G protein-coupled receptors (GPCRs) are seven transmembrane receptors with an N-terminus in the extracellular region and C-terminus in the intracellular region. When an agonist binds to a GPCR, a signal is transduced into a cell through the activation of trimeric G proteins. Recently, it has been shown that the activities of GPCRs are regulated by multiple mechanisms. One of the mechanisms is regulation through the binding proteins to the carboxy (C)-terminus of GPCRs. In the present study, the binding partners for the C-terminus of the parathyroid hormone receptor (PTHR) and thromboxane A 2 receptor (TP) were searched for using yeast two-hybrid screening, and the functions of these proteins were investigated. We identified t-complex testis expressed-1 (Tctex-1) and 4.1G as associated proteins for the PTHR. Tctex-1 is one of the light chains of cytoplasmic dynein, which is a motor protein across microtubles. We found that Tctex-1 was involved in agonist-induced internalization of the PTHR. 4.1G, a cytoskeletal protein, facilitated the cell surface localization of the PTHR and augmented PHTR-mediated signal transduction. TPs consists of two splicing variants, TPα and TPβ. As a result of yeast two-hybrid screening, two proteasomal proteins, proteasome activator PA28γ and proteasome subunit α7, were identified as direct interacting proteins for TPβ. TPβ has a tendency to be retained in the intracellular compartment, probably due to its binding to proteasomes. We also demonstrated that TPα and TPβ formed heterodimers, and the signal transduction through TPα was reduced by the formation of heterodimers. In conclusion, the proteins bound to GPCRs may regulate the intracellular traffic of GPCRs.
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