The chaperone-like activity of a small heat shock protein is lost after sulfoxidation of conserved methionines in a surface-exposed amphipathic alpha-helix

The small heat shock proteins (sHsps) possess a chaperone-like activity whi ch prevents aggregation of other proteins during transient heat or oxidativ e stress. The sHsps bind, onto their surface, molten globule forms of other proteins, thereby keeping them in a refolding competent state. In Hsp21, a . chloroplast-located sHsp in all higher plants, there is a highly conserve d region forming an amphipathic alpha -helix with several methionines on th e hydrophobic side according to secondary structure prediction. This paper describes how sulfoxidation of the methionines in this amphipathic alpha -h elix caused conformational changes and a reduction in the Hsp21 oligomer si ze, and a complete loss of the chaperone-like activity. Concomitantly, ther e was a loss of an outer-surface located alpha -helix as determined by limi ted proteolysis and circular dichroism spectroscopy. The present data indic ate that the methionine-rich amphipathic alpha -helix, a motif of unknown p hysiological significance which evolved during the land plant evolution, is crucial for binding of substrate proteins and has rendered the chaperone-l ike activity of Hsp21 very dependent on the chloroplast redox state. (C) 20 01 Elsevier Science B.V. All rights reserved.