Novel organization and divergent dockerin specificities in the cellulosome system of Ruminococcus flavefaciens

M T Rincon, S Y Ding, S I McCrae, J C Martin, V Aurilia, R Lamed, Y Shoham, E A Bayer, H J Flint

Research output: Contribution to journalArticlepeer-review

72 Citations (Scopus)

Abstract

The DNA sequence coding for putative cellulosomal scaffolding protein ScaA from the rumen cellulolytic anaerobe Ruminococcus flavefaciens 17 was completed. The mature protein exhibits a calculated molecular mass of 90,198 Da and comprises three cohesin domains, a C-terminal dockerin, and a unique N-terminal X domain of unknown function. A novel feature of ScaA is the absence of an identifiable cellulose-binding module. Nevertheless, native ScaA was detected among proteins that attach to cellulose and appeared as a glycosylated band migrating at around 130 kDa. The ScaA dockerin was previously shown to interact with the cohesin-containing putative surface-anchoring protein ScaB. Here, six of the seven cohesins from ScaB were overexpressed as histidine-tagged products in E. coli; despite their considerable sequence differences, each ScaB cohesin specifically recognized the native 130-kDa ScaA protein. The binding specificities of clockerins found in R.flavefaciens plant cell wall-degrading enzymes were examined next. The dockerin sequences of the enzymes EndA, EndB, XynB, and XynD are all closely related but differ from those of XynE and CesA. A recombinant ScaA cohesin bound selectively to dockerin-containing fragments of EndB, but not to those of XynE or CesA. Furthermore, dockerin-containing EndB and XynB, but not XynE or CesA, constructs bound specifically to native ScaA. XynE- and CesA-derived probes did however bind a number of alternative R. flavefaciens bands, including an similar to110-kDa supernatant protein expressed selectively in cultures grown on xylan. Our findings indicate that in addition to the ScaA dockerin-ScaB cohesin interaction, at least two distinct dockerin-binding specificities are involved in the novel organization of plant cell wall-degrading enzymes in this species and suggest that different scaffoldins and perhaps multiple enzyme complexes may exist in R. flavefaciens.

Original languageEnglish
Pages (from-to)703-713
Number of pages11
JournalJournal of Bacteriology
Volume185
Issue number3
DOIs
Publication statusPublished - Feb 2003

Keywords

  • cellulolytic complex cellusome
  • scaffolding protein CIPA
  • clostridium-thermocellum
  • binding protein
  • cell-surface
  • cellulase
  • gene
  • identification
  • degradation
  • domains

Cite this