Chaperone-client interactions between Hsp21 and client proteins monitored in solution by small angle X-ray scattering and captured by crosslinking mass spectrometry.

The small heat shock protein (sHsp) chaperones are important for stress survival, yet the molecular details of how they interact with client proteins are not understood. All sHsps share a folded middle domain to which is appended flexible N- and C-terminal regions varying in length and sequence between different sHsps which, in different ways for different sHsps, mediate recognition of client proteins. In plants there is a chloroplast-localized sHsp, Hsp21, and a structural model suggests that Hsp21 has a dodecameric arrangement with six N-terminal arms located on the outside of the dodecamer and six inwardly-facing. Here, we investigated the interactions between Hsp21 and thermosensitive model substrate client proteins in solution, by small-angle X-ray scattering (SAXS) and crosslinking mass spectrometry. The chaperone-client complexes were monitored and the Rg -values were found to increase continuously during 20 min at 45°, which could reflect binding of partially unfolded clients to the flexible N-terminal arms of the Hsp21 dodecamer. No such increase in Rg -values was observed with a mutational variant of Hsp21, which is mainly dimeric and has reduced chaperone activity. Crosslinking data suggest that the chaperone-client interactions involve the N-terminal region in Hsp21 and only certain parts in the client proteins. These parts are peripheral structural elements presumably the first to unfold under destabilizing conditions. We propose that the flexible and hydrophobic N-terminal arms of Hsp21 can trap and refold early-unfolding intermediates with or without dodecamer dissociation.