Homology swapping of intrinsic secondary structural elements between cellulosomal types I and II cohesins and their effect on dockerin binding

Abstract

The high-affinity cohesin–dockerin interaction dictates the suprastructural assembly of the multienzyme cellulosome complex. The interaction between these two complementary families of protein modules was found to be species-specific and type-dependent. The structure of the type II cohesin module possesses additional intrinsic secondary structural elements absent in the type I cohesin, i.e., an α-helix and two singular "β-flaps". The role of these extra secondary structures in dockerin recognition was studied in this work using gene swapping, in which corresponding homologous stretches of types I and II cohesins were interchanged. The specificity of binding of the resultant chimaeric cohesins was determined by enzyme-linked affinity assay. Several chimaeric cohesins retained dockerin recognition properties. Hence, these cohesins may undergo manipulations (insertion/deletion of peptide segments) without altering their affinity toward their counterpart dockerin, although type-dependent binding specificity cannot be converted by swapping of the additional secondary structural elements between the two cohesin types. The results further emphasize the strong affinity and plasticity between the cohesin and dockerin pair and are consistent with the known findings on specificity of the types I and II interactions. These studies provide insight into the structural and functional resilience of the cohesins and thus have direct bearing on their potential use in biotechnological applications.