Proparathyroid hormone-related protein is associated with the chaperone protein BiP and undergoes proteasome-mediated degradation.
Parathyroid hormone-related peptide (PTHrP) is an important causal factor for hypercalcemia associated with malignancy. In addition to the endocrine functions attributed to secretory forms of the peptide, PTHrP also plays a local role as a mediator of cellular growth and differentiation presumably at least in part through intracellular pathways. In studying the post-translational regulation of PTHrP, we observed that PTHrP was conjugated to multiple ubiquitin moieties. We report here that the proteasome is responsible for the degradation of the endoplasmic reticulum-associated precursor, pro-PTHrP. Cells expressing prepro-PTHrP and exposed to lactacystin accumulate pro-PTHrP assessed by anti-pro specific antibodies. Brefeldin A-treated cells also accumulate pro-PTHrP suggesting that degradation does not occur in the endoplasmic reticulum (ER) lumen. Subcellular fractionation of both lactacystin and brefeldin A-treated cells indicated that accumulated pro-PTHrP resides in microsomal fractions with a portion of the protein exposed to the cytosolic side of the ER membrane as assessed by protease protection experiments. Immunoprecipitation and Western blot analysis identified pro-PTHrP in association with the ER molecular chaperone protein BiP. We conclude that pro-PTHrP from the ER can gain access to the cytoplasmic side of the ER membrane where it can undergo ubiquitination and degradation by the proteasome.
Calcium Research Laboratory, Department of Medicine, McGill University, Montreal, Quebec H3A 1A1, Canada.
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Meerovitch
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Wing
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Goltzman
D000111:Acetylcysteine / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D020126:Brefeldin A; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D003517:Cyclopentanes / Q000494:pharmacology; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D006868:Hydrolysis; D018942:Macrolides; D018832:Molecular Chaperones / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism*; D044162:Parathyroid Hormone-Related Protein; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D011498:Protein Precursors / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism
D000900:Anti-Bacterial Agents; D002352:Carrier Proteins; D003517:Cyclopentanes; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D018942:Macrolides; D018832:Molecular Chaperones; D009097:Multienzyme Complexes; D044162:Parathyroid Hormone-Related Protein; D011498:Protein Precursors; D011506:Proteins; C067713:lactacystin; D020126:Brefeldin A; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D000111:Acetylcysteine