Ruminococcus champanellensis is a keystone species in the human gut that produces an intricate cellulosome system of various architectures. A variety of cellulosomal enzymes have been identified, which exhibit a range of hydrolytic activities on lignocellulosic substrates. We describe herein a unique R. champanellensis scaffoldin, ScaK, which is expressed during growth on cellobiose and comprises a cohesin module and a family 25 glycoside hydrolase (GH25). The GH25 is non-autolytic and exhibits lysozyme-mediated lytic activity against several bacterial species. Despite the narrow acidic pH curve, the enzyme is active along a temperature range from 2 to 85°C and is stable at very high temperatures for extended incubation periods. The ScaK cohesin was shown to bind selectively to the dockerin of a monovalent scaffoldin (ScaG), thus enabling formation of a cell-free cellulosome, whereby ScaG interacts with a divalent scaffodin (ScaA) that bears the enzymes either directly or through additional monovalent scaffoldins (ScaC and ScaD). The ScaK cohesin also interacts with the dockerin of a protein comprising multiple Fn3 domains that can potentially promote adhesion to carbohydrates and the bacterial cell surface. A cell-free cellulosomal GH25 lysozyme may provide a bacterial strategy to both hydrolyze lignocellulose and repel eventual food competitors and/or cheaters. This article is protected by copyright.
This research was supported by a grant to EAB, NMK and ECM from the United States-Israel Binational Science. Foundation (BSF), Jerusalem, Israel and by a grant (No.1349) to EAB from the Israel Science Foundation (ISF). Additional support was obtained from the establishment of an Israeli Center of Research Excellence (I-CORE Center No.152/11) managed by the Israel Science Foundation. The authors also appreciate the support of the European Union, Area NMP.2013.1.1-2: Self-assembly of naturally occurring nanosystems: CellulosomePlus Project number: 604530. HJF and SHD acknowledge financial support from the Scottish Government Rural and Environmental Sciences and Analytical Services (SG-RESAS) and from BBSRC grant no BB/L009951/1. EAB is the incumbent of The Maynard I. and Elaine Wishner Chair of Bio-organic Chemistry.