight multifunctional complex termed the cellulosome. This con sists of a number of independent enzyme components, each of which contains a conserved dockerin domain, which functions to bind the enzyme to a cohesi n domain within the protein scaffoldin protein. Here we describe the first three-dimensional structure of a fungal dockerin, the N-terminal dockerin o f Ce145A from the anaerobic fungus Piromyces equi. The structure contains a novel fold of 42 residues. The ligand binding site consists of residues Tr p 35, Tyr 8 and Asp 23, which are conserved in all fungal dockerins. The bi nding site is on the opposite side of the N- and C-termini of the molecule, implying that tandem dockerin domains, seen in the majority of anaerobic f ungal plant cell wall degrading enzymes, could present multiple simultaneou s binding sites and, therefore, permit tailoring of binding to catalytic de mands.