The noncanonical small heat shock protein HSP-17 from Caenorhabditis elegans is a selective protein aggregase.

Small heat shock proteins (sHsps) are conserved, ubiquitous members of the proteostasis network. Canonically, they act as "holdases" and buffer unfolded or misfolded proteins against aggregation in an ATP-independent manner. Whereas bacteria and yeast each have only two sHsps in their genomes, this number is higher in metazoan genomes, suggesting a spatiotemporal and functional specialization in higher eukaryotes. Here, using recombinantly expressed and purified proteins, static light-scattering analysis, and disaggregation assays, we report that the noncanonical sHsp HSP-17 of Caenorhabditis elegans facilitates aggregation of model substrates, such as malate dehydrogenase (MDH), and inhibits disaggregation of luciferase in vitro Experiments with fluorescently tagged HSP-17 under the control of its endogenous promoter revealed that HSP-17 is expressed in the digestive and excretory organs, where its overexpression promotes the aggregation of polyQ proteins and of the endogenous kinase KIN-19. Systemic depletion of hsp-17 shortens C. elegans lifespan and severely reduces fecundity and survival upon prolonged heat stress. HSP-17 is an abundant protein exhibiting opposing chaperone activities on different substrates, indicating that it is a selective protein aggregase with physiological roles in development, digestion, and osmoregulation.

The small heat-shock protein αB-crystallin uses different mechanisms of chaperone action to prevent the amorphous versus fibrillar aggregation of α-lactalbumin.

Stress conditions can destabilize proteins, promoting them to unfold and adopt intermediately folded states. Partially folded protein intermediates are unstable and prone to aggregation down off-folding pathways leading to the formation of either amorphous or amyloid fibril aggregates. The sHsp (small heat-shock protein) αB-crystallin acts as a molecular chaperone to prevent both amorphous and fibrillar protein aggregation; however, the precise molecular mechanisms behind its chaperone action are incompletely understood. To investigate whether the chaperone activity of αB-crystallin is dependent upon the form of aggregation (amorphous compared with fibrillar), bovine α-lactalbumin was developed as a model target protein that could be induced to aggregate down either off-folding pathway using comparable buffer conditions. Thus when α-lactalbumin was reduced it aggregated amorphously, whereas a reduced and carboxymethylated form aggregated to form amyloid fibrils. Using this model, αB-crystallin was shown to be a more efficient chaperone against amorphously aggregating α-lactalbumin than when it aggregated to form fibrils. Moreover, αB-crystallin forms high molecular mass complexes with α-lactalbumin to prevent its amorphous aggregation, but prevents fibril formation via weak transient interactions. Thus, the conformational stability of the protein intermediate, which is a precursor to aggregation, plays a critical role in modulating the chaperone mechanism of αB-crystallin.

αB-crystallin, a small heat shock protein, modulates NF-κB activity in a phosphorylation-dependent manner and protects muscle myoblasts from TNF-α induced cytotoxicity.

αB-crystallin, a member of the small heat shock protein family, has been implicated in various biological functions including response to heat shock, differentiation and apoptosis, the mechanisms of which have not been well understood. Myoblasts, the precursor cells in muscle regeneration, when subjected to growth factor deprivation differentiate to form myotubes or undergo apoptosis. During differentiation, myoblasts express elevated levels of αB-crystallin as well as TNF-α but the connecting link between these proteins in cell signaling is not clearly understood. We have therefore investigated the role of αB-crystallin in TNF-α induced regulation of NF-κB. We demonstrate that in response to TNF-α treatment, αB-crystallin associates with IKKß and activate its kinase activity, facilitating the degradation of phosphorylated I-kBα, a prime step in NF-κB activation. Reducing the level of αB-crystallin using the RNAi approach reduces the translocation of p65, further confirming the role of αB-crystallin in NF-κB activation. Our study shows that the ability of αB-crystallin to activate NF-κB depends on its phosphorylation status. The present study shows that αB-crystallin-dependent NF-κB activation protects myoblasts from TNF-α induced cytoxicity by enhancing the expression of the anti-apoptotic protein, Bcl 2. Thus, our study identifies yet another mechanism by which αB-crystallin exerts its anti-apoptotic activity.