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[摘要]：To date, very few beta gamma-crystallins have been identified and structurally characterized. Several of them have been shown to bind Ca2+ and thereby enhance their stability without any significant change in structure. Although Ca2+-induced conformational changes have been reported in two putative beta gamma-crystallins from Caulobacter crescentus and Yersinia pesos, they are shown to be partially unstructured, and whether they acquire a beta gamma-crystallin fold is not known. We describe here a beta gamma-crystallin domain, hahellin, its Ca2+ binding properties and NMR structure. Unlike any other beta gamma-crystallin, hahellin is characterized as a pre-molten globule (PMG) type of natively unfolded protein domain. It undergoes drastic conformational change and acquires a typical beta gamma-crystallin fold upon Ca2+ binding and hence acts as a Ca2+-regulated conformational switch. However, it does not bind Mg2+. The intrinsically disordered Ca2+-free state and the close structural similarity of Ca2+-bound hahellin to a microbial beta gamma-crystallin homologue, Protein S, which shows Ca2+-dependent stress response, make it a potential candidate for the cellular functions. This study indicates the presence of a new class of natively unfolded beta gamma-crystallins and therefore the commencement of the possible functional roles of such proteins in this superfamily. |
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