Abstract
Small heat shock proteins (sHSPs) are molecular chaperones ubiquitously present in all forms of life, but their function mechanisms remain controversial. Here we show by cryo-electron microscopy and single particle 3D reconstruction that, at the low temperatures (4–25°C), CeHSP17 (a sHSP from Caenorhabditis elegans) exists as a 24-subunit spherical oligomer with tetrahedral symmetry. Our studies demonstrate that CeHSP17 forms large sheet-like super-molecular assemblies (SMAs) at the high temperatures (45–60°C), and such SMAs are apparently the form that exhibits chaperone-like activity. Our findings suggest a novel molecular mechanism for sHSPs to function as molecular chaperones.
Highlights
Small heat shock proteins are molecular chaperones ubiquitously present in all forms of life, but their function mechanisms remain controversial
In line with our previous observation that the CeHSP17 protein enables the E. coli cells to grow at 50uC26, here we demonstrated that the heterologously-expressed CeHSP17 protein is able to confer thermotolerance on E. coli cells, resulting in a survival rate of about 100% and 10% upon heat shock treatment at 58uC and 62uC respectively, with the control cells being almost completely killed upon such heat shock treatments (Fig. 1a)
It should be noted that our previous microscopy analysis demonstrated that the E. coli cells heterologously expressing the CeHSP17 protein retained their normal morphology when growing at 50uC, with their cytoplasmic contents, inner and outer membranes being clearly visible; while the control cells had lost their normal morphology such that their periplasmic space become hardly visible and their cytoplasmic contents largely disappeared with only electron-dense putative protein aggregates visible[26]
Summary
Small heat shock proteins (sHSPs) are molecular chaperones ubiquitously present in all forms of life, but their function mechanisms remain controversial. A s a family of molecular chaperones, small heat shock proteins (sHSPs) are characterized by having a low subunit molecular mass of 12–43 kDa, possession of a conserved a-crystallin domain[1,2,3,4] and formation of large oligomers[5,6,7] They are known to act as ATP-independent ‘‘holdase’’ molecular chaperones that bind to the non-native substrate proteins and prevent them from forming irreversible aggregations, under stress conditions[8,9]; The client proteins would be released for refolding with the help of other ATP-dependent chaperones when the conditions become optimal[10,11,12,13,14]. Our findings unveiled a new structural form of sHSPs and shed light onto a novel molecular mechanism for sHSPs to function as molecular chaperones
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