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[摘要]：The topologically similar beta gamma-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 beta gamma-crystallins that possess either a canonical (mostly from microorganisms) or degenerate (principally found in vertebrate homologues) Ca(2+)-binding motif is not known. Using equilibrium unfolding of beta gamma-crystallin domains (26 wildtype 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 Ca(2+)-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 Ca(2+)-binding motif) to a vertebrate-type domain (with a degenerate Ca(2+)-binding motif) by mutating serine to arginine/lysine disables the Ca(2+)-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 Ca(2+)-binding. In a physiological context, we demonstrate that a mutation such as an arginine to serine (R77S) mutation in this motif of gamma-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 Ca(2+)-induced gain in stability, incorporating a stability gradient across beta gamma-crystallin members critical for their specialized functions. |
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