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.