Messenger RNA-Binding Properties of Nonpolysomal Ribonucleoproteins from Heat-Stressed Tomato Cells

Most cells experiencing heat stress reprogram their translational machinery to favor the synthesis of heat-stress proteins. Translation of other transcripts is almost completely repressed, but most untranslated messengers are not degraded. In contrast to yeast, Drosophila melanogaster, and HeLa cells, plant cells store repressed messengers in cytoplasmic nonpolysomal ribonucleoproteins (RNPs). To follow the fate of untranslated transcripts, we studied protein composition, mRNA content, and RNA-binding properties of nonpolysomal RNPs from heat-stressed tomato (Lycopersicon peruvianum) cells. Contrary to the selective interaction in vivo, RNPs isolated from tomato cells bound both stress-induced and repressed messengers, suggesting that the selection mechanism resides elsewhere. This binding was independent of a cap or a poly(A) tail. The possible role of proteasomes and heat-stress granules (HSGs) in mRNA storage is a topic of debate. We found in vitro messenger-RNA-binding activity in messenger RNP fractions free of C2-subunit-containing proteasomes and HSGs. In addition, mRNAs introduced into tobacco (Nicotiana plumbaginifolia) protoplasts were found in the cytoplasm but were not associated with HSGs.