A recent paper from the Martin lab explores the evolutionary connection between calcium-binding beta/gamma crystallin proteins and the structural beta/gamma crystallins of the eye lens. The tunicate beta/gamma crystallin from Ciona intestinalis, which is known to bind calcium, is also capable of binding a wide variety of other divalent cations, which is highly unusual for a metal-binding protein. Metal-ion binding occurs at the protein's calcium binding sites and greatly enhances its thermal stability. In contrast, the human lens protein gammaS-crystallin, which contains defunct calcim-binding sites binding sites, does not bind any divalent cations and aggregates readily in the presence of zinc. This type of information about how the vertebrate lens crystallins evolved away from ion-binding and toward independent stability is critical to understanding their function in the eye lens.
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