Evidence that endoplasmic reticulum (ER)-associated degradation of cystic fibrosis transmembrane conductance regulator is linked to retrograde translocation from the ER membrane
Xm. Xiong et al., Evidence that endoplasmic reticulum (ER)-associated degradation of cystic fibrosis transmembrane conductance regulator is linked to retrograde translocation from the ER membrane, J BIOL CHEM, 274(5), 1999, pp. 2616-2624
The ubiquitin-proteasome pathway has been implicated in the degradation of
newly synthesized, misfolded and unassembled proteins in the endoplasmic re
ticulum (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 pr
otein with 12 predicted transmembrane segments. Our results provide direct
evidence that length, glycosylated and membrane-integrated CFTR is a substr
ate for degradation and that degradation involves polyubiquitination and cy
tosolic proteolytic activity, CFTR ubiquitination was both temperature- and
ATP-dependent, Degradation was significantly inhibited by EDTA, apyrase, a
nd 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 lacta
cystin and N-tosyl lysyl chloromethyl ketone. In the presence of hemin, pol
yubiquitinated CFTR remained tightly associated with ER microsomes. However
, membrane-bound ubiquitinated CFTR could be subsequently degraded into tri
chloroacetic acid-soluble fragments upon incubation in hemin-free, ATP-cont
aining 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 w
hich the degradation of polytopic proteins such as CFTR is coupled to retro
grade translocation and removal of the polypeptide from the lipid bilayer.