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J. Bacteriol. doi:10.1128/JB.00143-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

A novel cell-surface anchored cellulose-binding protein encoded by the sca gene cluster of Ruminococcus flavefaciens

Marco T. Rincon, Tadej epeljnik, Jennifer C. Martin, Yoav Barak, Raphael Lamed, Edward A. Bayer, and Harry J. Flint^*

Microbial Ecology Group, Rowett Research Institute, Aberdeen, United Kingdom; Zootechnical Department, Biotechnical Faculty, University of Ljubljana, Slovenia; Department of Molecular Microbiology and Biotechnology, Tel Aviv University, Ramat Aviv, and Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot, Israel

^* To whom correspondence should be addressed. Email: H.Flint{at}rowett.ac.uk.

	Abstract

Ruminococcus flavefaciens produces a cellulosomal enzyme complex, based on the structural proteins ScaA, B and C, that was recently shown to attach to the bacterial cell surface via the wall-anchored protein ScaE. ScaA, B, C and E are all cohesin-bearing proteins encoded by linked genes in the sca cluster. The product of an unknown open reading frame within the sca cluster, herein designated CttA, is similar in sequence at its C-terminus with the corresponding region of ScaB, which contains an X-module together with a dockerin sequence. The ScaB-XDoc dyad was shown previously to interact tenaciously with the cohesin of ScaE. Likewise, avid binding was confirmed between purified, recombinant fragments of the CttA-XDoc dyad and the ScaE cohesin. In addition, the N-terminal regions of CttA were shown to bind to cellulose, thus suggesting that CttA is a cell-wall anchored, cellulose-binding protein. Proteomic analysis showed that the native CttA (130 kDa) corresponds to one of the three most abundant polypeptides from cellulose-grown R. flavefaciens 17 cultures that bind tightly to insoluble cellulose. Interestingly, this protein was also detected among cellulose-bound proteins in the related strain R. flavefaciens 007C, but not in a mutant derivative 007S that was previously shown to have lost the ability to grow on de-waxed cotton fibres. In R. flavefaciens, the presence of CttA on the cell surface is likely to provide an important mechanism for substrate binding, perhaps compensating for the absence of an identified cellulose-binding module in the major cellulosomal scaffolding proteins in this species.

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