A Low Molecular Mass Heat-Shock Protein Is Localized to Higher Plant Mitochondria

When pea (Pisum sativum L. var Douce Provence) plants are shifted from a normal growth temperature of 25°C up to 40°C for 3 h, a novel 22-kD protein is produced and accumulates in the matrix compartment of green leaf mitochondria. HSP22 was purified and used as antigen to prepare guinea pig antiserum. The expression of HSP22 was studied using immunodetection methods. HSP22 is a nuclear-encoded protein de novo synthesized in heat-stressed pea plants. The heat-shock response is rapid and can be detected as early as 30 min after the temperature is raised. On the other hand, HSP22 declines very slowly after pea leaves have been transferred back to 25°C. After 100 h at 25°C, the heat-shock pattern was undetectable. The precise localization of HSP22 was investigated and we demonstrated that HSP22 was found only in mitochondria, where it represents 1 to 2% of total matrix proteins. However, the induction of HSP22 does not seem to be tissue specific, since the protein was detected in green or etiolated pea leaves as well as in pea roots. Finally, examination of matrix extracts by nondenaturing polyacrylamide gel electrophoresis and immunoblotting with anti-HSP22 serum revealed a high-molecular mass heat-shock protein complex of 230 kD, which contains HSP22.