[Chaperonin: structure and function].

The folding of most newly synthesized proteins in the cell requires the interaction of a variety of protein cofactors known as molecular chaperones. These molecules recognize and bind to nascent polypeptide chains and partially folded intermediates of proteins, preventing their aggregation and misfolding. There are several families of chaperones; those most involved in protein folding are the 40-kDa heat shock protein (HSP40; DnaJ), 60-kDa heat shock protein (HSP60; GroEL), and 70-kDa heat shock protein (HSP70; DnaK) families. The availability of high-resolution structures has facilitated a more detailed understanding of the complex chaperone machinery and mechanisms, including the ATP-dependent reaction cycles of the GroEL and HSP70 chaperones. For both of these chaperones, the binding of ATP triggers a critical conformational change leading to release of the bound substrate protein. Whereas the main role of the HSP70/HSP40 chaperone system is to minimize aggregation of newly synthesized proteins, the HSP60 chaperones also facilitate the actual folding process by providing a secluded environment for individual folding molecules and may also promote the unfolding and refolding of misfolded intermediates.

Chaperonins↗

PubMed1998

Changing the nature of the initial chaperonin capture complex influences the substrate folding efficiency.

For the chaperonin substrates, rhodanese, malate dehydrogenase (MDH), and glutamine synthetase (GS), the folding efficiencies, and the lifetimes of folding intermediates were measured with either the nucleotide-free GroEL or the activated ATP.GroEL.GroES chaperonin complex. With both nucleotide-free and activated complex, the folding efficiency of rhodanese and MDH remained high over a large range of GroEL to substrate concentration ratios (up to 1:1). In contrast, the folding efficiency of GS began to decline at ratios lower than 8:1. At ratios where the refolding yields were initially the same, only a relatively small increase (1.6-fold) in misfolding kinetics of MDH was observed with either the nucleotide-free or activated chaperonin complex. For rhodanese, no change was detected with either chaperonin complex. In contrast, GS lost its ability to interact with the chaperonin system at an accelerated rate (8-fold increase) when the activated complex instead of the nucleotide-free complex was used to rescue the protein from misfolding. Our data demonstrate that the differences in the refolding yields are related to the intrinsic folding kinetics of the protein substrates. We suggest that the early kinetic events at the substrate level ultimately govern successful chaperonin-substrate interactions and play a crucial role in dictating polypeptide flux through the chaperonin system. Our results also indicate that an accurate assessment of the transient properties of folding intermediates that dictate the initial chaperonin-substrate interactions requires the use of the activated complex as the interacting chaperonin species.

Chaperonins↗