Non-conventional Trafficking of the Cystic Fibrosis Transmembrane Conductance Regulator through the Early Secretory Pathway

Abstract

The mechanism(s) of cystic fibrosis transmembrane conductance regulator (CFTR) trafficking from the endoplasmic reticulum (ER) through the Golgi apparatus, the step impaired in individuals afflicted with the prevalent CFTR-⌬F508 mutation leading to cystic fibrosis, is largely unknown. Recent morphological observations suggested that CFTR is largely absent from the Golgi in situ (Bannykh, S. I., Bannykh, G. I., Fish, K. N., Moyer, B. D., Riordan, J. R., and Balch, W. E. (2000) Traffic 1, 852-870), raising the possibility of a novel trafficking pathway through the early secretory pathway. We now report that export of CFTR from the ER is regulated by the conventional coat protein complex II (COPII) in all cell types tested. Remarkably, in a cell type-specific manner, processing of CFTR from the core-glycosylated (band B) ER form to the complex-glycosylated (band C) isoform followed a non-conventional pathway that was insensitive to dominant negative Arf1, Rab1a/Rab2 GTPases, or the SNAp REceptor (SNARE) component syntaxin 5, all of which block the conventional trafficking pathway from the ER to the Golgi. Moreover, CFTR transport through the non-conventional pathway was potently blocked by overexpression of the late endosomal target-SNARE syntaxin 13, suggesting that recycling through a late Golgi/endosomal system was a prerequisite for CFTR maturation. We conclude that CFTR transport in the early secretory pathway can involve a novel pathway between the ER and late Golgi/endosomal compartments that may influence developmental expression of CFTR on the cell surface in polarized epithelial cells.

Regulation of anterograde transport of adrenergic and angiotensin II receptors by Rab2 and Rab6 GTPases

“…It has also been demonstrated that manipulation of Rab6 function through expressing its GTP-bound, GDP-bound or guanidine nucleotide deficient mutants inhibits transport of the plasma membrane proteins hemagglutinin protein and CFTR in mammalian cells and rhodopsin in Drosophila [42,45,46]. Consistent with the role of Rab6 in the plasma membrane targeting, our data indicate that Rab6 is required for the transport to the cell surface of some GPCRs.…”

“…For example, expression of GTP-bound Rab2Q65L inhibits the maturation of vesicular stomatitis viral glycoprotein (VSVG) [36]. Interestingly, the trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) through the early secretory pathway is independent of Rab2 function [45]. Our data demonstrated that transient expression of Rab2Q65L significantly inhibited the cell-surface expression of α 2B -AR and β 2 -AR as determined by ligand binding of intact cells and membrane preparations.…”

Cystic Fibrosis Transmembrane Conductance Regulator Trafficking Is Mediated by the COPI Coat in Epithelial Cells

“…We found that ␤-COP co-immunoprecipitate with both WT-and ⌬F508-CFTR in transfected human embryonic kidney 293T cells. 3 However, the transient or stable nature of their association remains to be determined. Interestingly, CFTR harbors four RXR motifs and deletion of these motifs allows the rescue of the processing of the ⌬F508-CFTR mutant (35).…”

“…Recent studies suggested that CFTR processing is independent of components involved in conventional trafficking (2). It was reported that in some cells like baby hamster kidney cells, CFTR follows a non-conventional trafficking pathway where CFTR is transported from the ER to late Golgi/endosomal compartments before recycling back to Golgi compartments for oligosaccharide processing (3). Earlier studies show that the COPII machinery is required for CFTR to be exported out of the ER (4,5).…”

“…Yoo et al (3) found that in non-epithelial cells, CFTR trafficking could be COPI-dependent (i.e. HeLa and human embryonic kidney 293 cells) or COPI-independent (i.e.…”