PROTEIN-SYNTHESIS AND BREAKDOWN DURING HEAT-SHOCK OF CULTURED PEAR (PYRUS-COMMUNIS L) CELLS

Cultured pear (Pyrus communis L. cv Passe Crassane) cells were subject ed to temperatures of 39, 42, and 45 degrees C. Heat-shock protein (hs p) synthesis was greater at 39 degrees C than at temperatures above 40 degrees C and continued for up to 8 h. Both cellular uptake of radiol abeled methionine and total protein synthesis were progressively lower as the temperature was increased. Polysome levels decreased immediate ly when cells were placed at 39 or 42 degrees C, although at 39 degree s C the levels began to recover after 1 h. In cells from both temperat ures, reassembly occurred after transfer of cells to 25 degrees C. Fou r heat-shock-related mRNAs-hsp17, hsp70, and those of two ubiquitin ge nes-all showed greatest abundance at 39 degrees C and decreased at hig her temperatures. Protein degradation increased with time at 42 and 45 degrees C, but at 39 degrees C it increased for the first 2 h and the n decreased. In the presence of cycloheximide, which prevented hsp syn thesis, protein degradation at 39 degrees C was as great as that at 45 degrees C in the absence of cycloheximide. The data suggest that hsps may have a role in protecting proteins from degradation at the permis sive temperature of 39 degrees C. At temperatures high enough to inhib it hsp synthesis, protein degradation was enhanced. Although ubiquitin may play a role in specific protein degradation, it does not appear t o be involved in increased protein degradation occurring above 40 degr ees C.