ΔF508 CFTR Localizes in the Endoplasmic Reticulum–Golgi Intermediate Compartment in Cystic Fibrosis Cells

Abstract

We have studied the localization of mutant cystic fibrosis transmembrane regulator ΔF508CFTR in pancreatic adenocarcinoma cells (CFPAC), which naturally express the mutant protein. Our goal was to investigate whether ΔF508CFTR is strictly retained in the endoplasmic reticulum (ER) or alternatively whether it can be transported beyond the ER and reach the endoplasmic reticulum–Golgi intermediate compartment (ERGIC). This compartment, defined by the presence of the 53-kDa protein ERGIC-53, was identified by subcellular fractionation and by immunofluorescence. Part of the ΔF508CFTR population and ERGIC-53 showed similar distributions in membrane fractions analyzed on Nycodenz density gradients. Both proteins were present in density fractions distinct from the ones containing the ER marker proteins calnexin and Sec61. Immunofluorescence microscopy of CFPAC cells revealed some colocalization of ΔF508CFTR with ERGIC-53. Following incubation of CFPAC cells at 15°C, a condition known to block ER to Golgi transport, both ERGIC-53 and ΔF508CFTR subcellular localizations were altered. By contrast, this temperature shift had no effect on the localization of the ER marker Sec61. Our observations indicate that the abnormal protein ΔF508CFTR can leak out of the ER and reach the ERGIC. These results support the idea that this intermediate compartment plays a role in the trafficking events leading to retention and finally degradation of the misfolded ΔF508CFTR protein.