P-selectin glycoprotein ligand-1 (PSGL-1) is a homodimeric transmembrane mucin on leukocytes. During inflammation, reversible interactions of PSGL-1 with selectins mediate leukocyte rolling on vascular surfaces. The transmembrane domain of PSGL-1 is required for dimerization, and the cytoplasmic domain propagates signals that activate  2 integrins to slow rolling on integrin ligands. Leukocytes from knock-in "⌬CD" mice express a truncated PSGL-1 that lacks the cytoplasmic domain. Unexpectedly, they have 10-fold less PSGL-1 on their surfaces than WT leukocytes. Using glycosidases, proteases, Western blotting, confocal microscopy, cell-surface cross-linking, FRET, and pulse-chase metabolic labeling, we demonstrate that deleting the cytoplasmic domain impaired dimerization and delayed export of PSGL-1 from the endoplasmic reticulum (ER), markedly increasing a monomeric precursor in the ER and decreasing mature PSGL-1 on the cell surface. A monomeric full-length PSGL-1 made by substituting the transmembrane domain with that of CD43 exited the ER normally, revealing that dimerization was not required for ER export. Thus, the transmembrane and cytoplasmic domains cooperate to promote dimerization of PSGL-1. Furthermore, the cytoplasmic domain provides a key signal to export precursors of PSGL-1 from the ER to the Golgi apparatus en route to the cell surface.During inflammation, reversible interactions of selectins with their glycosylated ligands mediate leukocyte rolling on vascular surfaces (1). Cooperative signaling upon engagement of selectin ligands and chemokine receptors causes rolling leukocytes to decelerate and arrest through interactions of integrin ␣ L  2 on leukocytes with intercellular adhesion molecule-1 (ICAM-1) 4 on endothelial cells. Leukocytes then use integrins to migrate across blood vessels in response to chemokine gradients or other signals (2). P-selectin is expressed on activated endothelial cells and platelets; E-selectin is expressed on activated endothelial cells, and L-selectin is expressed on leukocytes (1). Each selectin mediates leukocyte rolling by interacting with a subset of membrane glycoproteins that must be appropriately glycosylated and, for ligands binding to P-and L-selectin, sulfated. P-selectin glycoprotein ligand-1 (PSGL-1) is a homodimeric, type I transmembrane mucin (1, 3, 4). It is the dominant ligand for P-and L-selectin on leukocytes (5-8), and it cooperates with other ligands to mediate rolling on E-selectin (9 -11). Dimerization of PSGL-1 enhances rolling on P-selectin (12). The subunits of the PSGL-1 homodimer are linked by a disulfide bond just outside the plasma membrane (13). Each extracellular domain has multiple Ser/Thr-linked O-glycans that cause it to adopt a highly extended conformation (14). PSGL-1 is localized on the tips of leukocyte microvilli (5) and has affinity for cholesterol-rich membrane domains or lipid rafts (15). It moves to the uropods as cells polarize after chemokine stimulation (16,17).The sequences of the transmembrane and cytoplasmic domai...