Ca2+ and βγ-crystallins: An affair that did not last?

Background: During the last three decades, lens beta- and gamma-crystallins have found a huge number of kin from numerous taxonomical sources. Most of these proteins from invertebrates and microbes have been demonstrated or predicted to bind Ca2+ involving a distinct double-clamp motif, which is largely degenerated in lens homologues. Scope of review: The various aspects of transformation of beta gamma-crystallins from a quintessential Ca2+-binding protein into a primarily structural molecule have been reviewed. Major conclusions: In lens members of beta gamma-crystallins, the residues involved in Ca2+ binding have diverged considerably from the classical consensus with consequent reduction in their Ca2+-binding properties. This evolutionary change is congenial to their new role as robust constituents of lens. The exact functions of the residual affinity for Ca2+ are yet to be established. General significance: This review highlights the significance of reduction in Ce2+-binding ability of the beta gamma-crystallins for lens physiology and why this residual affinity may be functionally important. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease. (C) 2015 Elsevier B.V. All rights reserved.