Histidine at position 5 is the specificity ''switch'' between two parathyroid hormone receptor subtypes
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The PTH and PTH-related protein (PTHrP) system consists of two hormones, at least two G protein-coupled seven-transmembrane domain receptors, and at least two intracellular signal transduction pathways for each receptor. The PTH/PTHrP receptor is present in the conventional target tissues of PTH action, namely kidney and bone. Both PTH and PTHrP bind to and activate the PTH/PTHrP receptor with equal affinity and efficacy. The newly discovered receptor, termed the human (h) PTH2 receptor, has 70% homology with the PTH/PTHrP receptor, but is found predominantly in brain and pancreas. It interacts selectively with PTH and not with PTHrP. PTH and PTHrP differ in several positions, including position 5 (Ile in PTH; His in PTHrP). To define the role of position 5 in receptor selectivity, we designed and synthesized a series of hybrid analogs containing specific elements of both the PTH and PTHrP sequences. Using human cell lines stably expressing either human receptor subtype, we evaluated the biological profile of the hybrids in assays of receptor binding and action. Both point-mutated hybrids, [Ile5]- (1-34) and [His5]PTH-(1-34), bind to and stimulate cAMP accumulation and the release of cytosolic free calcium in HEK293/C-21, a clonal human embryonic kidney cell line stably expressing the recombinant hPTH/PTHrP receptor. However, only [IIe5]PTHrP-(1-34), and not [His5]PTH-(1-34), binds to and stimulates cAMP accumulation and the release of cytosolic free calcium in HEK293/BP-16, a clonal human embryonic kidney cell line stably expressing the recombinant hPTH2 receptor. The segmental hybrid PTHrP-(1-14)-PTH-(15-34) binds to and activates the hPTH/PTHrP receptor, but not the hPTH2 receptor, similar to the biological profile of His5-containing ligands: PTHrP-(1-34) and [His5]PTH-(1-34). Exchanging Ile5 for His5 in the segmental hybrid produces the analog [Ile5]PTHrP-(1-14)-PTH-(15-34), which interacts with both receptor subtypes. We conclude that His5 in PTHrP is the major structural determinant of receptor subtype specificity in the hPTH/PTHrP and hPTH2 two-receptor system. The mechanism of the specificity 'switch' remains to be elucidated, but may result from a subtle perturbation of the bioactive conformation and/or from a direct steric hindrance at the hPTH2 receptor- ligand interface created by histidine at position 5. The hPTH2, but not the hPTH/PTHrP, receptor can discriminate between the two hormones based on the structural differences generated at position 5.
