HEAT-SHOCK INHIBITS RELEASE OF THE SIGNAL RECOGNITION PARTICLE FROM THE ENDOPLASMIC-RETICULUM IN BARLEY ALEURONE LAYERS

When barley (Hordeum vulgare) aleurone layers are subjected to heat sh ock there is a selective degradation of the normally stable mRNAs enco ding secreted proteins, Messages for nonsecreted proteins are not degr aded. The synthesis of heat shock proteins is not required for this se lective message degradation. Our hypothesis explaining this phenomenon is that a component of the early steps in the synthesis of secreted p roteins is damaged by heat shock, resulting in a selective halt in tra nslation 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 r ecognition particle. We have obtained cDNA clones and antibodies for t he barley 54-kDa subunit of the signal recognition particle. In cell f ractionation experiments, more signal recognition particle was bound t o the endoplasmic reticulum membranes and less was in the free particl e fraction following a heat shock. The results suggest that heat shock inhibits the release of the signal recognition particle from the endo plasmic reticulum. This would, in turn, inhibit the resumption of tran slation and may be the underlying cause of the secretory message degra dation.