Complex conformational dynamics are essential for the chaperone function of heat shock protein 90 (Hsp90), including transient, ATP-biased N-domain dimerization establishing ATPase competence. Biochemical data demonstrate that the intrinsic, but weak, ATP hydrolyzing activity of Hsp90 is markedly enhanced by the co-chaperone Aha1. However, cellular concentration of Aha1 is substoichiometric relative to Hsp90. In cells, interaction of this important co-chaperone with Hsp90 is up-regulated by posttranslational modifications (PTMs), including phosphorylation of a highly conserved tyrosine (Y313 in Hsp90a). Here we use chemical cross-linking with mass spectrometry to explore the the impacts of a phosphomimetic mutation (Y313E) and binding of a non-hydrolyzable ATP analog (AMP-PNP),on the structure Hsp90a and its interaction with Aha1.