Molecular chaperones—holding and folding

Any cellular function is based upon the proper function of native proteins. However, protein function requires a specific 3D structure. Establishing the correct three-dimensional conformation is therefore a premise for the exclusive biochemical function of a given protein. But how can this structure be achieved and maintained throughout the entire lifetime of a protein? In 1972, Christian Anfinsen was awarded with the Nobel Prize for chemistry for showing that the primary amino acid sequence contains all the information required for the native structure and that folding is an autonomous process, which does not require any additional energy. Based on in-vitro experiments with purified proteins, he suggested that the correct three-dimensional structure can form spontaneously in vivo once the newly synthesized protein leaves the ribosome. However, this view changed considerably over the last decade. It has become clear that a complex and sophisticated collection of proteins exists, which assists nascent protein folding and allows the functional state of proteins to be maintained under stress conditions in which they would normally unfold and aggregate. These proteins are collectively called “molecular chaperones”, because they prevent unwanted interactions between their immature clients, like their human counterparts in Victorian society of the 19th century, who as chaperones had to prevent bourgeoisie female adolescents from unwanted encounters with men. Experimental data indicate that several members of this group of proteins form heterooligomeric complexes, sometimes termed “chaperone machines”, interacting with each other to generate a network for maturation, assembly and intracellular targeting of proteins. In this review, the essential information on the structure and function of chaperone and chaperone complexes are summarized and the impact on the cellular stress response in plants will be discussed.