Degradation of a Mutant Secretory Protein, α1-Antitrypsin Z, in the Endoplasmic Reticulum Requires Proteasome Activity

Abstract

Degradation of proteins that are retained in the quality control apparatus of the endoplasmic reticulum (ER) has been attributed to a third proteolytic system, distinct from the lysosomal and the cytoplasmic ubiquitindependent proteosomal proteolytic pathways. However, several recent studies have shown that ER degradation of a mutant membrane protein, CFTR⌬F508, is at least in part mediated from the cytoplasmic side by the 26 S proteasome. In this study, we examined the possibility that ER degradation of mutant secretory protein ␣ 1 -antitrypsin (␣ 1 -AT) Z, the mutant protein associated with infantile liver disease and adult-onset emphysema of ␣ 1 -AT deficiency, is mediated by the proteasome. The results show that a specific proteasome inhibitor, lactacystin, inhibits ER degradation of ␣1-ATZ in transfected human fibroblast cell lines and in a cell-free microsomal translocation system. Although it is relatively easy to conceptualize how a transmembrane protein like CFTR⌬F508 might be accessible on the cytoplasmic aspect of the ER membrane for ubiquitination and degradation by the proteasome, it is more difficult to conceptualize how this might occur for a luminal polypeptide. The results show that, once within the lumen of the ER, ␣ 1 -ATZ interacts with the transmembrane molecular chaperone calnexin and specifically induces the polyubiquitination of calnexin. The results, therefore, provide evidence that the proteasome, from its cytoplasmic localization, induces the degradation of the luminal ␣ 1 -ATZ molecule by first attacking the cytoplasmic tail of calnexin molecules that are associated with ␣ 1 -ATZ.

Autophagic disposal of the aggregation-prone protein that causes liver inflammation and carcinogenesis in α-1-antitrypsin deficiency

“…[16][17][18] It is still not clear how the ATZ is transported from the lumen of the ER to the proteasome in the cytoplasm. Although retrograde translocation through the microsomal import channel has been proposed for some ER proteins, there is growing evidence for a mechanism in which the proteasome, as part of a multiprotein complex that forms a platform on the cytosolic side of the ER membrane, directly mediates extraction of substrates from the ER membrane.…”

“…Although retrograde translocation through the microsomal import channel has been proposed for some ER proteins, there is growing evidence for a mechanism in which the proteasome, as part of a multiprotein complex that forms a platform on the cytosolic side of the ER membrane, directly mediates extraction of substrates from the ER membrane. 19,20 Nevertheless, these early studies in yeast, mammalian cell lines and cell-free systems all indicated that the proteasomal pathway could not fully account for disposal of ATZ [16][17][18][21][22][23] -that is in the absence of proteasomal activity there was clear-cut residual and time-dependent degradation of ATZ.…”

Alpha-1-Antitrypsin Deficiency: A New Paradigm for Hepatocellular Carcinoma in Genetic Liver Disease *

“…Studies in many systems have indicated that the proteosomal system is involved. [17][18][19][20][21] This involvement appears to include both the classical ubiquitin-dependent proteosomal mechanism as well as a ubiquitin-independent proteosomal mechanism. 19 However, exactly how ␣1ATZ on the luminal side of the ER membrane is accessed by the proteosome from the cytoplasm is not clear.…”

Endoplasmic Reticulum Quality Control of Oligomeric Membrane Proteins: Topogenic Determinants Involved in the Degradation of the Unassembled Na,K-ATPase α Subunit and in Its Stabilization by β Subunit Assembly

“…For instance, interaction with the molecular ER chaperone BiP (binding protein) appears to be necessary for the retrograde transport and proteasomal degradation of soluble proteins such as mutant yeast carboxypeptidase ysc Y (Plemper et al, 1997) but not for that of polytopic membrane proteins such as the mutant ATP-binding cassette transporter Pdr5* . In the same line, interaction with calnexin, another ER chaperone, facilitates the degradation of the soluble mutant ␣1-anti-trypsin Z (Qu et al, 1996) and prepro-␣ factor (McCracken and Brodsky, 1996) but is not important for the degradation of the polytopic cystic fibrosis conductance regulator (CFTR) . On the other hand, soluble and membrane proteins may share common mechanisms for recognition and targeting to proteasomal degradation.…”