KEY CONCEPTS:
- Chaperones act on newly synthesized proteins, proteins that are passing through membranes, or proteins that have been denatured.
- Hsp70 and some associated proteins form a major class of chaperones that act on many target proteins.
- Group I and group II chaperonins are large oligomeric assemblies that act on target proteins they sequester in internal cavities.
- Hsp90 is a specialized chaperone that acts on proteins of signal transduction pathways.
The ability of chaperones to recognize incorrect protein conformations allows them to play two related roles concerned with protein structure:
- When a protein is initially synthesized, that is to say, as it exits the ribosome to enter the cytosol, it appears in an unfolded form. Spontaneous folding then occurs as the emerging sequence interacts with regions of the protein that were synthesized previously. Chaperones influence the folding process by controlling the accessibility of the reactive surfaces. This process is involved in initial acquisition of the correct conformation.
- When a protein is denatured, new regions are exposed and become able to interact. These interactions are similar to those that occur when a protein (transiently) misfolds as it is initially synthesized. They are recognized by chaperones as comprising incorrect folds. This process is involved in recognizing a protein that has been denatured, and either assisting renaturation or leading to its removal by degradation.
Chaperones may also be required to assist the formation of oligomeric structures and for the transport of proteins through membranes. A persistent theme in membrane passage is that control (or delay) of protein folding is an important feature. Figure 8.9 shows that it may be necessary to maintain a protein in an unfolded state before it enters the membrane because of the geometry of passage: the mature protein could simply be too large to fit into the available channel. Chaperones may prevent a protein from acquiring a conformation that would prevent passage through the membrane; in this capacity, their role is basically to maintain the protein in an unfolded, flexible state. Once the protein has passed through the membrane, it may require another chaperone to assist with folding to its mature conformation in much the same way that a cytosolic protein requires assistance from a chaperone as it emerges from the ribosome. The state of the protein as it emerges from a membrane is probably similar to that as it emerges from the ribosome—basically extended in a more or less linear condition.
Two major types of chaperones have been well characterized (for review see Frydman, 2001). They affect folding through two different types of mechanism:
- Figure 8.10 shows that the Hsp70 system consists of individual proteins that bind to, and act on, the substrates whose folding is to be controlled. It recognizes proteins as they are synthesized or emerge from membranes (and also when they are denatured by stress). Basically it controls the interactions between exposed reactive regions of the protein, enabling it to fold into the correct conformation in situ. The components of the system are Hsp70, Hsp40, and GrpE. The name of the system reflects the original identification of Hsp70 as a protein induced by heat shock. The Hsp70 and Hsp40 proteins bind individually to the substrate proteins. They use hydrolysis of ATP to provide the energy for changing the structure of the substrate protein, and work in conjunction with an exchange factor that regenerates ATP from ADP.
- Figure 8.11 shows that a chaperonin system consists of a large oligomeric assembly (represented as a cylinder). This assembly forms a structure into which unfolded proteins are inserted. The protected environment directs their folding. There are two types of chaperonin system. GroEL/GroES is found in all classes of organism. TRiC is found in eukaryotic cytosol.
The components of the systems are summarized in Figure 8.12. The Hsp70 system and the chaperonin systems both act on many different substrate proteins. Another system, the Hsp90 protein, functions in conjunction with Hsp70, but is directed against specific classes of proteins that are involved in signal transduction, especially the steroid hormone receptors and signaling kinases (for review see Moarefi and Hartl, 2001). Its basic function is to maintain its targets in an appropriate conformation until they are stabilized by interacting with other components of the pathway (Rutherford and Lindquist, 1998; Queitsch, Sangster, and Lindquist, 2002).
(The reason many of these proteins are named "hsp", which stands for "heat shock protein" is that increase in temperature causes production of heat shock proteins whose function is to minimize the damage caused to proteins by heat denaturation. Many of the heat shock proteins are chaperones and were first discovered, and named, as part of the heat shock response.)