Structural studies on the co-chaperone Hop and its complexes with Hsp90

The tetratricopeptide repeat domain (TPR)-containing co-chaperone Hsp-organising protein (Hop) plays a critical role in mediating interactions between Heat Shock Protein (Hsp)70 and Hsp90 as part of the cellular assembly machine. It also modulates the ATPase activity of both Hsp70 and Hsp90, thus facilitating client protein transfer between the two. Despite structural work on the individual domains of Hop, no structure for the full-length protein exists, nor is it clear exactly how Hop interacts with Hsp90, although it is known that its primary binding site is the C-terminal MEEVD motif. Here, we have undertaken a biophysical analysis of the structure and binding of Hop to Hsp90 using a variety of truncation mutants of both Hop and Hsp90, in addition to mutants of Hsp90 that are thought to modulate the conformation, in particular the N-terminal dimerisation of the chaperone. The results establish that whilst the primary binding site of Hop is the C-terminal MEEVD peptide of Hsp90, binding also occurs at additional sites in the C-terminal and middle domain. In contrast, we show that another TPR-containing co-chaperone, CyP40, binds solely to the C-terminus of Hsp90. Truncation mutants of Hop were generated and used to investigate the dimerisation interface of the protein. In good agreement with recently published data, we find that the TPR2a domain that contains the Hsp90-binding site is also the primary site for dimerisation. However, our results suggest that residues within the TPR2b may play a role. Together, these data along with shape reconstruction analysis from small-angle X-ray scattering measurements are used to generate a solution structure for full-length Hop, which we show has an overall butterfly-like quaternary structure. Studies on the nucleotide dependence of Hop binding to Hsp90 establish that Hop binds to the nucleotide-free, 'open' state of Hsp90. However, the Hsp90-Hop complex is weakened by the conformational changes that occur in Hsp90 upon ATP binding. Together, the data are used to propose a detailed model of how Hop may help present the client protein to Hsp90 by aligning the bound client on Hsp70 with the middle domain of Hsp90. It is likely that Hop binds to both monomers of Hsp90 in the form of a clamp, interacting with residues in the middle domain of Hsp90, thus preventing ATP hydrolysis, possibly by the prevention of association of N-terminal and middle domains in individual Hsp90 monomers.

Mechanistic studies on Hsp90 inhibition by ansamycin derivatives

Heat shock protein 90 (Hsp90) is a molecular chaperone that is required for the maturation and activation of a number of client proteins, many of which are involved in cancer development. The ansamycin family of natural products and their derivatives, such as geldanamycin (GA), are well-known inhibitors of the essential ATPase activity of Hsp90. Despite structural studies on the complexes of ansamycin derivatives with the ATPase domain of Hsp90, certain aspects of their inhibitory mechanism remain unresolved. For example, it is known that GA in solution exists in an extended conformation with a trans amide bond; however, it binds to Hsp90 in a significantly more compact conformation with a cis amide bond. GA and its derivatives have been shown to bind to Hsp90 with low micromolar affinity in vitro, in contrast to the low nanomolar anti-proliferative activity that these drugs exhibit in vivo. In addition, they show selectivity towards tumour cells. We have studied both the equilibrium binding, and the association and dissociation kinetics of GA derivative, 17-DMAG, and the fluorescently labelled analogue BDGA to both wild-type and mutant Hsp90. The mutants were made in order to test the hypothesis that conserved residues near the ATP-binding site may catalyse the trans-cis isomerisation of GA. Our results show that Hsp90 does not catalyse the trans-cis isomerisation of GA, and suggests that there is no isomerisation step before binding to Hsp90. Experiments with BDGA measured over a wide range of conditions, in the absence and in the presence of reducing agents, confirm recent studies that have suggested that the reduced dihydroquinone form of the drug binds to Hsp90 considerably more tightly than the non-reduced quinone species.

Effects of Acute Exposure to Endosulfan (Organochlorine Pesticides) on Hematology of African Mud Catfish, Clarias gariepinus (Burchell, 1822)

Changes in the blood parameters of fingerlings of Clarias gariepinus were investigated after 24 and 96-h of exposures to endosulfan. 180 fingerlings of C. gariepinus [mean weight (10.5±1.3 g); total length (11.2±1.2 cm)] were exposed to five different concentrations (1.00, 2.20, 4.80, 11.00, 23.00 µg/L) of endosulfan and a control for 96 h after being acclimatized for 21 days. After 24 h of exposure, microcytic hypochromic anemia was observed and all erythrocyte profiles tested showed significant variation (p<0.05) among the treatments except thrombocyte and mean corpuscular hemoglobin concentration. Macrocytic hyperchromic anemia was noticed after 96 h of exposure and all the hematological parameters varied significantly (p<0.05) except packed cell volume and red blood cell count. The study shows that endosulfan alters the hematology of C. gariepinus fingerlings. Therefore, awareness on the hazards associated with the use of endosulfan should be intensified and sound sustainable alternatives to endosulfan should be developed.

Generation of a neutralizing antibody against RD114-pseudotyped viral vectors

The feline endogenous RD114 glycoprotein has proved to be an attractive envelope to pseudotype both retroviral and lentiviral vectors. As a surface protein, its detection on packaging cells as well as viral particles would be useful in different fields of its use. To address this, we generated a monoclonal antibody against RD114 by immunization of rats, termed 22F10. Once seroconversion was confirmed, purified 22F10 was cloned into murine Fc and characterized with a binding affinity of 10nM. The antibody was used to detect RD114 and its variant envelopes on different stable viral packaging cell lines (FLYRD18 and WinPac-RD). 22F10 was also shown to prevent the infections of different strains of RD-pseudotyped vectors but not related envelope glycoproteins by blocking cell surface receptor binding. We are the first to report the neutralization of viral particles by a monoclonal αRD114 antibody.