Cohesin diversity revealed by the crystal structure of the anchoring cohesin from Ruminococcus flavefaciens.
The cellulosome is an intriguing multienzyme complex found in cellulolytic bacteria that plays a key role in the degradation of plant cell-wall polysaccharides. In Ruminococcus flavefaciens, a predominant fiber-degrading bacterium found in ruminants, the cellulosome is anchored to the bacterial cell wall through a relatively short ScaE scaffoldin. Determination of the crystal structure of the lone type-III ScaE cohesin from R. flavefaciens (Rf-CohE) was initiated as a part of a structural effort to define cellulosome assembly. The structure was determined at 1.95 A resolution by single-wavelength anomalous diffraction. This is the first detailed description of a crystal structure for a type-III cohesin, and its features were compared with those of the known type-I and type-II cohesin structures. The Rf-CohE module folds into a nine-stranded beta-sandwich with jellyroll topology, typically observed for cohesins, and includes two beta-flaps in the midst of beta-strands 4 and 8, similar to the type-II cohesin structures. However, the presence in Rf-CohE of an additional 13-residue alpha-helix located between beta-strands 8 and 9 represents a dramatic divergence from other known cohesin structures. The prominent alpha-helix is enveloped by an extensive N-terminal loop, not observed in any other known cohesin, which embraces the helix presumably enhancing its stability. A planar surface at the upper portion of the front face of the molecule, bordered by beta-flap 8, exhibits plausible dimensions and exposed amino acid residues to accommodate the dockerin-binding site.
Alber
Orly
O
Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel.
Noach
Ilit
I
Rincon
Marco T
MT
Flint
Harry J
HJ
Shimon
Linda J W
LJ
Lamed
Raphael
R
Frolow
Felix
F
Bayer
Edward A
EA
D000595:Amino Acid Sequence; D001665:Binding Sites; D018797:Cell Cycle Proteins / Q000737:chemistry*; D002482:Cellulose / Q000737:chemistry; D002868:Chromosomal Proteins, Non-Histone / Q000737:chemistry*; D048013:Acetivibrio thermocellus / Q000378:metabolism; D018360:Crystallography, X-Ray / Q000379:methods; D030562:Databases, Protein; D008958:Models, Molecular; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D010802:Phylogeny; D011485:Protein Binding; D017433:Protein Structure, Secondary; D045857:Ruminococcus / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D000097722:Cohesins
D018797:Cell Cycle Proteins; D002482:Cellulose; D002868:Chromosomal Proteins, Non-Histone; D000097722:Cohesins