Heat Shock Inhibits Release of the Signal Recognition Particle from the Endoplasmic Reticulum in Barley Aleurone Layers

Abstract

When barley (Hordeum vulgare) aleurone layers are subjected to heat shock there is a selective degradation of the normally stable mRNAs encoding secreted proteins. Messages for nonsecreted proteins are not degraded. The synthesis of heat shock proteins is not required for this selective message degradation. Our hypothesis explaining this phenomenon is that a component of the early steps in the synthesis of secreted proteins is damaged by heat shock, resulting in a selective halt in translation on secretory mRNAs, which may in turn lead to degradation of those messages. The first committed step in the synthesis of secreted proteins is the binding of the nascent signal sequence to the signal recognition particle. We have obtained cDNA clones and antibodies for the barley 54-kDa subunit of the signal recognition particle. In cell fractionation experiments, more signal recognition particle was bound to the endoplasmic reticulum membranes and less was in the free particle fraction following a heat shock. The results suggest that heat shock inhibits the release of the signal recognition particle from the endoplasmic reticulum. This would, in turn, inhibit the resumption of translation and may be the underlying cause of the secretory message degradation.When organisms are subjected to the stress of high temperature, synthesis of a set of new proteins, the heat shock proteins (hsps), 1 is induced (for reviews see Refs. 1-4). In some organisms, notably Drosophila, heat shock also suppresses the synthesis of normal cellular proteins (5, 6). The selective translation of hsps which is seen in Drosophila, however, is not a general feature of the heat shock response in plants (7), although it has been reported in soybean and tomato (3).In our studies on barley aleurone layers we have observed another response to heat stress which may have a profound effect on cellular metabolism. The aleurone layer of cereal grains is a specialized tissue whose main function in vivo is the secretion of hydrolytic enzymes during seed germination. In the barley aleurone layer system, heat shock has a specific effect on the mRNA levels for secreted proteins. In response to heat shock the normally stable message for ␣-amylase is rapidly degraded (8). At 25°C the half-life of the ␣-amylase mRNA was estimated to be over 100 h (9), yet within 30 min at 40°C over 60% of it was degraded (10). The same phenomenon has been observed for the mRNA levels for the secreted proteins endochitinase and protease, whereas the mRNA levels of the nonsecreted proteins actin and tubulin were not affected by heat shock (10). Through the use of RNA synthesis inhibitors we have determined that synthesis of the hsps is not required for this selective message degradation (11).To better understand this phenomenon we are focusing on those features of protein synthesis unique to secreted proteins. A feature common to most secreted proteins that distinguishes them from cytoplasmically localized proteins is the presence of a signal sequence. The signal sequence at the amino te...

Translational Regulation of Cytoplasmic mRNAs

“…In heat-shocked barley aleurone cells, for example, mRNAs for secreted proteins appear to be affected preferentially. Translation of these mRNAs stalls after synthesis of the signal peptide and targeting to the ER membrane (Chu et al, 1997). In species where the translational block by heat shock is more global, it is the heat shock protein mRNAs that remain polysomal, due to sequence elements that may reside in the 5' UTR (Dinkova et al, 2005).…”

A heat shock transcription factor in pea is differentially controlled by heat and virus replication

“…In this case, the degradation of the mRNAs has been linked to the destruction of the endoplasmic reticulum (ER) and the disorganization of the synthesizing machinery for secreted proteins (Belanger et al . 1986; Chu et al . 1997).…”

Characterization of genes expressed in Casuarina equisetifolia in response to elicitation by cell wall components of Trichosporium vesiculosum

“…SRPs are also associated with degradation of mis-targeted cytoplasmic membrane proteins. SRP mediated degradation of secretary proteins like amylase and defense-related proteins like endochitinase and proteases were demonstrated in barley (BRODL and HO, 1991) and heat shock inhibited the release of SRP from ER in aleurone layers of barley (CHU et al, 1997). Direct evidence on role of SRP during pathogen defense is not reported to our knowledge.…”