Temperature‐dependent binding to the thylakoid membranes of nuclear‐coded chloroplast heat‐shock proteins

Two nuclear‐coded heat‐shock proteins (HSP) of pea ( Pisum sativum ) are synthesized as larger precursors of 26 kDa and 30 kDa in vitro. They are transported post‐translationally into isolated, homologous chloroplasts where they are processed to mature proteins of 22 kDa and 25 kDa, respectively. When the chloroplasts used for the transport are isolated from control plants grown at 25°C the 22‐kDa and 25‐kDa HSPs are located in the stroma of the chloroplasts. However, when chloroplasts are prepared from heat‐shocked plants both proteins are found bound to the thylakoid membranes. The transition of the non‐binding to the binding status is comparatively sharp and occurs between 36°C and 40°C in the variety ‘Rosa Krone’. The transition temperature has been determined at 38°C for ‘Rosa Krone’ and at 40°C for the variety ‘Golf’. At 42°C, 15‐min treatment of the plants is sufficient to induce membrane binding, which persists for at least 4–6 h (but not for 24 h) after return to the ambient temperature. Once lost, membrane binding can be reinduced by a second heat‐shock treatment in vivo . High light intensities during the heat shock interfere with the binding capacity for heat‐shock proteins.