The cell division cycle protein 37 (Cdc37) and the 90-kDa heat shock protein (Hsp90) are molecular chaperones, which are crucial elements in the protein signaling pathway. The largest class of client proteins for Cdc37 and Hsp90 are protein kinases. The catalytic domains of these kinases are stabilized by Cdc37, and their proper folding and functioning is dependent on Hsp90. Here, we present the x-ray crystal structure of the 16-kDa middle domain of human Cdc37 at 1.88 Å resolution and the

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... ture of this domain in complex with the 23-kDa N-terminal domain of human Hsp90 based on heteronuclear solution state NMR data and docking. Our results demonstrate that the middle domain of Cdc37 exists as a monomer. NMR and mutagenesis experiments reveal Leu-205 in Cdc37 as a key residue enabling complex formation. These findings can be very useful in the development of small molecule inhibitors against cancer. * This work was supported by the SPINE (Structural Proteomics in Europe) project of the European commission and by the Cluster of Excellence: Macromolecular Complexes (Deutsche Forschungsgemeinschaft). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. The atomic coordinates and structure factors (codes 2W0G and 2K5B) have been deposited in the Protein Downloaded from FIGURE 6. Schematic representation of the interaction interface of NMR calculated Cdc37 M ⅐Hsp90 N complex structure. The positively charged, negatively charged, neutral, and hydrophobic amino acids are colored in red, blue, green, and gray, respectively. FIGURE 7. Cdc37 M -L205D mutant does not bind to Hsp90 N . 1 H, 15 N-HSQC spectral region showing the amide signal for Lys-202, for wild type-free (red), mutant-free (orange), wild type in complex with Hsp90 N (1:1) (green), and mutant in the presence of Hsp90 N (1:4) (blue).