Sec61p-independent degradation of the tail-anchored ER membrane protein Ubc6p.
Tail-anchored proteins are distinct from other membrane proteins as they are thought to insert into the endoplasmic reticulum (ER) membrane independently of Sec61p translocation pores. These pores not only mediate import but are also assumed to catalyze export of proteins in a process called ER-associated protein degradation (ERAD). In order to examine the Sec61p dependence of the export of tail-anchored proteins, we analyzed the degradation pathway of a tail-anchored ER membrane protein, the ubiquitin-conjugating enzyme 6 (Ubc6p). In contrast to other ubiquitin conjugating enzymes (Ubcs), Ubc6p is naturally short-lived. Its proteolysis is mediated specifically by the unique Ubc6p tail region. Degradation further requires the activity of Cue1p-assembled Ubc7p, and its own catalytic site cysteine. However, it occurs independently of the other ERAD components Ubc1p, Hrd1p/Der3p, Hrd3p and Der1p. In contrast to other natural ERAD substrates, proteasomal mutants accumulate a membrane-bound degradation intermediate of Ubc6p. Most interestingly, mutations in SEC61 do not reduce the turnover of full-length Ubc6p nor cause a detectable accumulation of degradation intermediates. These data are in accordance with a model in which tail-anchored proteins can be extracted from membranes independently of Sec61p.
Walter
J
J
Max-Delbrück-Centrum für Molekulare Medizin, Robert-Rössle-Strasse 10, D-13092 Berlin, Germany.
Urban
J
J
Volkwein
C
C
Sommer
T
T
D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D008025:Ligases / Q000235:genetics / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D000069816:SEC Translocation Channels; D029701:Saccharomyces cerevisiae Proteins; D044763:Ubiquitin-Conjugating Enzymes; D044767:Ubiquitin-Protein Ligases; D014452:Ubiquitins / Q000378:metabolism
C109280:CUE1 protein, S cerevisiae; D002352:Carrier Proteins; C098515:DER1 protein, S cerevisiae; C104622:HRD3 protein, S cerevisiae; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009097:Multienzyme Complexes; D000069816:SEC Translocation Channels; D029701:Saccharomyces cerevisiae Proteins; D014452:Ubiquitins; C489967:UBE2J1 protein, human; C079437:UBE2L3 protein, human; D044763:Ubiquitin-Conjugating Enzymes; C104621:HRD1 protein, S cerevisiae; D044767:Ubiquitin-Protein Ligases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D008025:Ligases