Snyder and Champness Molecular Genetics of Bacteria. Tina M. Henkin

Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin


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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).

“Schematic illustration of the 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 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.

      TRIGGER FACTOR AND OTHER CHAPERONES

      Given the prevalence and central role of DnaK in the cell, it came as a surprise that E. coli mutants that lack DnaK still multiply, albeit slowly. In fact, the only reason they are sick at all is because they are making too many copies of the other heat shock proteins, since DnaK also regulates the heat shock response (see chapter 12). One reason why cells lacking DnaK are not dead is that other chaperones can substitute for it. One of these is trigger factor. This type of chaperone has so far been found only in bacteria, and much less is known about it. It binds close to the exit pore of the ribosome and helps proteins fold as they emerge from the ribosome. It is also a prolyl isomerase. Of all the amino acids, only proline has an asymmetric carbon, which allows it to exist in two isomers. Trigger factor can convert the prolines in a protein from one isomer to the other. There are many other examples of chaperones that act as prolyl isomerases.

      CHAPERONINS


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