Non-Homologous Sequences of Parathyroid Hormone and the Parathyroid Hormone Related Peptide Bind to a CommonReceptor on ROS 17/2.8 Cells

Abstract

We and others have recently shown that amino terminal sequences of parathyroid hormones (PTH) and parathyroid hormone related peptide (PTHrP), which share a 62% homology within the first 13 residues, bind to the same receptor on ROS 17/2.8 cells. The remaining PTHrP sequence is markedly different from PTH, suggesting that receptor binding may be dependent on the first 13 amino acids of either peptide. However, since the amino acid residues 14-34 have previously been recognized as an important binding domain for PTH, conformational similarity within this portion''s secondary structure of both peptides could contribute to their capacity to bind to the same receptor. To test this hypothesis, we synthesized [Tyr36, Cys38]PTHrP-(14-38) and [Tyr34]bPTH(14-34)NH2, and studied binding of both peptides to the common PTH/PTHrP receptor on ORS 17/2.8 cells. Radioiodinated, HPLC-purified [Nle8,18, Tyr34]bPTH(1-34)NH2 (NlePTH) and [Tyr36]PTHrP-(1-36)NH2 were used to functionally define receptor binding requirements. [Tyr36, Cys38]PTHrP(14-38) and [Tyr34]bPTH(14-34)NH2 competed with 125I-NlePTH for binding sites on ROS 17/2.8 cells with apparent Kds of 10 .mu.M and 50 .mu.M respectively. Both peptides also competed with 125I-[Tyr36]PTHrP(1-36)NH2 with apparent Kds of 30 .mu.M and 10 .mu.M respectively. In the same assay system, NlePTH and [Tyr36,Cys38]PTHrP(1-38) inhibited binding of either radioiodinated ligand with apparent Kds of 0.3 and 1.0 nM. These studies indicate that although [Tyr34]bPTH(14-34)NH2 and [Tyr36,Cys38]PTHrP(14-38) share virtually no sequence homology, their secondary structures must be sufficiently similar to permit binding to a common PTH/PTHrP receptor.