Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 212

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.

Another set of chaperones, including ClpA, ClpB, and ClpX, form cylinders and unfold misfolded proteins by sucking them through the cylinder. This takes energy, which is derived from cleavage of ATP. Some of them, including ClpA and ClpX, can also feed the unfolded proteins directly into an associated protease called ClpP, which degrades the unfolded protein. Association with ClpP switches the function of ClpA and ClpX from protein folding to protein degradation. ClpB, another cylindrical chaperone, does not associate with a protease but seems to cooperate with the small heat shock proteins IbpA and IbpB to help redissolve precipitated proteins so that they can be refolded by DnaK (see Mogk et al., Suggested Reading).