| When the temperature is increased from 28(DEGREES)C to 40(DEGREES)C, a set of "heat shock proteins" (hsp) is induced in maize. This response is presumed to be homeostatic, although the precise function of the proteins is unknown. This work investigates the regulation of hsp synthesis and potential physiological roles of the hsp by determining their subcellular location and their relationship to the establishment of thermotolerance.;A correlative relationship exists between the induction of hsp and the acquisition of thermotolerance. Pretreatment at 40(DEGREES)C affords protection against an otherwise lethal 48(DEGREES)C challenge. The amount of protection is proportional to the length of the pretreatment.;Hsp were subcellularly localized by separating organelles on linear sucrose density gradients and analyzing collected fractions for enrichment in hsp by SDS polyacrylamide gel electrophoresis. Hsp79-83 are soluble, while hsp29 is mitochondria-associated. Hsp25 and 72 are associated with the endoplasmic reticulum, and hsp18 and 70 are plasma membrane-associated. The specificity of the association of hsp18, 25, 70, and 72 to particular membranes was demonstrated by means of the "Mg('+2) shift" technique. The buoyant density of the endoplasmic reticulum can be altered by the retention or removal of ribosomes from the membranes, causing a corresponding change in the equilibrium position of the endoplasmic reticulum in a sucrose gradient. The position of hsp25 and 72 is found to shift with the position of the endoplasmic reticulum. The positions of hsp18 and 70 remain with the plasma membrane.;Heat shock strongly perturbs the plasma membrane-associated process of H('+) efflux/K('+) influx, but normal ion movement is restored even if heat shock is prolonged. The association of hsp18 and 70 with plasma membrane suggests their involvement in the repair of this and possibly other thermally-damaged, membrane-associated processes.;Increases in hsp synthesis result from increased levels of mRNA coding for the hsp. Hsp synthesis declines if the tissue is returned to 28(DEGREES)C or if 40(DEGREES)C treatment extends beyond four hours. The hsp undergo no major post-translational modifications. The hsp are stable, but do not accumulate to high levels. All tissues except pollen, the most heat-sensitive part of the plant, synthesize hsp. |
