Identification of AUF1 as a Parathyroid Hormone mRNA 3′-Untranslated Region-binding Protein That Determines Parathyroid Hormone mRNA Stability*

Parathyroid hormone (PTH) mRNA levels are post-transcriptionally increased by hypocalcemia and decreased by hypophosphatemia, and this is mediated by cytosolic proteins binding to the PTH mRNA 3′-untranslated region (UTR). The same proteins are also present in other tissues, such as brain, but only in the parathyroid is their binding regulated by calcium and phosphate. The function of the PTH mRNA 3′-UTR-binding proteins was studied using an in vitro degradation assay. Competition for the parathyroid-binding proteins by excess unlabeled 3′-UTR destabilized the full-length PTH transcript in this assay, indicating that these proteins protect the RNA from RNase activity. The PTH RNA 3′-UTR-binding proteins were purified by RNA affinity chromatography of rat brain S-100 extracts. The eluate from the column was enriched in PTH RNA 3′-UTR binding activity. Addition of eluate to the in vitro degradation assay with parathyroid protein extracts stabilized the PTH transcript. A major band from the eluate at 50 kDa was sequenced and was identical to AU-rich binding protein (AUF1). Recombinant AUF1 bound the full-length PTH mRNA and the 3′-UTR. Added recombinant AUF1 also stabilized the PTH transcript in thein vitro degradation assay. Our results show that AUF1 is a protein that binds to the PTH mRNA 3′-UTR and stabilizes the PTH transcript.