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1-antitrypsin Z in the endoplasmic
reticulum
Departments of 1 Pediatrics and 4 Cell Biology and Physiology, Washington University School of Medicine, and 2 Division of Gastroenterology and Nutrition, Children's Hospital, St. Louis, Missouri 63110; and 3 Department of Biochemistry and Molecular Biology, University of Massachusetts, Boston, Massachusetts 01655
A delay in
intracellular degradation of the mutant
1-antitrypsin
(
1AT)Z molecule is associated with greater retention
within the endoplasmic reticulum (ER) and susceptibility to liver
disease in a subgroup of patients with
1AT deficiency.
Recent studies have shown that
1ATZ is ordinarily
degraded in the ER by a mechanism that involves the proteasome, as
demonstrated in intact cells using human fibroblast cell lines
engineered for expression of
1ATZ and in a cell-free
microsomal translocation assay system programmed with purified
1ATZ mRNA. To determine whether the ubiquitin system is
required for proteasomal degradation of
1ATZ and whether
specific components of the ubiquitin system can be implicated, we have
now used two approaches. First, we overexpressed a dominant-negative
ubiquitin mutant (UbK48R-G76A) by transient transfection in the human
fibroblast cell lines expressing
1ATZ. The results
showed that there was marked, specific, and selective inhibition of
1ATZ degradation mediated by UbK48R-G76A, indicating that the ubiquitin system is at least in part involved in ER
degradation of
1ATZ. Second, we subjected reticulocyte
lysate to DE52 chromatography and tested the resulting
well-characterized fractions in the cell-free system. The results
showed that there were both ubiquitin-dependent and -independent
proteasomal mechanisms for degradation of
1ATZ and that
the ubiquitin-conjugating enzyme E2-F1 may play a role in the
ubiquitin-dependent proteasomal mechanism.
1-antitrypsin deficiency; liver disease; emphysema; quality- control apparatus; protein degradation; endoplasmic reticulum
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