REVERSIBLE MEMBRANE ASSOCIATION OF HEAT-SHOCK PROTEIN-22 IN CHLAMYDOMONAS-REINHARDTII DURING HEAT-SHOCK AND RECOVERY

The process of reversible membrane association of the nuclear-encoded heat-shock protein hsp22 in Chlamydomonas reinhardtii cells during rec overy from heat stress has been investigated. hsp22 associates with a chloroplast membrane-enriched fraction, dissociates from the membranes during recovery from heat shock and rebinds during a subsequent heat- shock treatment in vivo. The protein remains in the cell soluble fract ion for at least 22 h after heat-stress treatment. Dissociation of mem brane-bound hsp22 occurs only at 25-38 degrees C and reassociation occ urs only at the hsp22 induction temperature (38-42 degrees C). Hsp22 d issociation from the membrane fraction is not related to de novo prote in synthesis in vivo and does not occur in vitro. Based on the derived amino acid sequence, hsp22 is not considered a typical chloroplast-as sociated heat-shock protein [Vierling, E. (1991) Annu. Rev. Plant Phys iol. Plant Mel. Biol. 42, 579-620] and may be associated with the chlo roplast envelope membrane. However, the reversible association of hsp2 2 with the chloroplast-enriched membrane fraction indicates similar pr operties to those of pea low-molecular-mass heat-shock proteins [Glacz inski, H. and Kloppstech, K. (1988) Eur. J. Biochem. 173, 579-583] and may be related the transient response of the chloroplast to heat stre ss.