Heat shock induced by an increase in temperature from 28 to 37 degrees C led to changes in synthesis and phosphorylation of cytoplasmic and nuclear proteins in the aquatic fungus Achlya. In the cytoplasmic fraction a marked increase in [35S]methionine labelling of proteins in the molecular weight range of 96 000, 85 000, 74 000, and 70 000 was observed. Two-dimensional electrophoresis resolved each of these classes of proteins into several components. Major changes in the nuclear fraction included the increased [35S]methionine labelling of 43 000 and 28 000-23 000 proteins. A marked decline in the synthesis of many other proteins was also evident. The heat-shock-induced changes in labelling patterns became evident as early as 20 to 60 min after treatment, but they were transient. With continued incubation at the heat-shock temperature, the cells appeared to adapt to the new temperature conditions. Both cytoplasmic and nuclear proteins returned to nearly normal labelling patterns within 100 to 140 min at 37 degrees C. Changes in phosphorylation of histone and nonhistone nuclear proteins were also noted. Achlya histone H3 and the putative oomycete-specific histone "alpha" appeared highly phosphorylated after heat shock. Since phosphorylation of histone H3 is primarily associated with chromatin condensation, it is possible that rapid chromatin condensation is an initial response to heat shock in Achlya.

Heat shock led to marked changes in the apparent levels of phosphorylation of nuclear proteins in the fungus Achlya ambisexualis. We characterized these heat shock-induced changes in nuclear proteins on two types of two-dimensional polyacrylamide gel systems. We report here that one of two Achlya H3 histones (H3.1) and also the oomycete histone a appear to be highly phosphorylated with heat shock. Additional changes observed in acid-soluble nuclear proteins included an apparent increase in the 32p labeling of a 43,000-molecular-weight protein and the dephosphorylation of a major group of Achlya phosphoproteins in the 30,000-to-32,000-molecular-weight range. The changes in protein phosphorylation were accompanied by striking changes in the morphology of Achlya nuclei. Nuclei in the heat-shocked cells, but not in control cells, exhibited marked chromatin condensation and contained bundles of filaments which were approximately 4 nm in diameter. Concomitantly, the bulk of chromatin from heat-shocked nuclei showed a decreased sensitivity to digestion with the enzyme DNase I relative to chromatin from control cells.We have previously reported that in the aquatic fungus Achlya ambisexualis at least 10 molecular weight classes of proteins (96,000 [96K], 85K, 70K to 74K, 43K, 28K, 27K, 24K, 23K, and 18K) are synthesized within 20 to 60 min after a rapid elevation of temperature (heat shock) from 28 to 37°C (29,30). This heat shock response was accompanied by changes in the levels of phosphorylation of several nuclear proteins, including acid-insoluble proteins, as well as the apparent marked phosphorylation of two Achlya histones.In higher eucaryotes and also in the slime mold Physarum polycephalum, the phosphorylation of histone H3 and histone Hi is associated with the condensation of chromatin during mitosis (3,7,13,14). The apparent phosphorylation of Achlya histones during heat shock was therefore of particular interest to us, since Achlya nuclei normally contain no discernible heterochromatin and Achlya chromosomes like those of the related water mold Saprolegnia ferax (15) are not thought to condense even when cells are undergoing mitosis.We therefore undertook a study of the possible heat shock-induced morphological changes in Achlya nuclei. We also characterized further the heat shock-induced changes in 32p labeling of Achlya nuclear proteins. We report here that both a subtype of Achlya histone H3 and also the oomycete histone a appear to be highly phosphorylated with heat shock. This is accompanied by striking changes in the morphology of Achlya nuclei. Although control nuclei contained no discernible heterochromatin, nuclei in the heatshocked cells exhibited marked chromatin condensation. In addition, the heat-shocked nuclei contained bundles of parallel-oriented filaments which were not observed in the control nuclei. Consistent with the observed condensation of chromatin, the bulk of chromatin from heat-shocked nuclei showed a decreased sensitivity to digestion with the enzyme DNase I relative to chromatin f...

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