Cytoplasmic Heat Shock Granules Are Formed from Precursor Particles and Are Associated with a Specific Set of mRNAs

Abstract

In heat-shocked tomato cell cultures, cytoplasmic heat shock granules (HSGs) are tightly associated with a specific subset of mRNAs coding mainly for the untranslated control proteins. This messenger ribonucleoprotein complex was banded in a CsCl gradient after fixation with formaldehyde (~1.30 g/cm³). It contains all the heat shock proteins and most of the RNA applied to the gradient. During heat shock, a reversible aggregation of HSGs from 15S precursor particles can be shown. These pre-HSGs are not identical to the 19S plant prosomes. Ultrastructural analysis supports the ribonucleoprotein nature of HSGs and their composition of ~10-nm precursor particles. A model summarizes our results. It gives a reasonable explanation for the striking conservation of untranslated mRNAs during heat shock and may apply also to animal cells.