A Change in Domain Cooperativity Drives the Function of Calnuc

With the increasing incidence of neurodegenerative disorders, there is an urgent need to understand the protein folding process. Examining the folding process of multidomain proteins remains a prime challenge, as their complex conformational dynamics make them highly susceptible to misfolding and/or aggregation. The presence of multiple domains in a protein can lead to interaction between the partially folded domains, thereby driving misfolding and/or aggregation. Calnuc is one such multidomain protein for which Ca²⁺ binding plays a pivotal role in governing its structural dynamics and stability and, presumably, in directing its interactions with other proteins. We demonstrate differential structural dynamics between the Ca²⁺-free and Ca²⁺-bound forms of calnuc. In the absence of Ca²⁺, full-length calnuc displays equilibrium structural transitions with four intermediate states, reporting a sum of the behavioral properties of its individual domains. Fragment-based studies illustrate the sequential events of structure adoption proceeding in the following order: EF domain followed by the NT and LZ domains in the apo state. On the other hand, Ca²⁺ binding increases domain cooperativity and enables the protein to fold as a single unit. Single-tryptophan mutant proteins, designed in a domain-dependent manner, confirm an increase in the number of interdomain interactions in the Ca²⁺-bound form as compared to the Ca²⁺-free state of the protein, thereby providing insight into its folding process. The attenuated domain crosstalk in apo-calnuc is likely to influence and regulate its physiologically important intermolecular interactions.