The Hdj-2/Hsc70 chaperone pair facilitates early steps in CFTR biogenesis

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Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride ion channel constructed from two membrane‐spanning domains (MSDs), two nucleotide‐binding domains (NBD) and a regulatory (R) domain. The NBDs and R‐domain are cytosolic and how they are assembled with the MSDs to achieve the native CFTR structure is not clear. Human DnaJ 2 (Hdj‐2) is a co‐chaperone of heat shock cognate 70 (Hsc70) which is localized to the cytosolic face of the ER. Whether Hdj‐2 directs Hsc70 to facilitate the assembly of cytosolic regions on CFTR was investigated. We report that immature ER forms of CFTR and ΔF508 CFTR can be isolated in complexes with Hdj‐2 and Hsc70. The ΔF508 mutation is localized in NBD1 and causes the CFTR to misfold. Levels of complex formation between ΔF508 CFTR and Hdj‐2/Hsp70 were ∼2‐fold higher than those with CFTR. The earliest stage at which Hdj‐2/Hsc70 could bind CFTR translation intermediates coincided with the expression of NBD1 in the cytosol. Interestingly, complex formation between Hdj‐2 and nascent CFTR was greatly reduced after expression of the R‐domain. In experiments with purified components, Hdj‐2 and Hsc70 acted synergistically to suppress NBD1 aggregation. Collectively, these data suggest that Hdj‐2 and Hsc70 facilitate early steps in CFTR assembly. A putative step in the CFTR folding pathway catalyzed by Hdj‐2/Hsc70 is the formation of an intramolecular NBD1–R‐domain complex. Whether this step is defective in the biogenesis of ΔF508 CFTR will be discussed.