Synthesis, transport and localization of a nuclear coded 22-kd heat-shock protein in the chloroplast membranes of peas and Chlamydomonas reinhardi

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Abstract
The synthesis, transport and localization of a nuclear coded 22‐kd heat‐shock protein (HSP) in the chloroplast membranes was studied in pea plants and Chlamydomonas reinhardi. HSPs were detected in both systems by in vivo labeling and in vitro translation of poly(A)+RNA, using the wheat‐germ and reticulocyte lysate systems. Heat‐shock treatment of pea plants for 2 h at 42‐45°C induces the expression of ˜10 nuclear coded proteins, among which several (18 kd, 19 kd, 22 kd) are predominant. A 22‐kd protein is synthesized as a 26‐kd precursor protein and is localized in a chloroplast membrane fraction in vivo. Following post‐translational transport into intact chloroplasts in vitro of the 26‐kd precursor, the protein is processed but the resulting 22‐kd mature protein is localized in the chloroplast stroma. If, however, the in vitro transport is carried out with chloroplasts from heat‐shocked plants, the 22‐kd protein is preferentially transported to the chloroplast membrane fraction. In C. reinhardi the synthesis of poly(A)+RNAs coding for several HSPs is progressively and sequentially induced when raising the temperature for 1.5 h from 36°C to 42°C, while that of several preexisting RNAs is reduced. Various pre‐existing poly(A)+RNAs endure in the cells at 42°C up to 5 h but are no longer translated in vivo, whereas some poly(A)−RNAs persist and are translated. As in pea, a poly(A)+RNA coded 22‐kd HSP is localized in the chloroplast membranes in vivo, although it is translated as a 22‐kd protein in vitro. The in vitro translated protein is not transported in isolated pea chloroplast which, however, processes and transports other nuclear coded chloroplast proteins of Chlamydomonas. The poly(A)+RNA coding for the 22‐kd HSP appears after 1 h at 36°C. Its synthesis increases with the temperature of incubation up to 42°C, although it decreases after ˜2 h of heat treatment and the already synthesized RNA is rapidly degraded. The degradation is faster upon return of the cells to 26°C. None of the heat‐induced proteins is identical to the light‐inducible proteins of the chloroplast membranes.