a functional enhanced green fluorescent protein egfptagged angiotensin ii at1a receptor recruits the endogenous galphaq11 protein to the membrane and induces its specific internalization independently of receptorg protein coupling in hek293 cells the angiotensin ii ang ii at1a receptor was tagged at its c terminus with the enhanced green fluorescent protein egfp and the corresponding chimeric cdna was expressed in hek293 cells this tagged receptor presents wildtype pharmacological and signaling properties and can be immunodetected by western blotting and immunoprecipitation using egfp antibodies therefore this egfptagged at1a receptor is the perfect tool for analyzing in parallel the subcellular distributions of the receptor and its interacting g protein and their trafficking using confocal microscopy morphological observation of both the fluorescent receptor and its cognate galphaq11 protein identified by indirect immunofluorescence and the development of a specific software for digital image analysis together allow examination and quantification of the cellular distribution of these proteins before and after the binding of different agonist or antagonist ligands these observations result in several conclusions 1 expression of increasing amounts of the at1a receptor at the cell surface is associated with a progressive recruitment of the cytosolic galphaq11 protein at the membrane 2 internalization of the egfptagged at1a induced by peptide ligands but not nonpeptide ligands is accompanied by a galphaq11 protein intracellular translocation which presents a similar kinetic pattern but occurs predominantly in a different compartment and 3 this galphaq11 protein cellular translocation is dependent on receptor internalization process but not g protein coupling and signal transduction mechanisms as assessed by pharmacological data using agonists and antagonists and the characterization of at1a receptor mutants d74n and delta329 for which the coupling and internalization functions are modified