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

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chlorid e ion channel constructed from two membrane-spanning domains (MSDs), two nu cleotide-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 th e native CFTR structure is not clear. Human DnaJ 2 (Hdj-2) is a co-chaperon e 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 cytos olic regions on CFTR was investigated. We report that immature ER forms of CFTR and Delta F508 CFTR can be isolated in complexes with Hdj-2 and Hsc70. The Delta F508 mutation is localized in NBD1 and causes the CFTR to misfol d, Levels of complex formation between Delta F508 CFTR and Hdj-2/Hsp70 were similar to 2-fold higher than those with CFTR, The earliest stage at which Hdj-2/Hsc70 could bind CFTR translation intermediates coincided with the e xpression 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 synergisti cally to suppress NBD1 aggregation. Collectively, these data suggest that H dj-2 and Hsc70 facilitate early steps in CFTR assembly, A putative step in the CFTR folding pathway catalyzed by Hdj-2/Hsc70 is thc:formation of an in tramolecular NBD1-R-domain complex. Whether this step is defective in the b iogenesis of Delta F508 CFTR will be discussed.