Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 213

In addition to the relatively simple protein chaperones, cells contain much larger structures that help proteins fold. These large structures are called chaperonins, and they exist in all forms of life, including the archaea and eukaryotes. They are composed of two large cylinders with hollow chambers held together back to back with openings at their ends (Figure 2.37). They help fold a misfolded protein by taking it up into one of the chambers. A cap, called a cochaperonin, is then put on the chamber, and the protein folds within the more hospitable environment of the chamber. A more detailed model for what happens in the chamber and how this helps a protein fold is suggested by the structure (see Wang and Boisvert, Suggested Reading). When the misfolded protein is first taken up, the lining of the chamber consists of mostly hydrophobic amino acids that bind to the exposed hydrophobic regions of the misfolded protein. When the cochaperonin cap is put onto this chamber (the cis-chamber) and the bound ATP is cleaved to ADP, the lining may switch to being mostly hydrophilic amino acids, driving the more hydrophobic regions of the misfolded protein to the interior of the protein, where they usually reside in the folded protein. Binding of ATP and an unfolded protein to the other chamber (the trans-chamber) causes the cap to come off the cis-chamber, releasing the folded protein and preparing the other chamber to take up the misfolded protein and take its turn being the cis-chamber. This process takes a lot of energy, and a number of ATP molecules (one for each subunit of one chamber of the chaperonin, which is seven in E. coli [see below]) are cleaved to ADP in each cycle. It is a mystery why chaperonins have two chambers and why the folding has to alternate between the two chambers. Chaperonins composed of only one chamber function more poorly, although they might retain some of their activity (see Sun et al., Suggested Reading). The communication to one chamber that ATP and unfolded protein are bound to the opposite chamber is one of the most remarkable examples of allosterism known, especially considering that the chambers, while touching back to back, are composed of separate polypeptides.

The first chaperonin was discovered in E. coli, where it was named GroEL because it helps assemble the E protein of phage λ into the phage head. The GroEL chaperonin consists of 14 identical polypeptides (7 making up each cylinder) of 60 kDa. Its cochaperonin cap is called GroES, which is also made up of 7 subunits, each 10 kDa in size. Unlike DnaK and the other chaperones, GroEL and GroES are required for E. coli growth, even at lower temperatures. The GroEL chaperonin is known to be required for folding of some essential proteins, which explains why GroEL is essential. The chaperonins come in two general types called the group I and group II chaperonins. The group I chaperonins, related to GroEL, are composed of 60-kDa subunits and are found in all bacteria and in the mitochondria and chloroplasts of eukaryotes; this makes sense, since these organelles are derived from bacteria (see the introductory chapter). These chaperonins and their cochaperonins are induced by heat shock and other stresses, so they are called the Hsp60 proteins and Hsp10 proteins for heat shock 60-kDa and 10-kDa proteins (see Bukau and Horwich, Suggested Reading). The group II chaperonins are found in the archaea and in the cytoplasm of eukaryotes. They have very little amino acid sequence in common with the group I chaperonins and are not composed of identical subunits (i.e., they are mixed multimers) and often have eight or more polypeptide subunits per cylinder. Furthermore, if they have a cochaperonin cap, it might be attached to the opening of the chamber rather than being detachable, like GroES. They may also be more dedicated and fold only a small subset of proteins, including actin in the eukaryotic cytoplasm. Nevertheless, the two types form similar cylindrical structures and presumably use a similar two-stroke mechanism to help fold proteins. Note that this is yet another example where the archaea and eukaryotes are similar to each other and different from the bacteria (see the introductory chapter).

Figure 2.37 Chaperonins. The GroEL (Hsp60)-type chaperonin multimers form two connected cylinders. A denatured or unfolded protein enters the chamber in one of the cylinders, and the chamber is capped by the cochaperonin GroES (Hsp10). The denatured protein can then be helped to fold in the chamber and is released. Entry of a second unfolded protein into the second cylinder helps to trigger release of the folded protein.