Solution structure of Plasmodium falciparum Hsp90 indicates a high flexible dimer

Hsp90 is a ubiquitous, homodimer and modular molecular chaperone. Each Hsp90 protomer has three different domains, named the N-terminal domain (NTD), middle domain (MD) and C-terminal domain (CTD). The Hsp90 molecular cycle involves ATP binding and hydrolysis, which drive conformational changes. Hsp90 is critical for the viability of eukaryotic organisms, including the protozoan that causes the severe form of malaria, Plasmodium falciparum, the growth and differentiation of which are compromised when Hsp90 is inhibited. Here, we characterize the structure of a recombinant P. falciparum Hsp90 (PfHsp90) protein, as well as its MD (PfHsp90MD) and NTD plus MD (PfHsp90NMD) constructs. All the proteins were obtained with high purity and in the folded state. PfHsp90 and PfHsp90NMD interacted with adenosine nucleotides via the NTD, and Mg2+ was critical for strong binding. PfHsp90 behaved mostly as elongated and flexible dimers in solution, which dissociate with a sub-micromolar dissociation constant. The PfHsp90MD and PfHsp90NMD constructs behaved as globular and elongated monomers, respectively, confirming the importance of the CTD for dimerization. Small angle X-ray scattering data were obtained for all the constructs, and ab initio models were constructed, revealing PfHsp90 in an open conformation and as a greatly elongated and flexible protein.