title: Functional asymmetry in cohesin binding belies inherent symmetry of the dockerin module: insight into cellulosome assembly revealed by systematic mutagenesis.
abstract: The cellulosome is an intricate multi-enzyme complex, known for its efficient degradation of recalcitrant cellulosic substrates. Its supramolecular architecture is determined by the high-affinity intermodular cohesin-dockerin interaction. The dockerin module comprises a calcium-binding, duplicated 'F-hand' loop-helix motif that bears striking similarity to the EF-hand loop-helix-loop motif of eukaryotic calcium-binding proteins. In the present study, we demonstrate by progressive truncation and alanine scanning of a representative type-I dockerin module from Clostridium thermocellum, that only one of the repeated motifs is critical for high-affinity cohesin binding. The results suggest that the near-symmetry in sequence and structure of the repeated elements of the dockerin is not essential to cohesin binding. The first calcium-binding loop can be deleted entirely, with almost full retention of binding. Likewise, significant deletion of the second repeated segment can be achieved, provided that its calcium-binding loop remains intact. Essentially the same conclusion was verified by systematically mutating the highly conserved residues in the calcium-binding loop. Mutations in one of the calcium-binding loops failed to disrupt cohesin recognition and binding, whereas a single mutation in both loops served to reduce the affinity significantly. The results are mutually compatible with recent crystal structures of the type-I cohesin-dockerin heterodimer, which demonstrate that the dockerin can bind in an equivalent manner to its cohesin counterpart through either its first or second repeated motif. The observed plasticity in cohesin-dockerin binding may facilitate cellulosome assembly in vivo or, alternatively, provide a conformational switch that promotes access of the tethered cellulosomal enzymes to their polysaccharide substrates.
authors.affiliation: Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot, 76100 Israel.
authors.forename: Alon
authors.initials: A
authors.lastname: Karpol
authors.forename: Yoav
authors.initials: Y
authors.lastname: Barak
authors.forename: Raphael
authors.initials: R
authors.lastname: Lamed
authors.forename: Yuval
authors.initials: Y
authors.lastname: Shoham
authors.forename: Edward A
authors.initials: EA
authors.lastname: Bayer
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002118:Calcium; D018797:Cell Cycle Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002482:Cellulose / Q000096:biosynthesis*; D002868:Chromosomal Proteins, Non-Histone / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D048013:Acetivibrio thermocellus / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics; D007700:Kinetics; D008958:Models, Molecular; D009097:Multienzyme Complexes / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D009687:Nuclear Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D020641:Polymorphism, Single Nucleotide; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017384:Sequence Deletion; D000097722:Cohesins
chemical_list: D001426:Bacterial Proteins; D002118:Calcium; D018797:Cell Cycle Proteins; D002482:Cellulose; D002868:Chromosomal Proteins, Non-Histone; D009097:Multienzyme Complexes; D009687:Nuclear Proteins; D011994:Recombinant Proteins; D000097722:Cohesins