FUNCTION AND REGULATION OF THE HEAT-SHOCK RESPONSE 
Susan Lindquist, Ph.D., Investigator 
When cells of all types are exposed to mildly ele- 
vated temperatures, ethanol, anoxia, heavy metal 
ions, or a wide variety of other stresses, they re- 
spond by producing a small number of proteins, 
the heat-shock proteins. This response is one of 
the most highly conserved genetic regulatory sys- 
tems known. Dr. Lindquist's research is focused on 
1) use of the response as a model system to investi- 
gate mechanisms of genetic regulation, particularly 
post-transcriptional regulation, 2) investigation of 
the functions of the heat-shock proteins in protect- 
ing cells from the toxic effects of stress and in nor- 
mal growth and development, and 3) application of 
certain aspects of the response to other biological 
systems to solve a variety of practical problems. 
I. Regulation of Heat-Shock Protein Expression. 
The heat-shock response of Drosophila cells is 
particularly intense. Within minutes of a shift from 
25°C to 37°C, the entire pattern of protein synthe- 
sis in this organism is shifted from the production 
of normal cellular proteins to the production of 
heat-shock proteins. After heat shock the full pat- 
tern of normal protein synthesis is restored. In the 
past year the laboratory's studies in Drosophila 
have concentrated on the regulation of hsp70 syn- 
thesis. This protein is almost undetectable in cells 
growing at normal temperatures, but after heat 
shock it is the most abundantly synthesized pro- 
tein. The hsp70 message is very stable during heat 
shock but is rapidly degraded during recovery. This 
degradation appears to be highly specific and oc- 
curs while most other cellular messages are being 
reactivated for translation. Degradation is also 
highly regulated and only occurs after a specific 
quantity of protein— a quantity appropriate to the 
particular level of heat-stress applied to the cells— 
is produced. When the hsp70 message was ex- 
pressed at normal temperatures, from a heterolo- 
gous promoter, it was found to be very unstable. 
Thus heat shock inactivates a preexisting mecha- 
nism for degradation, and recovery restores it. To 
examine the mechanism of regulated hsp70 mes- 
sage degradation. Dr. Lindquist and her co-workers 
constructed and studied a variety of chimeric HSP70 
genes. It was determined that the 3 -untranslated 
region of the hsp70 message is sufficient to transfer 
regulated degradation to a heterologous message. 
The 3 -untranslated region shares sequence ele- 
ments with unstable messages in other systems, 
such as the c-myc and c-fos messages in mammalian 
cells. Moreover, the c-myc message is stabilized by 
heat shock in Drosophila cells. Repression of 
hsp70 may be accomplished through a highly con- 
served mechanism employed by other cells in other 
circumstances. 
II. Function of Heat-Shock Proteins. 
Studies on the function of heat-shock proteins 
have focused on the yeast Saccharomyces cerevi- 
siae, because of the ability to perform site-directed 
mutagenesis in this organism. Mutations that elimi- 
nate synthesis of hsp26 have no effect on the ability 
of cells to grow at high temperatures or to with- 
stand short exposures to extreme temperatures. 
The HSP83 gene exists in two copies. Disruption of 
either gene prevents cells from growing at high 
temperatures. Deletion of both genes is lethal. 
Thus this protein plays a vital role at all tempera- 
tures but is required by cells in higher concentra- 
tions for growth at higher temperatures. Cloning 
and sequencing of the HSP35 gene demonstrated 
that it encodes glyceraldehyde-3-phosphate dehy- 
drogenase. It may be induced at high temperatures 
to help restore normal ATP concentrations. Finally, 
and perhaps most importantly, hspl06 was found 
to be required for induced thermotolerance. Mu- 
tant hspl06 cells grow as well as wild-type cells at 
all temperatures and are killed at the same rate as 
wild-type cells when shifted directly from 25°C to 
50°C. However, when wild-type cells are given a 
mild pretreatment at 37°C, they become tolerant to 
exposure to 50°C. The mutants do not. 
III. Practical Applications of the Heat-Shock 
Response. 
Another interest of the laboratory has been the 
development of a heat-inducible site-directed re- 
combination system for Drosop/;^/^. Specifically, the 
site-specific flip (FLP) recombinase of the yeast 2|ul 
plasmid and its recombination targets (FRTs) were 
transferred into the genome of Drosophila mela- 
nogaster. Drosophila were independently trans- 
formed, using P-element vectors, with two con- 
structs: 1) an FLP gene under the control of hsplO 
regulatory sequences and 2) a white gene flanked 
by FRTs. When flies carrying both constructs were 
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