Increase in cell-surface localization of parathyroid hormone receptor by cytoskeletal protein 4.1G

The cell-surface localization of GPCRs (G-protein-coupled receptors) has emerged as one of critical factors of the GPCR-mediated signal transduction. It has been reported that the C-termini of GPCRs contain the sequences for sorting the receptors to cell surface. In the present study, we have searched for proteins that interact with the C-terminus of PTH (parathyroid hormone)/ PTH-related protein receptor (PTHR) by using the yeast two-hybrid system, and identified a cytoskeletal protein 4.1G (general-type 4.1 protein) as an interactant with the C-terminus. Immunohistochemical study revealed that both PTHR and 4.1G were co-localized on plasma membranes, when they were transiently expressed in COS-7 cells. When 4.1G or the C-terminal domain of 4.1G (4.1G-CTD), a dominant-negative form of 4.1G, was co-expressed with PTHR in COS-7 cells, 4.1G, but not 4.1G-CTD, facilitated the cell-surface localization of PTHR, determined by cell-surface biotinylation assay. PTH-(1-34) caused phosphorylation of ERK (extracellular-signal-regulated kinase) 1/2 in PTHR-expressed cells mainly mediated through EGF (epidermal growth factor) receptor. The phosphorylation was enhanced by the expression of 4.1G, but not 4.1G-CTD. PTH-(1-34) elevated [Ca²⁺]_i (intracellular Ca²⁺ concentration) independent of EGF receptor activation, and the elevation was enhanced by the expression of 4.1G, but not 4.1G-CTD. These data indicate that 4.1G facilitates the cell-surface localization of PTHR through its interaction with the C-terminus of the receptor, resulting in the potentiation of PTHR-mediated signal transduction.