Heat Shock and Heat Shock Proteins

Heat Shock

In biochemistry, heat shock is the effect of subjecting a cell to a higher temperature than that of the ideal body temperature of the organism from which the cell line was derived. In fish that survive at 0°C, heat shock can be induced with temperatures as low as 5°C, whereas thermophilic bacteria that proliferate at 50°C will not express heat shock proteins until temperatures reach approximately 60°C. The process of heat shocking can be done in a CO2 incubator, O2 incubator, or a hot water bath.

Induction of heat shock is a method by which genes can be introduced into cells via a vector. This is done by mixing the vector with competent bacteria in a microcentrifuge tube. First, the tube is cooled to a low temperature for several minutes, usually with an ice bath. The tube is then quickly moved into warm water, preferably around 42°C. This sudden change in temperature causes the pores to open up to larger sizes, allowing DNA molecules to enter. After a brief interval, the tube is quickly cooled to a low temperature again. This closes up the pores, and traps the DNA inside. With this, the cells would have been transformed. However, as with almost all transformation techniques, this method is far from 100% efficient.

from Wikipedia (accessed on 10/28/2009)

Heat Shock Proteins

Heat shock proteins (HSPs) are present in cells under normal conditions (1), but are expressed at high levels when exposed to a sudden temperature jump or other stress (2).

(1) HSPs act like ‘chaperones,’ making sure that the cell’s proteins are in the right shape and in the right place at the right time. For example, HSPs help new or distorted proteins fold into shape, which is essential for their function. Heat shock proteins also shuttle proteins from one compartment to another inside the cell, and transport old proteins to ‘garbage disposals’ inside the cell. Heat shock proteins are also believed to play a role in the presentation of pieces of proteins (or peptides) on the cell surface to help the immune system recognize diseased cells. HSPs appear to serve a significant cardiovascular role.

Chaperones are proteins that assist the non-covalent folding/unfolding and the assembly/disassembly of other macromolecular structures, but do not occur in these structures when the latter are performing their normal biological functions. Chaperones do not necessarily convey steric information required for proteins to fold: thus statements of the form `chaperones fold proteins` can be misleading. One major function of chaperones is to prevent both newly synthesised polypeptide chains and assembled subunits from aggregating into nonfunctional structures. It is for this reason that many chaperones, but by no means all, are also heat shock proteins because the tendency to aggregate increases as proteins are denatured by stress. However, 'steric chaperones' directly assist in the folding of specific proteins by providing essential steric information.

Heat shock proteins trigger immune response through activities that occur both inside the cell (intracellular) and outside the cell (extracellular).

• Intracellular activities: Because of the normal functions of heat shock proteins inside the cell (such as helping proteins fold, preparing proteins for disposal, etc.), HSPs end up binding virtually every protein made within the cell. This means that at any given time, HSPs can be found inside the cell bound to a wide array of peptides that represent a ‘library’ of all the proteins inside the cell. This library contains normal peptides that are found in all cells as well as abnormal peptides that are only found in sick cells. Research suggests that inside the cell, heat shock proteins take the peptides and hand them over to another group of molecules. These other molecules take the abnormal peptides that are found only in sick cells and move them from inside the cell to outside on the cell’s surface. When the abnormal peptides are displayed in this way, they act as red flags, warning the immune system that the cell has become sick. These abnormal peptides are called antigens — a term that describes any substance capable of triggering an immune response.
• Extracellular activities: Heat shock proteins are normally found inside cells. When they are found outside the cell, it indicates that a cell has become so sick that it has died and spilled out all of its contents. This kind of messy, unplanned death is called necrosis, and only occurs when something is very wrong with the cell. Extracellular HSPs are one of the most powerful ways of sending a ‘danger signal’ to the immune system in order to generate a response that can help to get rid of an infection or disease.

(2) HSPs are a part of the cell's internal repair mechanism. They are also called stress-proteins. They respond to heat, cold and oxygen deprivation by activating several cascade pathways. In fact, HSPs stabilize proteins and are involved in the folding of denatured proteins. High temperatures and other stresses, such as altered pH and oxygen deprivation, make it more difficult for proteins to form their proper structures and cause some already structured proteins to unfold. Left uncorrected, mis-folded proteins form aggregates that may eventually kill the cell. HSPs are induced rapidly at high levels to deal with this problem. Increased expression of HSPs is mediated at multiple levels: mRNA synthesis, mRNA stability, and translation efficiency.

Heat shock factor 1 (HSF-1) is the major regulator of heat shock protein transcription in eukaryotes. In the absence of cellular stress, HSF-1 is inhibited by association with heat shock proteins and is therefore not active. Cellular stresses, such as increased temperature, can cause proteins in the cell to misfold. Heat shock proteins bind to the misfolded proteins and dissociate from HSF-1. This allows HSF1 to form trimers and translocate to the cell nucleus and activate transcription.

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