Agonist treatment of cells expressing the chemokine receptor, CXCR2, induces receptor phosphorylation and internalization through a dynamin-dependent mechanism. In the present study, we demonstrate that a carboxyl terminus-truncated mutant of CXCR2 (331T), which no longer undergoes agonist-induced phosphorylation, continues to undergo ligand-induced internalization in HEK293 cells. This mutant receptor exhibits reduced association with β-arrestin 1 but continues to exhibit association with adaptin 2 α and β subunits. Replacing Leu320-321 and/or Ile323-Leu324 with Ala (LL320,321AA, IL323,324AA, and LLIL320,321,323,324AAAA) in wild-type CXCR2 or 331T causes little change in ligand binding and signaling through Ca 2+ mobilization but greatly impairs the agonist-induced receptor sequestration and ligand-mediated chemotaxis. The LL320,321AA, IL323,324AA, and LLIL320,321,323,324AAAA mutants of CXCR2 exhibit normal binding to β-arrestin 1 but exhibit decreased binding to adaptin 2α and β. These data demonstrate a role for the LLKIL motif in the carboxyl terminus of CXCR2 in receptor internalization and cell chemotaxis and imply a role for adaptin 2 in the endocytosis of CXCR2.The chemokine receptor, CXCR2, 1 is a member of a superfamily of G protein-coupled seventransmembrane receptors (GPCRs) that transduce intracellular signals via heterotrimeric guanine nucleotide-binding proteins (G proteins). Upon stimulation by agonists, such as interleukin 8 (IL-8) or melanoma growth-stimulatory activity (MGSA)/growth-regulatory protein (GRO), CXCR2 activates a series of G protein-mediated events, including phosphatidylinositide hydrolysis, to generate inositol 1,4,5-trisphosphate and diacylglycerol, as well as mobilization of intracellular free Ca 2+ to initiate a series of cellular responses (1). In addition, CXCR2 mediates cell chemotaxis, a distinct function of chemokine receptors (2). Like many other types of GPCRs, CXCR2 undergoes a dynamic trafficking between the cell surface and the intracellular compartments (3). Such trafficking may be involved in both transmission and termination of the receptor signals and may play an important role in mediating cell chemotaxis. For CXCR2, the most remarkable trafficking process is agonistinduced receptor internalization. 1 Abbreviations: CXCR2, receptor for CXC chemokines formerly defined as interleukin 8 receptor B; IL-8, interleukin 8; MGSA/GRO, melanoma growth-stimulatory activity/growth-regulatory protein; GPCRs, G protein-coupled receptors; G proteins, guanine nucleotidebinding proteins; β2-AR, β2-adrenergic receptor; AP-2, adaptin 2; HEK293 cells, human embryonic kidney 293 cells; RBL-2H3 cells, rabbit basophilic leukemia cells; DSP, dithiobis(succinimido propionate); GRKs, G protein-coupled receptor kinases; FITC, fluorescein isothiocyanate; BSA, bovine serum albumin; DMEM, Dulbecco's modified Eagle's medium; FBS, fetal bovine serum; PBS, phosphatebuffered saline; SDS, sodium dodecyl sulfate. Agonist-induced phosphorylation of the carboxyl terminus by G protein-couple...