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 fac tor for hypercalcemia associated with malignancy. In addition to the e ndocrine functions attributed to secretory forms of the peptide, PTHrP also plays a local role as a mediator of cellular growth and differen tiation 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 t hat the proteasome is responsible for the degradation of the endoplasm ic 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 pr o-PTHrP suggesting that degradation does not occur in the endoplasmic reticulum (ER) lumen. Subcellular fractionation of both lactacystin an d brefeldin A-treated cells indicated that accumulated pro-PTHrP resid es in microsomal fractions with a portion of the protein exposed to th e 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 cytopla smic side of the ER membrane where it can undergo ubiquitination and d egradation by the proteasome.