Dockerin Domains

Abstract

Bacterial protein domains known as dockerins serve as anchoring or tethering modules for the assembly of large extracellular cellulase complexes called cellulosomes. Dockerins function through interactions with another class of protein‐interaction module, the cohesins, and both domain families are currently divided into two subclasses (type I and type II) originally based on binding specificity. Dockerin sequences contain a pair of 22‐residue motifs with homology to calcium‐binding loops of the EF‐hand family. When exposed to calcium, the dockerin adopts a novel fold distinct from EF‐hand proteins that binds its cognate cohesin with high affinity. Conversely, in the absence of calcium, the dockerin domain is unfolded and does not bind cohesin. Thus, dockerin domains serve as calcium‐regulated mediators of protein complex formation. Analysis of conserved sequence features in the context of the three‐dimensional dockerin structure may identify elements that confer class and species specificity of dockerin–cohesin binding reactions.

Functional asymmetry in cohesin binding belies inherent symmetry of the dockerin module: insight into cellulosome assembly revealed by systematic mutagenesis

“…For example, a flexible linker and the conserved glycine residue may be important for correct orientation of the catalytic module of the parent enzyme in the cellulosome complex [34,35]. Regarding deletion of the loop-helix motif, the near-perfect internal symmetry of the dockerin, with respect both to sequence and structure [13,36,37], enables it, theoretically and practically, to bind to a cohesin in two independent and opposite orientations [18,19,[38][39][40][41].…”