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The Hsp70 family of chaperones is the most prevalent and ubiquitous type of general chaperone, existing in all types of cells with the possible exception of some archaea (see Bukau and Horwich, Suggested Reading). These chaperones are highly conserved evolutionarily. Chaperones in this family are called the Hsp70 proteins because they are about 70 kDa in size and because more of them are made (along with many other proteins) if cells are subjected to a sudden increase in temperature, or “heat shock” (see chapter 12); other stresses that denature proteins (such as ethanol) can have the same effect. Synthesis of chaperones increases after such stresses to help refold proteins that have been denatured by the environmental stress, although they also help to fold proteins under normal conditions. The Hsp70 type of chaperone was first discovered in E. coli, where it was given the name DnaK because it is required to assemble the DNA replication apparatus of phage λ and so is required for λ DNA replication. This name for the Hsp70 chaperone in E. coli is still widely used in spite of being a misnomer, because the chaperone has nothing directly to do with DNA but functions more generally in protein folding. In its role as a heat shock protein, the DnaK protein of E. coli also functions as a cellular thermometer, regulating the synthesis of other proteins in response to heat shock (see chapter 12).

To understand how Hsp70 chaperones, including DnaK, help fold proteins, it is necessary to understand something about the structure of most proteins. Proteins are made up of chains of amino acids that are folded up into well-defined structures, which are often rounded or globular. The amino acids that make up proteins can be charged, polar, or hydrophobic (see the inside front cover for a list). Amino acids that are charged (either acidic or basic) or polar tend to be more soluble in water and are called hydrophilic (water loving). Amino acids that are not charged or polar are hydrophobic (water fearing) and tend to be in the inside of the globular protein among other hydrophobic amino acids and away from the water on the surface. If the hydrophobic amino acids are exposed, they tend to associate with hydrophobic amino acids on other proteins and cause the proteins to precipitate. This is essentially what happens when you cook an egg. High temperatures cause the proteins in the egg to unfold, exposing their hydrophobic regions, which then associate with each other, causing the proteins to precipitate into a solid white mass.

Figure 2.36 Polarity in transcription of a polycistronic mRNA transcribed from pYZ. (A) The rut site in gene Y is normally masked by ribosomes translating the gene Y mRNA. (B) If translation is blocked in gene Y by a mutation that changes the codon CAG to UAG (boxed in red), the ρ factor can bind to the mRNA and cause transcription termination before the RNA polymerase reaches gene Z. (C) Only fragments of the gene Y protein and mRNA are produced, and gene Z is not even transcribed into mRNA.

The Hsp70-type chaperones help proteins fold by binding to the hydrophobic regions in denatured proteins and nascent proteins as they emerge from the ribosome and keeping these regions from binding to each other prematurely as the protein folds. The Hsp70 proteins have an ATPase activity that, by cleaving bound ATP to ADP, helps the chaperone to sequentially bind to, and dissociate from, the hydrophobic regions of the protein they are helping to fold. The Hsp70-type chaperones are directed in their protein-folding role by smaller proteins called cochaperones. The major cochaperones in E. coli were named DnaJ and GrpE, again for historical reasons. The DnaJ cochaperone helps DnaK to recognize some proteins and to cycle on and off of the proteins by regulating its ATPase activity. It can also sometimes function as a chaperone by itself. The GrpE protein is a nucleotide exchange protein that helps regenerate the ATP-bound form of DnaK from the ADP-bound form, allowing the cycle to continue.