Structure of ER chaperone complex GRP170-ATP-BiP suggests a new model for substrate engagement

Molecular chaperones are essential for maintaining protein homeostasis in all living cells. In the endoplasmic reticulum (ER), BiP and GRP170 are the sole representatives of Hsp70 and Hsp110 family and are critical for ER function. GRP170 is a relatively large and unusual chaperone that possess both nucleotide exchange and chaperoning activity. The molecular mechanism by which these chaperones collaborate to engage substrate protein and how GRP170 couples its dual functionalities are not currently known. Here, we report the 2.7 Angstrom cryo-electron microscopy structure of GRP170-ATP-BiP chaperone complex purified from HEK293 cells that reveals a C-terminal curved hook domain, suggesting a role in substrate engagement in coordination with BiP. Additionally, we uncover the structural basis for GRP170 pseudo-ATPase chaperone activity - making it, to our knowledge, the first chaperone with this type of regulation. Our endogenous complex structure, together with prior cellular data, suggests a new paradigm for how GRP170-BiP chaperones collaborate in ER protein quality control, broadening our understanding of how BiP/Hsp70 chaperones engage with substrates.