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, regulatory functions and responsible for several diseases. Although, they are abundant in nature, only a small fraction of it has been characterized till 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 factors contributing the transitions through a mutational study by employing replica exchange molecular dynamics simulation on a βγ-crystallin. Most of the proteins within this superfamily are inherently ordered. However, Hahellin, although a member of βγ-crystallin, it is intrinsically disordered in its apo-form which takes a well-ordered βγ-crystallin fold upon binding to Ca2+. It is intriguing that the mutation at the 5th position of the canonical motif to Arg increases the domain stability in several ordered microbial βγ-crystallins with concomitant loss in Ca2+ binding affinity. We carried out similar Ser to Arg mutations at 5th 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 Ca2+ binding sites play crucial role in transforming the disordered conformation into ordered βγ-crystallin.