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Publication Abstract from PubMed

The rumen bacterium Ruminococcus flavefaciens produces a highly organized multienzyme cellulosome complex that plays a key role in the degradation of plant cell wall polysaccharides, notably cellulose. The R. flavefaciens cellulosomal system is anchored to the bacterial cell wall through a relatively small ScaE scaffoldin subunit, which bears a single type-IIIe cohesin responsible for the attachment of two major dockerin-containing scaffoldin proteins: ScaB and the cellulose-binding protein CttA. While ScaB recruits the catalytic machinery onto the complex, CttA mediates attachment of the bacterial substrate via its two putative carbohydrate-binding modules (CBMs). In an effort to understand the structural basis for assembly and cell-surface attachment of the cellulosome in R. flavefaciens, we determined the crystal structure of the high-affinity complex (Kd = 20.83 nM) between the ScaE cohesin module (CohE) and its cognate X-dockerin modular dyad (XDoc) from CttA at 1.97-A resolution. The structure reveals an atypical calcium-binding loop containing a 13-residue insert. The results further pinpoint two charged specificity-related residues on the surface of the cohesin module, which are responsible for specific vs. promiscuous cross-strain binding of the dockerin module. In addition, a combined functional role for the three enigmatic dockerin inserts was established, whereby these extraneous segments serve as structural buttresses that reinforce the stalk-like conformation of the X-module, thus segregating its tethered complement of cellulosomal components from the cell surface. The novel structure of the RfCohE-XDoc complex sheds light on divergent dockerin structure and function and provides insight into the specificity features of the type-IIIe cohesin-dockerin interaction.

Atypical cohesin-dockerin complex responsible for cell-surface attachment of cellulosomal components: binding fidelity, promiscuity, and structural buttresses.,Salama-Alber O, Jobby MK, Chitayat S, Smith SP, White BA, Shimon LJ, Lamed R, Frolow F, Bayer EA J Biol Chem. 2013 Apr 11. PMID:23580648^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.