The dynamic interactome: A proteomic investigation of ligand-dependent HSP90 complexes

The molecular chaperone HSP90 and its many co-chaperones function to fold and fully activate a plethora of proteins involved in virtually every cellular process including oncogenesis. The ATPase activity of HSP90 is essential for its cellular functions and it is believed that conformational changes elicited by ATP binding and hydrolysis are required for client protein loading and refolding in a process referred to as the ATPase cycle. The discovery that HSP90 binds ADP at a 10-fold higher affinity than ATP led to the possibility that ADP binding may also have some biological significance and the particular nucleotide bound to HSP90 may control its interactions with client proteins and co-chaperones and therefore altering functions within the cell. To better understand the biological functions of HSP90, several groups have attempted to systematically identify HSP90 interacting proteins using proteomic and genetic approaches; however, these studies were conducted under single experimental conditions-producing a static picture of the HSP90 interactome without regard to the nucleotide-bound state of HSP90. With the development of quantitative proteomic tools, it is now possible to observe changes in the HSP90 interaction network on a proteome-wide scale under various experimental conditions. In this work, we show the human HSP90 interactome to be dynamic with respect to nucleotide and drug ligands. We purified TAP-tagged HSP90 complexes in the presence of the HSP90 ligands ATP, ADP and the pharmacological inhibitor geldanamycin and observed changes in the relative abundance of constituents of these complexes using label-free quantitative mass spectrometry. In addition to identifying new HSP90 interacting proteins, we identified several putative ligand-dependent interactions including a geldanamycin-dependent HSP90 complex enriched for core subunits of the transcriptional apparatus, suggesting possible mechanisms of action for the activity of this drug. We go further to characterize a novel ADP-dependent interaction with the co-chaperone CHORDC1 (Chp1) and link the CHORD domain to ADP-stimulated interactions.