The permanently chaperone-active small heat shock protein Hsp17 from C. elegans exhibits topological separation of its N-terminal regions

Small Heat shock proteins (sHsps) are a family of molecular chaperones that bind non-native proteins in an ATP-independent manner. C. 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 non-stress 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（small Heat shock proteins，sHsps）是一类以ATP非依赖方式结合非天然蛋白的分子伴侣家族。秀丽隐杆线虫（C. elegans）编码16种不同的sHsps，其中Hsp17在进化上与该线虫体内的其他sHsps存在显著差异。目前，Hsp17的结构与作用机制，以及其与其他sHsps的差异之处仍未明确。本研究发现，Hsp17具有独特的表达模式、结构组织方式与分子伴侣功能。与多数其他sHsps不同，且与其在非应激条件下稳定存在的特征相符，我们通过体外实验证实，Hsp17是一种单分散、永久活化的分子伴侣，在生理条件下可与秀丽隐杆线虫体内数百种不同蛋白质发生相互作用。此外，我们解析的Hsp17冷冻电镜（cryo-EM）结构显示，在其24聚体复合物中，有12个N端区域参与了分子伴侣功能。这些柔性区域位于球形寡聚体的外侧，而其余12个N端区域则参与了内部的稳定相互作用。这使得Hsp17中的相同区域可根据拓扑环境的不同执行不同的功能。综上，本研究结果揭示了永久活化型sHsps的结构与功能特征，进一步明确了其结构基础。