The successful execution of cellular processes depends on well-folded proteins in all cellular compartments. Although some nascent proteins are able to fold spontaneously,alarge number of proteins are less stable and vulnerable to misfolding and aggregation in the highlyc rowded cellular environment. Many proteins are just marginally stable in their physiological environment and susceptible to misfolding and aggregation,e specially under cellular stress conditions. Therefore, cells have evolved multiple different molecular chaperones to bind cellular proteins,b oth in unfolded and folded forms. Consequently,m olecular chaperones are implicated in aw ide variety of cellular processes, including protein biogenesis, protection of the proteome from stress;r ecovery of individual proteins from aggregates;p rotein folding;f acilitation of protein translocation across membranes;a nd more specialized roles, such as disassembly of particularprotein complexes. [1][2][3] In the case of cellular stress conditions, such as upon heat shock, in cancer cells, or infected cells, the expression levels of molecular chaperones is highly upregulated. [4,5] Unlike classical specific interactors, such as antibody-antigen pairs, chaperones are heavy-dutym olecular machines that operate on a wide clientome, that is, ar ange of many different clients, including nascenta nd native proteins. [6,7] For instance, in prokaryotic cells, the chaperone trigger factor (TF) binds at least 68 different full-length proteins from Escherichia coli lysates, [8] or in eukaryotic cells, the chaperone machinery Hsp70/Hsp90 binds 60 %o fk inases, 30 %o fu biquitin ligases, and 7% of transcription factor. [9][10][11] This interaction betweenc haperones and native client proteinsc an occur in an ATP-dependent or -independent manner and can serve the purpose of preventing aggregation and to assemble clients into quaternary structures and complexes. [11,12] In eukaryotes, chaperones binding folded clientsa re also involved in recruiting proteins to thep roteasome for turnover,b ringing proteins to the endosome or lysosome for autophagy, [13] and promoting client protein binding with ligands. [14] Descriptionso fc haperone-clienti nteractions at the atomic level are necessary to fully understand chaperone function. Considerable knowledge has been accumulated over the past decadeo nt he molecular mechanisms of chaperones aiding nascent protein folding, preventing protein aggregation, [15][16][17] yet many fundamental questions remain unanswered.S pecifically,u ntil recently,w eknew little about how chaperones recognizen ative client proteins, [18][19][20][21] and how these chaperones can act in ag eneric manner on many different proteins.T hese questionsa re difficultt oa ddress because the underlying molecular mechanisms involvei nteractions that are transient and dynamic. For studies of protein-protein interactions, am ultitude of experimental methods exist, including X-ray crystallography;c ryo-electronm icroscopy;a nd biophysical methods, such as small-angle ...