The cytoplasmic tail of the G-protein-coupled receptor for parathyroid hormone and parathyroid hormone-related protein contains positive and negative signals for endocytosis

Abstract

The present studies were done to evaluate the role of the cytoplasmic tail of the G-protein-coupled receptor for parathyroid hormone (PTH) and PTH- related protein (PTHrP) in the endocytosis of agonist-occupied receptors. PTH/PTHrP receptor mutants progressively truncated from the C terminus were expressed in COS-7 cells, and their ability to internalize 125I- PTHrP(134)amide was determined. Most of the C-terminal tail (91 of 127 residues) could be deleted without affecting internalization. However, further truncation removing residues 475-494 resulted in a 50-60% decrease in ligand internalization. A mutant with an internal deletion of these 20 amino acids showed a similar reduction in internalization, confirming the presence of a positive endocytic signal. No additional positive signals were found in the membrane-proximal region of the tail. However, alanine mutagenesis of the membrane-proximal residues 459-461 (EVQ → AAA) resulted in a mutant PTH/PTHrP receptor displaying a 40% increase in ligand endocytosis, indicating that EVQ functions as a negative signal. Treatment of COS-7 cells with hypertonic sucrose (to disrupt clathrin lattices) markedly suppressed (by >80%) PTH/PTHrP receptor internalization. These results demonstrate the presence of both positive and negative endocytic signals in the membrane- proximal cytoplasmic tail of the PTH/PTHrP receptor and suggest that these signals regulate the ability of the receptor to accumulate in clathrin- coated pits.