Phosphorylation at the cyclin-dependent kinases site (Thr85) 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 is accumulating for it to act in an intracrine manner. We have determined that PTHrP localizes to the nucleus at the G(1) phase of the cell cycle and is transported to the cytoplasm when cells divide. PTHrP contains a putative nuclear localization sequence (NLS) (residues 61-94) similar to that of SV40 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 phosphorylated by CDC2-CDK2 and phosphorylation was cell cycle-dependent. Mutation of Thr(85) to Ala(85) resulted in nuclear accumulation of PTHrP, while mutation to GIu(85) to mimic a phosphorylated residue resulted in localization of PTHrP to the cytoplasm. Combined, the data demonstrate that the intracellular localization of PTHrP is phosphorylation- and cell cycle-dependent, and such control further supports a potential intracellular role (10, 34, 35) for PTHrP.