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SUMMARYThe transmembrane Hsp40 DNAJB12 and cytosolic Hsp70 cooperate on the ER’s cytoplasmic face to facilitate the triage of nascent polytopic membrane proteins for folding versus degradation. N1303K is the second most common mutation in the ion channel CFTR, but unlike F508del-CFTR, biogenic and functional defects in N1303K-CFTR are resistant to correction bolding modulators. N1303K is reported to arrest CFTR folding at a late stage after partial assembly of its N-terminal domains. N1303K-CFTR intermediates are clients of JB12-Hsp70 complexes, maintained in a detergent soluble-state, and have a relatively long 3-hour half-life. ERAD-resistant pools of N1303K-CFTR are concentrated in ER-tubules that associate with autophagy initiation sites containing WIPI1, FlP200, and LC3. Destabilization of N1303K-CFTR or depletion of JB12 prevents entry of N1303K-CFTR into the membranes of ER-connected phagophores and autolysosomes. Whereas, the stabilization of intermediates with the modulator VX-809 promotes the association of N1303K-CFTR with autophagy initiation machinery. N1303K-CFTR is excluded from the ER-exits site, and its passage from the ER to autolysosomes does not require ER-phagy receptors. DNAJB12 operates in biosynthetically active ER-microdomains to triage in a conformation-specific manner membrane protein intermediates for secretion versus degradation via ERAD or selective-ER associated autophagy.

The Cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, crucial to epithelial salt and water homeostasis, and defective due to mutations in its gene in patients with cystic fibrosis is a unique member of the large family of ATP-binding cassette transport proteins. Regulation of CFTR channel activity is stringently controlled by phosphorylation and nucleotide binding. Structural changes that underlie transitions between active and inactive functional states are not yet fully understood. Indeed the first 3D structures of dephosphorylated, ATP-free and phosphorylated ATP-bound states were only recently reported. Here we have determined the structure of inactive and active states of a thermally stabilized CFTR with very high channel open probability, confirmed after reconstitution into proteoliposomes. The unique repositioning of the TMHs and R domain density that we observe provide insights into the structural transition between active and inactive functional states of CFTR.HighlightsStructures of thermostabilized avian CFTR in dephosphorylated or phosphorylated forms at 4.3 Å and 6.6 Å resolution, respectively.Conformational differences of transmembrane helices 7 & 8 compared to zebra fish and human CFTR structures reveal an extracellular vestibule that may provide anion access to the pore.R-domain density appears to “plug” the intercellular vestibule in the dephosphorylated avian CFTR cryo-EM map.