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Unconventional Mode of Attachment of the Ruminococcus flavefaciens Cellulosome to the Cell Surface

Marco T. Rincon,^1* Tadej epeljnik,² Jennifer C. Martin,¹ Raphael Lamed,³ Yoav Barak,⁴ Edward A. Bayer,⁴ and Harry J. Flint¹

Microbial Ecology Group, Rowett Research Institute, Aberdeen, United Kingdom,¹ Zootechnical Department, Biotechnical Faculty, University of Ljubljana, Slovenia,² Department of Molecular Microbiology and Biotechnology, Tel Aviv University, Ramat Aviv,³ Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot, Israel⁴

Received 10 June 2005/ Accepted 24 August 2005

Sequence extension of the scaffoldin gene cluster from Ruminococcus flavefaciens revealed a new gene (scaE) that encodes a protein with an N-terminal cohesin domain and a C terminus with a typical gram-positive anchoring signal for sortase-mediated attachment to the bacterial cell wall. The recombinant cohesin of ScaE was recovered after expression in Escherichia coli and was shown to bind to the C-terminal domain of the cellulosomal structural protein ScaB, as well as to three unknown polypeptides derived from native cellulose-bound Ruminococcus flavefaciens protein extracts. The ScaB C terminus includes a cryptic dockerin domain that is unusual in its sequence, and considerably larger than conventional dockerins. The ScaB dockerin binds to ScaE, suggesting that this interaction occurs through a novel cohesin-dockerin pairing. The novel ScaB dockerin was expressed as a xylanase fusion protein, which was shown to bind tenaciously and selectively to a recombinant form of the ScaE cohesin. Thus, ScaE appears to play a role in anchoring the cellulosomal complex to the bacterial cell envelope via its interaction with ScaB. This sortase-mediated mechanism for covalent cell-wall anchoring of the cellulosome in R. flavefaciens differs from those reported thus far for any other cellulosome system.

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