The (1-14) fragment of parathyroid hormone (PTH) activates intact and amino-terminally truncated PTH-1 receptors

Recent mutagenesis and cross-linking studies suggest that residues in the c arboxyl-terminal portion of PTH(1-34)interact with the amino-terminal extra cellular domain of the receptor and thereby contribute strongly to binding energy; and that residues in the amino-terminal portion of the ligand inter act with the receptor region containing the transmembrane helices and extra cellular loops and thereby induce second messenger signaling. We investigat ed the latter component of this hypothesis using the short amino-terminal f ragment PTH(1-14) and a truncated rat PTH-1 receptor (r Delta Nt) that lack s most of the amino-terminal extracellular domain. The binding of PTH(1-14) to LLC-PK1, or COS-7 cells transfected with the intact PTH-1 receptor was too weak to detect; however, PTH(1-14) dose-dependently stimulated cAMP for mation in these cells over the dose range of 1-100 mu M. PTH(1-14) also sti mulated cAMP formation in COS-7 cells transiently transfected with r Delta Nt, and its potency with this receptor was nearly equal to that seen with t he intact receptor. In contrast, PTH(1-34) was similar to 100-fold weaker i n potency with r Delta Nt than it was with the intact receptor. Alanine sca nning of PTH(1-14)revealed that for both the intact and truncated receptors , the 1-9 segment of PTH forms a critical receptor activation domain. Taken together, these results demonstrate that the amino-terminal portion of PTH (1-34) interacts with the juxtamembrane regions of the PTH-1 receptor and t hat these interactions are sufficient for initiating signal transduction.