Mechanistic Insights from Replica Exchange Molecular Dynamics Simulations into Mutation Induced Disordered-to-Ordered Transition in Hahellin, a βγ-Crystallin.

Intrinsically disordered proteins (IDPs) form a special category because they lack a unique well-folded 3D structure under physiological conditions. They play crucial role in cell signaling and regulatory functions and are responsible for several diseases. Although they are abundant in nature, only a small fraction of them have been characterized until date. Such proteins adopt a range of conformations and can undergo transformation from disordered-to-ordered state or vice versa upon binding to ligand. Insights of such conformational transition is perplexing in several cases. In the present study, we characterized disordered as well as ordered states and the interactions contributing the transitions through a mutational study by employing replica exchange molecular dynamics simulation with generalized Born implicit solvent model on a protein from the βγ-crystallin superfamily. Most of the proteins within this superfamily are inherently ordered and highly stable. However, Hahellin, although a member of the βγ-crystallin family, is intrinsically disordered in its apo-form which takes a well-ordered βγ-crystallin fold upon binding to Ca. It is intriguing that the mutation at the fifth position of the canonical motif to Arg increases the domain stability in several ordered microbial βγ-crystallins with concomitant loss in Ca binding affinity. We carried out similar Ser to Arg mutations at fifth position of the canonical motif for the first time in an intrinsically disordered protein to understand the mechanistic insights of conformational transition. Our study revealed that newly formed ionic and hydrogen bonding interactions at the canonical Ca binding sites play a crucial role in transforming the disordered conformation into ordered βγ-crystallin.