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A Natively Unfolded βγ-Crystallin Domain from Hahella chejuensis

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posted on 16.11.2010, 00:00 authored by Atul K. Srivastava, Yogendra Sharma, Kandala V. R. Chary
To date, very few βγ-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 βγ-crystallins from Caulobacter crescentus and Yersinia pestis, they are shown to be partially unstructured, and whether they acquire a βγ-crystallin fold is not known. We describe here a βγ-crystallin domain, hahellin, its Ca2+ binding properties and NMR structure. Unlike any other βγ-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 βγ-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 βγ-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 βγ-crystallins and therefore the commencement of the possible functional roles of such proteins in this superfamily.