Structural and functional characterization of a novel type-III dockerin from Ruminococcus flavefaciens.

Phylogenetic analysis of known dockerins in Ruminococcus flavefaciens revealed a novel subtype, type-III, in the scaffoldin proteins, ScaA, ScaB, ScaC and ScaE. In this study, we explored the Ca²⁺-binding properties of the type-III dockerin from the ScaA scaffoldin (ScaADoc) using a battery of structural and biophysical approaches including circular dichroism spectroscopy, isothermal titration calorimetry, differential scanning calorimetry, and nuclear magnetic resonance spectroscopy. Despite the lack of a second canonical Ca²⁺-binding loop, the behaviour of ScaADoc is similar with respect to other dockerin protein modules in terms of its responsiveness to Ca²⁺ and affinity for the cohesin from the ScaB scaffoldin. Our results highlight the robustness of dockerin modules and how their Ca²⁺-binding properties can be exploited in the construction of designer cellulosomes.