Cytoplasmic Distribution of Heat Shock Proteins in Soybean

Previous analyses of the distribution of heat shock (hs) proteins in soybean (Glycine max L. Merr., var Wayne) have demonstrated that a fraction of the low molecular weight hs protein associates with ribosomes during hs. To more specifically characterize the nature of this association, isokinetic centrifugation of ribosomes through sucrose gradients was used to separate monosomes from polysomes. The present analysis demonstrated that hs proteins were bound to polysomes but not monosomes. Treatment of polysomes with puromycin, K(+), and Mg(2+), which caused dissociation of ribosomes into 40S and 60S subunits, also caused dissociation of the hs proteins. Using the procedure of Nover et al. (1983, Mol. Cell Biol, 3: 1628-1655), a hs granule fraction was also isolated. As in tomato cells, hs granules from soybean seedlings contained the low molecular weight hs proteins as a primary component and a number of other non-hs proteins of relative molecular mass 30 to 40 kilodaltons and 70 to 90 kilodaltons. On metrizamide gradients they exhibited a buoyant density of 1.20 to 1.21 grams per cubic centimeter, typical of ribonucleoprotein particles. Heat shock granules were characterized as unique cytoplasmic particles based on protein composition and buoyant density. Isopycnic centrifugation of ribosome preparations demonstrated that they contained hs granules, but the hs proteins bound to polysomes were not released by KCI/EDTA treatment. Thus, the polysome-bound hs proteins and the granule-bound hs proteins appear to represent two distinct populations of hs proteins in the cytoplasm. Heat shock granules were not distinguishable from ribosomes at the level of resolution used in transmission electron microscopy.