Heat shock protein 90 (HSP90) is a highly conserved and essential molecular chaperone involved in maturation and activation of signaling proteins in eukaryotes. HSP90 operates as a dimer in a conformational cycle driven by ATP binding and hydrolysis. HSP90 often functions together with co-chaperones that regulate the conformational cycle and/or load a substrate "client" protein onto HSP90. In plants, immune sensing NLR (nucleotide-binding domain and leucine-rich repeat containing) proteins are among the few known client proteins of HSP90. In the process of chaperoning NLR proteins, co-chaperones, RAR1 and SGT1 function together with HSP90. Recent structural and functional analyses indicate that RAR1 dynamically controls conformational changes of the HSP90 dimer, allowing SGT1 to bridge the interaction between NLR proteins and HSP90. Here, we discuss the regulation of NLR proteins by HSP90 upon interaction with RAR1 and SGT1, emphasizing the recent progress in our understanding of the structure and function of the complex. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).
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