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Plant and Cell Physiology Advance Access originally published online on November 21, 2006
Plant and Cell Physiology 2007 48(1):31-41; doi:10.1093/pcp/pcl037
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© The Author 2006. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Heat Shock-Induced Changes in Lipid and Protein Metabolism in the Endoplasmic Reticulum of Barley Aleurone Layers

Mark K. Johnston1, Nitya P. Jacob1,3 and Mark R. Brodl2,*

1Department of Biology, Knox College, Galesburg, IL 61401, USA
2Department of Biology, Trinity University, San Antonio, TX 78212, USA

*Corresponding author: E-mail, mbrodl{at}trinity.edu; Fax, +1-210-999-7246.


  Abstract

Heat shock in barley aleurone layers induces heat shock protein synthesis and suppresses secretory protein synthesis by selectively destabilizing their mRNAs. In addition, the endoplasmic reticulum (ER) membranes upon which secretory protein mRNAs are translated become vesiculated during heat shock, leading to the hypothesis that ER dissociation and targeted mRNA destabilization are linked mechanistically. Supporting this, ER can be heat adapted, and heat-adapted ER has higher levels of fatty acid saturation in membrane phospholipids which do not vesiculate upon heat shock. Secretory protein mRNAs are also more stable in heat-adapted cells. To understand better heat shock-induced changes in ER membranes, we examined ER membrane proteins and enzymes involved in phosphatidylcholine biosynthesis and phospholipid turnover in heat-shocked aleurone cells. Heat shock significantly increased the activity of phospholipases A2 and D, and shortly thereafter significant but gradual increases in choline kinase and phosphocholine glyceride transferase activities and a sharp increase in phosphorylcholine citidyl transferase activity were observed. Only minor changes were observed in SDS–PAGE analyses of proteins from sonicated ER membranes fractionated on continuous sucrose gradients. Overall, heat shock reduced total lipid in ER membranes relative to protein, and in intact, ultracentrifuged aleurone cells examined by light and electron microscopy the ER band appeared to increase in density. The changes in phospholipid metabolism coupled with the suppression of secretory protein synthesis indicate that in addition to inducing a classic heat shock response, high temperature also induces a classic unfolded protein response in the ER of this secretory cell.

Keywords: Barley aleurone layers - Endoplasmic reticulum - ER integral membrane proteins - Heat shock - Phosphatidylcholine metabolism - Unfolded protein response

Abbreviations: CK, choline kinase; ER, endoplasmic reticulum; FFA, free fatty acid; GA, gibberellic acid; HSF, heat shock factor; HSP, heat shock protein; NEN, New England Nuclear; PC, phosphatidylcholine; PLA2, phospholipase A2; PLC, phospholipase C; PLD, phospholipase D; PCT, phosphorylcholine citidyl transferase; PGT, phosphorylcholine glyceride transferase; PL, phospholipid; SRP, signal recognition particle; TCA, trichloroacetic acid; UPR, unfolded protein response

3 Present address: Division of Natural Science and Mathematics, Oxford College of Emory University, Oxford, GA 30054, USA.

(Received August 29, 2006; Accepted November 12, 2006)
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