Evidence that endoplasmic reticulum (ER)-associated degradation of cystic fibrosis transmembrane conductance regulator is linked to retrograde translocation from the ER membrane

The ubiquitin-proteasome pathway has been implicated in the degradation of newly synthesized, misfolded and unassembled proteins in the endoplasmic reticulum (ER), Using a cell-free reticulocyte lysate system we have examined the relationship between biosynthesis and ER-associated degradation of the cystic fibrosis transmembrane conductance regulator (CFTR), a polytopic protein with 12 predicted transmembrane segments. Our results provide direct evidence that length, glycosylated and membrane-integrated CFTR is a substrate for degradation and that degradation involves polyubiquitination and cytosolic proteolytic activity, CFTR ubiquitination was both temperature- and ATP-dependent, Degradation was significantly inhibited by EDTA, apyrase, and the proteasome inhibitors hemin and MG132, Degradation was inhibited to a lesser extent by clasto-lactacystin beta-lactone, ALLN, and N-alpha-tosyl-L-phenylalanine chloromethyl ketone and was relatively unaffected by lactacystin and N-tosyl lysyl chloromethyl ketone. In the presence of hemin, polyubiquitinated CFTR remained tightly associated with ER microsomes. However, membrane-bound ubiquitinated CFTR could be subsequently degraded into trichloroacetic acid-soluble fragments upon incubation in hemin-free, ATP-containing lysate, Thus ER-associated degradation of CFTR occurs via a membrane-bound, rather than cytosolic, intermediate and likely involves recruitment of degradation machinery to the ER membrane, Our data suggest a model in which the degradation of polytopic proteins such as CFTR is coupled to retrograde translocation and removal of the polypeptide from the lipid bilayer.