Molecular determinants of tuberoinfundibular peptide of 39 residues (TIP39) selectivity for the parathyroid hormone-2 (PTH2) receptor. N-terminal truncation of TIP39 reverses PTH2 receptor/PTH1 receptor binding selectivity.

Abstract

Tuberoinfundibular peptide of 39 residues (TIP39) and the parathyroid hormone-2 (PTH2) receptor form part of an extended family of related signaling molecules that includes the PTH1 receptor, which responds to PTH and PTH-related protein. TIP39 does not appreciably activate the PTH1 receptor, but in this study it is shown to bind the receptor with moderate affinity (59 nm). In this study, we investigated the molecular determinants of both ligand and receptor for the PTH2 receptor selectivity of TIP39 and quantitatively evaluated the role of molecular elements in the binding of TIP39 to the PTH2 and PTH1 receptors. A chimeric receptor composed of the N-terminal extracellular domain of the PTH1 receptor and the remainder (juxtamembrane domain) of the PTH2 receptor (P2-NP1) was fully activated by TIP39 (E(max) = 98% of the rPTH-(1-34), E(max), EC(50) = 2.0 nm). This receptor chimera bound TIP39 with an equivalent affinity to the wild-type PTH2 receptor (2. 3 and 2.0 nm, respectively). The reciprocal chimeric receptor (P1-NP2) was not activated by TIP39 and bound the ligand with an affinity equivalent to that of the PTH1 receptor. Thus, the juxtamembrane receptor domain specifies the signaling and binding selectivity of TIP39 for the PTH2 receptor over the PTH1 receptor. Removing six N-terminal residues of TIP39 eliminated activation of the PTH2 receptor and reduced binding affinity 70-fold. In contrast, this truncation increased affinity for the PTH1 receptor 10-fold, reversing the PTH2/PTH1 receptor binding selectivity and resulting in a high affinity interaction of TIP-(7-39) with the PTH1 receptor (6 nm). These findings can be explained by a strong interaction between the N-terminal region of TIP39 and the juxtamembrane domain of the PTH2 receptor, with the corresponding domain of the PTH1 receptor acting as a selectivity barrier against high affinity binding of TIP39. As a result, TIP-(7-39) is a highly potent, selective antagonist for the PTH1 receptor.