Phosphorylation at the cyclin-dependent kinases site (Thr(85)) of parathyroid hormone-related protein negatively regulates its nuclear localization

Parathyroid hormone-related protein (PTHrP) is expressed by a wide variety of cells and is considered to act as a secreted factor; however, evidence i s accumulating for it to act in an intracrine manner. We have determined th at PTHrP localizes to the nucleus at the G(1) phase of the cell cycle and i s transported to the cytoplasm when cells divide. PTHrP contains a putative nuclear localization sequence (NLS) (residues 61-94) similar to that of SV 40 T-antigen, which may be implicated in the nuclear import of the molecule . We identified that Thr(85) immediately prior to the NLS of PTHrP was phos phorylated by CDC2-CDK2 and phosphorylation was cell cycle-dependent. Mutat ion of Thr(85) to Ala(85) resulted in nuclear accumulation of PTHrP, while mutation to GIu(85) to mimic a phosphorylated residue resulted in localizat ion of PTHrP to the cytoplasm. Combined, the data demonstrate that the intr acellular localization of PTHrP is phosphorylation- and cell cycle-dependen t, and such control further supports a potential intracellular role (10, 34 , 35) for PTHrP.