Abstract
Relative contributions of folding kinetics versus protein quality control (QC) activity in the partitioning of non-native proteins between life and death are not clear. Cystic fibrosis transmembrane conductance regulator (CFTR) biogenesis serves as an excellent model to study this question because folding of nascent CFTR is inefficient and deletion of F508 causes accumulation of CFTRΔF508 in a kinetically trapped, but foldable state. Herein, a novel endoplasmic reticulum (ER)-associated Hsp40, DNAJB12 (JB12) is demonstrated to play a role in control of CFTR folding efficiency. JB12 cooperates with cytosolic Hsc70 and the ubiquitin ligase RMA1 to target CFTR and CFTRΔF508 for degradation. Modest elevation of JB12 decreased nascent CFTR and CFTRΔF508 accumulation while increasing association of Hsc70 with ER forms of CFTR and the RMA1 E3 complex. Depletion of JB12 increased CFTR folding efficiency up to threefold and permitted a pool of CFTRΔF508 to fold and escape the ER. Introduction of the V510D misfolding suppressor mutation into CFTRΔF508 modestly increased folding efficiency, whereas combined inactivation of JB12 and suppression of intrinsic folding defects permitted CFTRΔF508 to fold at 50% of wild-type efficiency. Therapeutic correction of CFTRΔF508 misfolding in cystic fibrosis patients may require repair of defective folding kinetics and suppression of ER QC factors, such as JB12.