Frances W. Farrelly

hsp82 is one of the most highly conserved and abundantly synthesized heat shock proteins of eucaryotic cells. The yeast Saccharomyces cerevisiae contains two closely related genes in the HSP82 gene family. HSC82 was expressed constitutively at a very high level and was moderately induced by high temperatures. HSP82 was expressed constitutively at a much lower level and was more strongly induced by heat. Site-directed disruption mutations were produced in both genes. Cells homozygous for both mutations did not grow at any temperature. Cells carrying other combinations of the HSP82 and HSC82 mutations grew well at 25 degrees C, but their ability to grow at higher temperatures varied with gene copy number. Thus, HSP82 and HSC82 constitute an essential gene family in yeast cells. Although the two proteins had different patterns of expression, they appeared to have equivalent functions; growth at higher temperatures required higher concentrations of either protein. Biochemical analysis of hsp82 from vertebrate cells suggests that the protein binds to a variety of other cellular proteins, keeping them inactive until they have reached their proper intracellular location or have received the proper activation signal. We speculate that the reason cells require higher concentrations of hsp82 or hsc82 for growth at higher temperatures is to maintain proper levels of complex formation with these other proteins.

hsp82 is one of the most highly conserved and abundantly synthesized heat shock proteins of eucaryotic cells. The yeast Saccharomyces cerevisiae contains two closely related genes in the HSP82 gene family. HSC82 was expressed constitutively at a very high level and was moderately induced by high temperatures. HSP82 was expressed constitutively at a much lower level and was more strongly induced by heat. Site-directed disruption mutations were produced in both genes. Cells homozygous for both mutations did not grow at any temperature. Cells carrying other combinations of the HSP82 and HSC82 mutations grew well at 25 degrees C, but their ability to grow at higher temperatures varied with gene copy number. Thus, HSP82 and HSC82 constitute an essential gene family in yeast cells. Although the two proteins had different patterns of expression, they appeared to have equivalent functions; growth at higher temperatures required higher concentrations of either protein. Biochemical analysis of hsp82 from vertebrate cells suggests that the protein binds to a variety of other cellular proteins, keeping them inactive until they have reached their proper intracellular location or have received the proper activation signal. We speculate that the reason cells require higher concentrations of hsp82 or hsc82 for growth at higher temperatures is to maintain proper levels of complex formation with these other proteins.

We have sequenced the yeast HSP90 gene, which encodes the (apparent) Mr = 90,000 heat shock-inducible protein of this organism. All sequences required for the heat shock-regulated expression of this gene reside on a segment of DNA containing no more than 273 nucleotides 5' to the transcription origin. Transcript mapping reveals that the mature hsp90 mRNA contains a 59-nucleotide 5' untranslated segment, a coding segment of 2130 nucleotides (sufficient to encode a protein of Mr = 81,419), and a 3' untranslated segment of 128 nucleotides. Although the sequences surrounding the coding region of the HSP90 gene are quite similar to those surrounding other sequenced yeast genes, we find as well a limited homology between sequences located 5' to this gene and the putative heat shock-regulatory sequences located 5' to the heat shock-inducible genes of Drosophila.