Heat shock temperature acclimation of normal secretory protein synthesis in barley aleurone cells

Abstract

ABSTRACTExposure of barley (Hordeum vulgare L. cv. Himalaya) aleurone layers to 40°C for a period of 3 h results in the selective suppression of the synthesis and secretion of hydrolytic enzymes; other normal cellular protein synthesis continues during heat shock. This suppression is correlated with secretory protein mRNA destabilization and the dissociation of stacked ER lamellae during heat shock (Belanger et al. 1986, Proceedings of the National Academy of Sciences USA 83, pp. 1354–1358). In this report we examined the effect of exposure to extended periods of heat shock. If exposure to 40°C was continued for a period of 18 h, the synthesis of α‐amylase, the predominant secreted hydrolase, resumed. This was accompanied by increased α‐amylase mRNA levels and the reformation of ER lamellae. Though initial exposure (3 h) to 40°C reduced protein secretion to ∼10% of that observed in aleurone cells maintained at 25°C, exposure for prolonged periods (16–20 h) permitted the resumption of protein secretion to ∼66% of non‐heat‐shocked control levels. The resumption of normal secretory protein synthesis during prolonged exposure to 40°C was correlated with an increase in the incorporation of [14C]glycerol into phosphatidylcholine and an increase in the ratio of saturated to unsaturated fatty acids in lipids isolated from ER membrane preparations. Increased fatty acid saturation has been demonstrated to enhance thermostability in biological membranes, and such changes in membrane composition may be important to the recovery of secretory protein synthesis at the ER.