InterPro: IPR002883 Dockerin cellulose-binding domain

This domain is found in two distinct sets of proteins with different functions. Those found in aerobic bacteria bind cellulose (or other carbohydrates); but in anaerobic fungi they are protein binding domains, referred to as dockerin domains or docking domains. They are believed to be responsible for the assembly of a multiprotein cellulase/hemicellulase complex, similar to the cellulosome found in certain anaerobic bacteria.

The recycling of photosynthetically fixed carbon in plant cell walls is a key microbial process. Enzyme systems that attack the plant cell wall contain noncatalytic carbohydrate-binding modules that mediate attachment to this composite structure and play a pivotal role in maximizing the hydrolytic process. In anaerobes, the degradation is carried out by a high molecular weight, multifunctional complex termed the cellulosome. This consists of a number of independent enzyme components, each of which contains a conserved 40-residue dockerin domain, which functions to bind the enzyme to a cohesin domain within the scaffoldin protein [1, 2].

In anaerobic bacteria that degrade plant cell walls, exemplified by Clostridium thermocellum, the dockerin domains of the catalytic polypeptides can bind equally well to any cohesin from the same organism. More recently, anaerobic fungi, typified by Piromyces equi, have been suggested to also synthesise a cellulosome complex, although the dockerin sequences of the bacterial and fungal enzymes are completely different [3]. For example, the fungal enzymes contain one, two or three copies of the dockerin sequence in tandem within the catalytic polypeptide. In contrast, all the C. thermocellum cellulosome catalytic components contain a single dockerin domain. The anaerobic bacterial dockerins are homologous to EF hands (calcium-binding motifs) and require calcium for activity whereas the fungal dockerin does not require calcium. Finally, the interaction between cohesin and dockerin appears to be species specific in bacteria, there is almost no species specificity of binding within fungal species and no identified sites that distinguish different species.

The structure of dockerin from P. equi contains two helical stretches and four short beta-strands which form an antiparallel sheet structure adjacent to an additional short twisted parallel strand. The N- and C-termini are adjacent to each other.

Aerobic bacteria contain related regions, however these appear to function as cellulose/carbohydrate binding domains.