Our group identified angiotensin II type 1 (AT1) receptor-associated protein (ATRAP) in a yeast two-hybrid screen for proteins that bind to the carboxyl-terminal cytoplasmic domain of the AT1. In this work, we characterize ATRAP as a transmembrane protein localized in intracellular trafficking vesicles and plasma membrane that functions as a modulator of angiotensin II-induced signal transduction. ATRAP contains three hydrophobic domains at the amino-terminal end of the protein, encompassing the amino acid residues 14–36, 55–77, and 88–108 and a hydrophilic cytoplasmic carboxyl-terminal tail from residues 109–161. Endogenous and transfected ATRAP cDNA shows a particulate distribution; electron microscopy reveals the presence of ATRAP in prominent perinuclear vesicular membranes; and colocalization analysis by immunofluorescence shows that ATRAP colocalizes in an intracellular vesicular compartment corresponding to endoplasmic reticulum, Golgi, and endocytic vesicles. Real-time tracking of ATRAP vesicles shows constitutive translocation toward the plasma membrane. Using epitope-tagged forms of ATRAP at either the amino or carboxyl end of the molecule, we determined the orientation of the amino end as being outside the cell. Mutant forms of ATRAP lacking the carboxyl end are unable to bind to the AT1 receptor, leading to the formation of prominent perinuclear vesicle clusters. Functional analysis of the effects of ATRAP on angiotensin II-induced AT1 receptor signaling reveals a moderate decrease in the generation of inositol lipids, a marked decrease in the angiotensin II-stimulated transcriptional activity of the c-fos promoter luciferase reporter gene, and a decrease in cell proliferation.