Mediates the association of the molecular chaperones HSP70 and HSP90. Mediates nuclear encoded chloroplast preproteins binding to HSP90 prior to chloroplastic sorting (By similarity). Co-chaperone that forms a complex with HSP70 and HSP90 and preproteins (e.g. chloroplast preproteins) (By similarity). The tetratricopeptide repeat (TPR) domain, forming a carboxylate clamp (CC), mediates interaction with the highly conserved 'EEVD' motif at the C-terminal ends of HSP90 and HSP70. Phosphorylated. Acetylated.