Small Heat shock proteins (sHsps) are a family of molecular chaperones that bind nonnative proteins in an ATP-independent manner. Caenorhabditis elegans encodes 16 different sHsps, among them Hsp17, which is evolutionarily distinct from other sHsps in the nematode. The structure and mechanism of Hsp17 and how these may differ from other sHsps remain unclear. Here, we find that Hsp17 has a distinct expression pattern, structural organization, and chaperone function. Consistent with its presence under nonstress conditions, and in contrast to many other sHsps, we determined that Hsp17 is a mono-disperse, permanently active chaperone in vitro, which interacts with hundreds of different C. elegans proteins under physiological conditions. Additionally, our cryo-EM structure of Hsp17 reveals that in the 24-mer complex, 12 N-terminal regions are involved in its chaperone function. These flexible regions are located on the outside of the spherical oligomer, whereas the other 12 N-terminal regions are engaged in stabilizing interactions in its interior. This allows the same region in Hsp17 to perform different functions depending on the topological context. Taken together, our results reveal structural and functional features that further define the structural basis of permanently active sHsps.

小热休克蛋白 (sHsps) 是一个分子伴侣家族，它以不依赖 ATP 的方式结合非天然蛋白质。秀丽隐杆线虫编码 16 种不同的 sHsps，其中包括 Hsp17，它在进化上不同于线虫中的其他 sHsps。Hsp17 的结构和机制以及这些与其他 sHsp 的不同之处仍不清楚。在这里，我们发现 Hsp17 具有独特的表达模式、结构组织和伴侣功能。与其在非应激条件下的存在一致，并且与许多其他 sHsps 相比，我们确定 Hsp17 是一种单分散的、永久活跃的体外伴侣，它与数百种不同的C. elegans相互作用生理条件下的蛋白质。此外，我们的 Hsp17 低温电子显微镜结构表明，在 24 聚体复合物中，12 个 N 末端区域参与其伴侣功能。这些柔性区域位于球形低聚物的外部，而其他 12 个 N 末端区域在其内部参与稳定相互作用。这允许 Hsp17 中的同一区域根据拓扑上下文执行不同的功能。总之，我们的结果揭示了结构和功能特征，进一步定义了永久活跃的 sHsps 的结构基础。