FKBP8 connects the Hsp70-Hsp90 chaperone machinery to the folding of membrane proteins

Through a genome-wide CRISPR-Cas9 screen, we identified the ER-anchored FK506 binding protein 8 (FKBP8) as a chaperone essential for membrane protein folding and assembly. Using ABC transporters as model substrates, we show that FKBP8 cooperates with Hsp70-Hsp90 machinery to remodel nascent or misfolded cytosolic domains into their native conformations.

Cryo-EM analysis reveals that FKBP8 employs a conserved hydrophobic ϕ94ϕ96ϕ97/ϕ158ϕ162 cluster to help form a large client-binding cavity within the FKBP8–Hsp90 complex that captures folding intermediates. FKBP8 deficiency, disruption of this cluster, or disease-associated mutations within FKBP8 abolish substrate maturation, leading to ER retention and degradation. Reconstitution with purified components demonstrates that FKBP8 and Hsp40-Hsp70-HOP-Hsp90 constitute a minimal machinery capable of restoring the native structure of a misfolded ABC transporter.