Scaffoldin-borne family 3b carbohydrate-binding module from the cellulosome of Bacteroides cellulosolvens: structural diversity and significance of calcium for carbohydrate binding.

The potent cellulose-binding modules of cellulosomal scaffoldin subunits belong to the greater family of carbohydrate-binding modules (CBMs). They have generally been classified as belonging to family 3a on the basis of sequence similarity. They form nine-stranded β-sandwich structures with jelly-roll topology. The members of this family possess on their surface a planar array of aromatic amino-acid residues (known as the linear strip) that form stacking interactions with the glucose rings of cellulose chains and have a conserved Ca(2+)-binding site. Intriguingly, the CBM3 from scaffoldin A (ScaA) of Bacteroides cellulosolvens exhibits alterations in sequence that make it more similar to the CBMs of free cellulolytic enzymes, which are classified into CBM family 3b. X-ray structural analysis was undertaken in order to examine the structural consequences of the sequence changes and the consequent family affiliation. The CBM3 crystallized in space group I4(1)22 with one molecule in the asymmetric unit, yielding diffraction to a resolution of 1.83 Å using X-ray synchrotron radiation. Compared with the known structures of other scaffoldin-borne CBMs, a sequence insertion and deletion appear to compensate for each other as both contained an aromatic residue that is capable of contributing to cellulose binding; hence, even though there are alterations in the composition and localization of the aromatic residues in the linear strip its binding ability was not compromised. Interestingly, no Ca(2+) ions were detected in the conserved calcium-binding site, although the module was properly folded; this suggests that the structural role of Ca(2+) is less important than originally supposed. These observations indicate that despite their conserved function the scaffoldin-borne CBMs are more diverse in their sequences and structures than previously assumed.

Yaniv O ，Shimon LJ ，Bayer EA ，Lamed R ，Frolow F ... -  《-》

Structure of a family 3b' carbohydrate-binding module from the Cel9V glycoside hydrolase from Clostridium thermocellum: structural diversity and implications for carbohydrate binding.

Petkun S ，Jindou S ，Shimon LJ ，Rosenheck S ，Bayer EA ，Lamed R ，Frolow F ... -  《-》

A single mutation reforms the binding activity of an adhesion-deficient family 3 carbohydrate-binding module.

Yaniv O ，Petkun S ，Shimon LJ ，Bayer EA ，Lamed R ，Frolow F ... -  《-》

Crystal structure of a type-II cohesin module from the Bacteroides cellulosolvens cellulosome reveals novel and distinctive secondary structural elements.

Noach I ，Frolow F ，Jakoby H ，Rosenheck S ，Shimon LW ，Lamed R ，Bayer EA ... -  《JOURNAL OF MOLECULAR BIOLOGY》

Interactions between family 3 carbohydrate binding modules (CBMs) and cellulosomal linker peptides.

Family 3 carbohydrate-binding modules (CBM3s) are among the most distinctive, diverse, and robust. CBM3s, which are numerous components of both free cellulases and cellulosomes, bind tightly to crystalline cellulose, and thus play a key role in cellulose degradation through their substrate targeting capacity. In addition to the accepted cellulose binding surface of the CBM3 molecule, a second type of conserved face (the "shallow groove") is retained on the opposite side of the molecule in all CBM3 subfamilies, irrespective of the loss or modification of the cellulose-binding function. The exact function of this highly conserved shallow groove is currently unknown. The cellulosomal system contains many linker segments that interconnect the various modules in long polypeptides chains. These linkers are varied in length (5-700 residues). The long linkers are commonly composed of repeated sequences that are often rich in Ser, Pro, and Thr residues. The exact function of the linker segments in the cellulosomal system is currently unknown, although they likely play several roles. In this chapter, we document the binding interaction between the conserved shallow-groove region of the CBM3s with selected cellulosomal linker segments, which may thus induce conformational changes in the quaternary structure of the cellulosome. These conformational changes would presumably promote changes in the overall arrangement of the cellulosomal enzymes, which would in turn serve to enhance cellulosome efficiency and degradation of recalcitrant polysaccharide substrates. Here, we describe two different methods for determining the interactions between a model CBM3 and cellulosomal linker peptides.

Yaniv O ，Frolow F ，Levy-Assraf M ，Lamed R ，Bayer EA ... -  《-》