The Hsp90 Complex in Microbes and Man
Model for how proteins get reactivated by the collaborative activities of Hsp90Ec and the DnaK system in E.coli. (A) First, the microbial chaperones in the DnaK system (DnaK, DnaJ, or CbpA and GrpE) interact with an inactive or denatured protein and initiate the ATP-dependent remodeling process. (B) Then, Hsp90Ec interacts with both DnaK and the protein, and together, in a nucleotide-dependent manner, the two chaperones promote remodeling. (C) Finally, the reactivated protein is released by the chaperones.
Why would cancer researchers be interested in how a bacteria named Escherichia coli (E. coli) rebuilds its cellular proteins after they have been inactivated by environmental stress such as heat? The answer lies in a protein remodeling mechanism that is shared by microbes and man.
When cellular proteins are denatured in E. coli and in man, molecular chaperones come to the rescue. These proteins assist the folding, unfolding, and refolding of other proteins—called clients —that are critical to survival. One family of chaperones present in all organisms, including man, is called the heat shock protein 90 (Hsp90) family. It uses an ATP (adenosine triphosphate) energy source to reshape and activate hundreds of client proteins with the assistance of several co-chaperones. A similar Hsp90 complex (Hsp90Ec) exists in E. coli, so Olivier Genest, Ph.D., working with Sue Wickner, Ph.D., in CCR’s Laboratory of Molecular Biology, decided to work with E. coli to better understand its Hsp90 complex’s mechanism of action. As they recently reported in the Proceedings of the National Academy of Science, Hsp90Ec promotes reactivation of a heat-inactivated protein called luciferase by cooperating with another chaperone complex, the Hsp70 homolog in E. coli, called DnaK.
In man, geldanamycin inhibits Hsp90 by blocking the ATP energy source. In E. coli, geldanamycin also inhibits luciferase reactivation by blocking ATP. The Wickner team discovered that the reactivation of client proteins in E. coli requires ATP-dependent chaperone activity of the DnaK system before the troubled client protein even reaches the Hsp90Ec complex. The DnaK system acts first, and then Hsp90Ec swings into action. The DnaK system and Hsp90Ec collaborate synergistically to reshape client proteins. This collaboration occurs both in vivo and in vitro.
The Wickner team also discovered that the Hsp90Ec and DnaK complexes are quite particular about which proteins they will remodel. Hsp90Ec and the DnaK system reactivated several, but not all, denatured protein substrates. So the Hsp90Ec-DnaK team has substrate specificity.
Although chaperones Hsp90Ec and the DnaK system reactivate proteins in vitro without additional known co-chaperones being present, and the two complexes appear to interact in vivo and in vitro, other co-chaperones may participate in protein reactivation, and they just have not yet been identified. With their E. coli model system in hand, and with hints from some of the known co-chaperones in counterpart multicellular organisms, the Wickner team can get an answer.Summary Posted: 05/2011
www.pnas.org/content/early/2011/04/26/1104703108.full.pdf Reviewed by Donna Kerrigan