Cell-surface Attachment of Bacterial Multienzyme Complexes Involves Highly Dynamic Protein-Protein Anchors

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• Cellulosomes are complex nanomachines essential for breaking down carbohydrates.
• Protein-protein interactions, specifically cohesin-dockerin modules, drive cellulosome assembly.
• Cellulosomes are typically anchored to cell surfaces via type II interactions.

Purpose of the Study:

• To elucidate the structural basis of type I cohesin-dockerin interactions involving the adaptor scaffoldin ScaB in Acetivibrio cellulolyticus.
• To investigate the binding modes and affinity of the ScaB dockerin with the ScaC cohesin.
• To identify residues that modulate interaction specificity in this system.

Main Methods:

• Crystal structure determination of the type I ScaB dockerin in complex with a type I ScaC cohesin.
• Analysis of the complex interface and structural symmetry.
• Characterization of interaction affinity and specificity.

Main Results:

• The crystal structure revealed the ScaB dockerin in complex with ScaC in two distinct orientations.
• The ScaB dockerin exhibits structural symmetry with two identical binding surfaces.
• The interaction demonstrates an ultra-high affinity (Ka ~10^12 M) due to an extensive interface.
• Specific ScaB residues were identified as key for modulating interaction specificity.

Conclusions:

• The ScaB dockerin's dual binding mode and structural symmetry contribute to conformational flexibility in cellulosome assembly.
• Ultra-high affinity interactions mediated by ScaB are critical for the stability of complex cellulosome structures.
• This study reveals novel mechanisms for cellulosome surface recruitment and quaternary structure regulation.