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VCP/p97 AAA-ATPase Does Not Interact with the Endogenous Wild-Type Cystic Fibrosis Transmembrane Conductance Regulator

Department of Cell Biology, Department of Pharmacology and Toxicology, Comprehensive Cancer Center, and Gregory Fleming James Cystic Fibrosis Center, University of Alabama at Birmingham, Birmingham, Alabama

Correspondence and requests for reprints should be addressed to Dr. James Collawn, Department of Cell Biology, University of Alabama at Birmingham, 1918 University Blvd., MCLM 350, Birmingham, AL 35294-0005. E-mail: jcollawn{at}uab.edu

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that is defective in cystic fibrosis. The most common mutation, F508 CFTR, is retained in the endoplasmic reticulum, retrotranslocated into the cytosol, and degraded by the proteasome. In a proteomics screen to identify F508 CFTR interacting proteins, we found that valosin-containing protein (VCP)/p97, a Type II AAA ATPase that is a component of the retrotranslocation machinery, binds F508 CFTR, and this interaction is stabilized by proteasomal inhibition. Since wild-type (WT) CFTR has been reported to be inefficiently processed during biogenesis with as much as 75% of the newly synthesized protein degraded by the proteasome, we examined the VCP interaction in Calu-3, T-84, and 16HBE, three epithelial cell lines that endogenously express WT CFTR. The results indicate that when WT CFTR processing is efficient, as demonstrated in Calu-3 cells, VCP does not interact. Interestingly, overexpression of recombinant WT CFTR in Calu-3 cells results in inefficient processing and VCP interaction, demonstrating that CFTR processing efficiency and the VCP interaction are tightly coupled. Furthermore, induction of ER stress and activation of the unfolded protein response result in inefficient processing of WT CFTR in Calu-3 cells and promote the WT CFTR–VCP interaction. The results support the hypothesis that components of the retrotranslocation machinery such as VCP do not interact with CFTR in epithelial cells that endogenously express WT CFTR, since under normal conditions the processing of the WT protein is efficient.

Key Words: CFTR • p97/VCP • epithelium • ubiquitination • biogenesis

CLINICAL RELEVANCE Currently it is thought that, like mutant F508 cystic fibrosis transmembrane conductance regulator (CFTR), most wild-type (WT) CFTR is degraded during processing. In this study, WT CFTR is degraded only under endoplasmic reticulum stress conditions, providing stronger support for WT CFTR's efficient processing in native epithelium.