Cecilia Emanuelsson - The chloroplast-localized Hsp21 – structure and interactions

The small heat shock protein (sHsp) chaperones can prevent aggregation of other proteins. sHsps may be among the most highly upregulated proteins in heat-stressed cells and in plants the sHsps confer an especially pronounced and important part of the stress response. The sHsps rapidly sequester destabilized client proteins, thereby overcoming the kinetic competition with aggregation. The sHsps typically act on early unfolding intermediates and capture unfolded conformations present for only a small fraction of the time. There are three structurally and functionally distinct regions in sHsps: the α-crystallin domain (ACD) that defines the family of sHsps, the C-terminal region (CTR), with a conserved I/V-X-I/V motif and the N-terminal region (NTR), which differs in length and composition between the different sHsps. The chloroplast-localized sHsp, referred to as Hsp21, evolved when the land-plants developed in response to the selection pressure in a non-water environment. In Hsp21 the NTR contains a unique set of methionines in an amphipathic α-helix-motif. Hsp21 plays a crucial role in resistance to heat and oxidative stress in Arabidopsis thaliana, during which the Hsp21 dodecamer undergoes a conformational change coupled to methionine oxidation. Some Hsp21 may also translocate into the thylakoid membranes during plant heat stress. We have recently obtained a structural model of Hsp21, suggesting that the CTR stabilizes the dodecamer while the NTR can interact with client proteins on the outside of the dodecamer, and we have developed an approach with crosslinking mass spectrometry to investigate the interactions between Hsp21 and client proteins.