The Third Intracellular Loop of the Human Somatostatin Receptor 5 Is Crucial for Arrestin Binding and Receptor Internalization after Somatostatin Stimulation

Somatostatin (SS) is a widely distributed polypeptide that exerts inhibitory effects on hormone secretion and cell proliferation by interacting with five different receptors (SST1-SST5). β-Arrestins have been implicated in regulating SST internalization, but the structural domains mediating this effect are largely unknown. The aim of this study was to characterize the intracellular mechanisms responsible for internalization of human SST5 in the rat pituitary cell line GH3 and to identify the SST5 structural domains involved in this process. To this purpose we evaluated, by fluorescence microscopy and biochemical assay, the ability of wild-type, progressive C-terminal truncated and third cytoplasmatic loop mutants SST5-DsRed to associate with β-arrestin-enhanced green fluorescent protein and to internalize under SS28 stimulation. The truncated mutants were comparable to the wild-type receptor with respect to recruitment of β-arrestin-2 and internalization, whereas the third loop mutants R240W, S242A, and T...