Differential interaction of plakoglobin and beta-catenin with the ubiquitin-proteasome system.
Beta-catenin and plakoglobin are closely related armadillo family proteins with shared and distinct properties; Both are associated with cadherins in actin-containing adherens junctions. Plakoglobin is also found in desmosomes where it anchors intermediate filaments to the desmosomal plaques. Beta-catenin, on the other hand, is a component of the Wnt signaling pathway, which is involved in embryonic morphogenesis and tumorigenesis. A key step in the regulation of this pathway involves modulation of beta-catenin stability. A multiprotein complex, regulated by Wnt, directs the phosphorylation of beta-catenin and its degradation by the ubiquitin-proteasome system. Plakoglobin can also associate with members of this complex, but inhibition of proteasomal degradation has little effect on its levels while dramatically increasing the levels of beta-catenin. Beta-TrCP, an F-box protein of the SCF E3 ubiquitin ligase complex, was recently shown to play a role in the turnover of beta-catenin. To elucidate the basis for the apparent differences in the turnover of beta-catenin and plakoglobin we compared the handling of these two proteins by the ubiquitin-proteasome system. We show here that a deletion mutant of beta-TrCP, lacking the F-box, can stabilize the endogenous beta-catenin leading to its nuclear translocation and induction of beta-catenin/LEF-1-directed transcription, without affecting the levels of plakoglobin. However, when plakoglobin was overexpressed, it readily associated with beta-TrCP, efficiently competed with beta-catenin for binding to beta-TrCP and became polyubiquitinated. Fractionation studies revealed that about 85% of plakoglobin in 293 cells, is Triton X-100-insoluble compared to 50% of beta-catenin. These results suggest that while both plakoglobin and beta-catenin can comparably interact with beta-TrCP and the ubiquitination system, the sequestration of plakoglobin by the membrane-cytoskeleton system renders it inaccessible to the proteolytic machinery and stabilizes it.
Sadot
E
E
Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot, Israel.
Simcha
I
I
Iwai
K
K
Ciechanover
A
A
Geiger
B
B
Ben-Ze'ev
A
A
D000818:Animals; D001692:Biological Transport; D016466:CHO Cells / Q000378:metabolism; D002451:Cell Compartmentation; D006224:Cricetinae; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D003598:Cytoskeletal Proteins / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D051180:Desmoplakins; D003907:Dexamethasone / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D051821:Lymphoid Enhancer-Binding Factor 1; D009097:Multienzyme Complexes / Q000378:metabolism*; D017830:Octoxynol / Q000737:chemistry; D046988:Proteasome Endopeptidase Complex; D011994:Recombinant Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D015534:Trans-Activators; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014452:Ubiquitins / Q000378:metabolism*; D051176:beta Catenin; D044844:beta-Transducin Repeat-Containing Proteins; D051185:gamma Catenin
C471644:BTRC protein, human; C495270:CTNNB1 protein, human; D003598:Cytoskeletal Proteins; D004268:DNA-Binding Proteins; D051180:Desmoplakins; D051821:Lymphoid Enhancer-Binding Factor 1; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D014452:Ubiquitins; D051176:beta Catenin; D044844:beta-Transducin Repeat-Containing Proteins; D051185:gamma Catenin; D003907:Dexamethasone; D017830:Octoxynol; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D019204:GTP-Binding Proteins