Chaperonin 60 and the stability of polypeptides synthesized in isolated chloroplasts of Euglena gracilis at moderately elevated temperatures

Abstract

Summary Chloroplast function and continuity is affected when exponential cultures of Euglena gracilis Z are grown at the moderately elevated temperature of 33°C. Since protein instability could be a key factor in the response of chloroplasts to temperature, we carried out pulse-chase experiments with isolated chloroplasts. We have identified a polypeptide of 20.5 ku (kDa) that is degraded quite rapidly when isolated chloroplasts are pulsed at 33°C and then chased at either 23°C or at 33°C. By contrast, most polypeptides synthesized by isolated chloroplasts do not seem to be degraded during the chase period. We believe that the labile nature of the 20.5 ku is exacerbated at 33°C because the amounts of chaperonin 60 detected in Euglena chloroplasts appear to be low and probably insufficient for protection of newly synthesized proteins at elevated temperatures.