Structural characterization of the N-terminal kinase-interacting domain of an Hsp90-cochaperone Cdc37 by CD and solution NMR spectroscopy.

Cdc37 is a protein kinase-targeting molecular chaperone, which cooperates with Hsp90 to assist the folding, assembly and maturation of various signaling kinases. It consists of three distinct domains: the N-terminal, middle, and C-terminal domain. While the middle domain is an Hsp90-binding domain, the N-terminal domain is recognized as a kinase-interacting domain. The N-terminal domain contains a well-conserved Ser residue at position 13, and the phosphorylation at this site has been shown to be a prerequisite for the interaction between Cdc37 and signaling kinases. Although the phosphorylation of Ser13 might induce some conformational change in Cdc37 molecule, little is known about the structure of the N-terminal domain of Cdc37. We examined the structural and dynamic properties of several fragment proteins corresponding to the N-terminal region of Cdc37 by circular dichroism and solution NMR spectroscopy. We found that the N-terminal domain of Cdc37 exhibits highly dynamic structure, and it exists in the equilibrium between α-helical and more disordered structures. We also found that phosphorylation at Ser13 did not significantly change the overall structure of N-terminal fragment protein of Cdc37. The results suggested that more complicated mechanisms might be necessary to explain the phosphorylation-activated interaction of Cdc37 with various kinases.