title: Insights into the structural determinants of cohesin-dockerin specificity revealed by the crystal structure of the type II cohesin from Clostridium thermocellum SdbA.
abstract: The plant cell wall degrading enzymes expressed by anaerobic microorganisms form large multienzyme complexes (cellulosomes). Cellulosomes assemble by the Type I dockerins on the catalytic subunits binding to the reiterated Type I cohesins in the molecular scaffold, while Type II dockerin-cohesin interactions anchor the complex onto the bacterial cell surface. Type I and Type II cohesin, dockerin pairs show no cross-specificity. Here we report the crystal structure of the Type II cohesin (CohII) from the Clostridium thermocellum cell surface anchoring protein SdbA. The protein domain contains nine beta-strands and a small alpha-helix. The beta-strands assemble into two elongated beta-sheets that display a typical jelly roll fold. The structure of CohII is very similar to Type I cohesins, and the dockerin binding site, which is centred at beta-strands 3, 5 and 6, is likely to be conserved in the two proteins. Subtle differences in the topology of the binding sites and a lack of sequence identity in the beta-strands that comprise the core of the dockerin binding site explain why Type I and Type II cohesins display such distinct specificities for their target dockerins.
authors.affiliation: REQUIMTE/CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
authors.forename: Ana L
authors.initials: AL
authors.lastname: Carvalho
authors.forename: Virginia M R
authors.initials: VM
authors.lastname: Pires
authors.forename: Tracey M
authors.initials: TM
authors.lastname: Gloster
authors.forename: Johan P
authors.initials: JP
authors.lastname: Turkenburg
authors.forename: José A M
authors.initials: JA
authors.lastname: Prates
authors.forename: Luís M A
authors.initials: LM
authors.lastname: Ferreira
authors.forename: Maria J
authors.initials: MJ
authors.lastname: Romão
authors.forename: Gideon J
authors.initials: GJ
authors.lastname: Davies
authors.forename: Carlos M G A
authors.initials: CM
authors.lastname: Fontes
authors.forename: Harry J
authors.initials: HJ
authors.lastname: Gilbert
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D044603:Cellulosomes / Q000737:chemistry; D048013:Acetivibrio thermocellus / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008565:Membrane Proteins / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry
chemical_list: D001426:Bacterial Proteins; D008565:Membrane Proteins; D011994:Recombinant Proteins; C105449:SdbA protein, bacteria