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 degrees C up to 40 degrees C for 3 h, a novel 22-kD protein is produced and accumulates in the matrix com partment of green leaf mitochondria. HSP22 was purified and used as an tigen to prepare guinea pig antiserum. The expression of HSP22 was stu died using immunodetection methods. HSP22 is a nuclear-encoded protein de novo synthesized in heat-stressed pea plants. The heat-shock respo nse is rapid and can be detected as early as 30 min after the temperat ure is raised. On the other hand, HSP22 declines very slowly after pea leaves have been transferred back to 25 degrees C. After 100 h at 25 degrees C, the heat-shock pattern was undetectable. The precise locali zation of HSP22 was investigated and we demonstrated that HSP22 was fo und only in mitochondria, where it represents 1 to 2% of total matrix proteins. However, the induction of HSP22 does not seem to be tissue s pecific, since the protein was detected in green or etiolated pea leav es as well as in pea roots. Finally, examination of matrix extracts by nondenaturing polyacrylamide gel electrophoresis and immunoblotting w ith anti-HSP22 serum revealed a high-molecular mass heat-shock protein complex of 230 kD, which contains HSP22.