Presence of a Plasma Membrane Targeting Sequence in the Amino-Terminal Region of the Rat Somatostatin Receptor 3

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Abstract
Although peptide hormone receptors commonly exert their actions at the plasma membrane the cellular mechanisms that route the receptor proteins to the cell surface during biosynthesis are not well characterized. Here we report on the identification of a plasma membrane targeting sequence of rat somatostatin receptor subtype 3. While type 3 somatostatin receptors are present almost exclusively at the cell surface, type 1 receptors localize in addition largely in intracellular vesicular compartments. Chimeric receptors were constructed between rat somatostatin receptors 3 and 1. They were tagged by recombinant DNA techniques with a herpes simplex virus glycoprotein D epitope at the carboxyl-termini to facilitate their detection using fluorescence microscopic methods. Following transfection of the constructs in human embryonic kidney and rat insulinoma cells the chimeric receptors were analyzed by indirect immunofluorescence using anti-epitope monoclonal antibody and confocal laser scanning microscopy. The results demonstrate that the amino-terminal domain of somatostatin receptor 3 suffices to guide chimeric receptors to the cell surface. In marked contrast, chimeric receptors that lack this sequence but contain instead the amino-terminus of somatostatin type 1 receptor localize in an intracellular vesicular compartment.