Evolutionary remodeling of βγ-crystallins for domain stability at cost of Ca2+ binding

The topologically similar βγ-crystallins that are prevalent in all kingdoms of life have evolved for high innate domain stability to perform their specialized functions. The evolution of stability and its control in βγ-crystallins that possess either a canonical (mostly from microorganisms) or degenerate (principally found in vertebrate homologues) Ca2+-binding motif is not known. Using equilibrium unfolding of βγ-crystallin domains (26 wild-type domains and their mutants) in apo- and holo-forms, we demonstrate the presence of a stability gradient across these members, which is attained by the choice of residues in the (N/D)(N/D)XX(S/T)S Ca2+-binding motif. The occurrence of a polar, hydrophobic, or Ser residue at the 1st, 3rd, or 5th position of the motif is likely linked to a higher domain stability. Partial conversion of a microbe-type domain (with a canonical Ca2+-binding motif) to a vertebrate-type domain (with a degenerate Ca2+-binding motif) by mutating serine to arginine/lysine disables the Ca2+-binding but significantly augments its stability. Conversely, stability is compromised when arginine (in a vertebrate-type disabled domain) is replaced by serine (as a microbe type). Our results suggest that such conversions were acquired as a strategy for desired stability in vertebrate members at the cost of Ca2+-binding. In a physiological context, we demonstrate that a mutation such as an arginine to serine (R77S) mutation in this motif of γ-crystallin (partial conversion to microbe-type), implicated in cataracts, decreases the domain stability. Thus, this motif acts as a "central tuning knob" for innate as well as Ca2+-induced gain in stability, incorporating a stability gradient across βγ-crystallin members critical for their specialized functions.