Receptor for the C-X-C chemokine CXCL12/SDF-1 that transduces a signal by increasing intracellular calcium ion levels and enhancing MAPK1/MAPK3 activation (PubMed:8962122, PubMed:9295051, PubMed:9103415). Involved in the AKT signaling cascade (By similarity). Plays a role in regulation of cell migration, e.g. during wound healing. Acts as a receptor for extracellular ubiquitin; leading to enhanced intracellular calcium ions and reduced cellular cAMP levels. Binds bacterial lipopolysaccharide (LPS) et mediates LPS-induced inflammatory response, including TNF secretion by monocytes (By similarity). Involved in hematopoiesis and in cardiac ventricular septum formation (PubMed:9634237, PubMed:9634238, PubMed:9689100). Also plays an essential role in vascularization of the gastrointestinal tract, probably by regulating vascular branching and/or remodeling processes in endothelial cells (PubMed:9634237). Involved in cerebellar development. In the CNS, could mediate hippocampal-neuron survival (PubMed:9634238, PubMed:9689100). Monomer. Can form homodimers. Interacts with CD164. Interacts with ARRB2; the interaction is dependent on the C-terminal phosphorylation of CXCR4 and allows activation of MAPK1 and MAPK3. Interacts with ARR3; the interaction is dependent on the C-terminal phosphorylation of CXCR4 and modulates calcium mobilization. Interacts with RNF113A; the interaction, enhanced by CXCL12, promotes CXCR4 ubiquitination and subsequent degradation. Interacts (via the cytoplasmic C-terminal) with ITCH (via the WW domains I and II); the interaction, enhanced by CXCL12, promotes CXCR4 ubiquitination and leads to its degradation. Interacts with extracellular ubiquitin. Interacts with DBN1; this interaction is enhanced by antigenic stimulation. Following LPS binding, may form a complex with GDF5, HSP90AA1 and HSPA8. In unstimulated cells, diffuse pattern on plasma membrane. On agonist stimulation, colocalizes with ITCH at the plasma membrane where it becomes ubiquitinated (By similarity). In the presence of antigen, distributes to the immunological synapse forming at the T-cell-APC contact area, where it localizes at the peripheral and distal supramolecular activation cluster (SMAC) (By similarity). Lymphocytes, macrophages, neutrophils, microglial cells and astrocytes. Found in spleen, thymus, bone marrow, lymph nodes and, at lower levels in brain, small intestine, stomach and kidney. CXCR4-A is predominant in all tissues tested. During embryonic development, high levels are detected in the endothelium of developing blood vessels and in many regions of the developing brain including the olfactory epithelium, olfactory bulb, hippocampus, cerebellum and spinal cord. High expression during embryonic development does not seem to be associated with the differentiation of any particular cell type, but is widely utilized when there is a requirement for cell movement. Frequently associated with less differentiated cell types and down-regulated with subsequent differentiation. Detected in sites with hemopoietic potential: the yolk sac (7.5, 8.5 and 12.5 dpc) and fetal liver (12.5 dpc). During gastrulation, at 7.2 to 7.8 dpc, expressed in the mesoderm and the definitive endoderm. As gastrulation pattern fades (8.5 dpc), expression in the mesoderm is down-regulated, while it becomes predominant in neural ectoderm. Endodermal expression is retained in the foregut and later in a subset of foregut derivatives, including the stomach (10.5 dpc), the cystic ducts of the gall bladder and the lung epithelium (12.5 dpc). In neuronal tissue: at 10.5 and 12.5 dpc, expressed in the dorsal root ganglia, in the ventral mantle layer of the spinal cord (or basal plates), in the hindbrain. At 14.5 dpc, expression more tightly confined to the neural epithelium lining the ventricular space and to the external granular layer of the ventral rhombic lip (the developing cerebellum). Expressed in the outpocketing of the diencephalic floor at 10.5 dpc and in the developing thalamus and, to a lesser extent, the developing hypothalamus. At 14.5 dpc, restricted to the region where thalamus and hypothalamus meet. Detected in a discrete band of cells at the edge of the olfactory bulb. In the vascular system: expressed in the endothelium of numerous blood vessels, but not all, at 10.5, 11.5 and 12.5 dpc, such as vitelline/umbilical vessels, cardiac ventricular wall capillaries, facial vessels and, at 14.5 dpc, in the vasculature of the herniated gut. Expression seems to be associated with expanding vascular networks. In the heart development, expressed at 10.5 dpc in the precursor to the aortopulmonary (AP) septum. At 12.5 dpc, detected in the AP septum at the base of the outflow tract and in the atrioventricular valves. Detected in cranofacial ectoderm from 10.5 to 14.5 dpc. At 10.5 and 11.5 dpc, expressed in the Rathke pouch. Phosphorylated on agonist stimulation. Rapidly phosphorylated on serine and threonine residues in the C-terminal. Phosphorylation at Ser-331 and Ser-332 leads to recruitment of ITCH, ubiquitination and protein degradation. Ubiquitinated after ligand binding, leading to its degradation. Ubiquitinated by ITCH at the cell membrane on agonist stimulation. The ubiquitin-dependent mechanism, endosomal sorting complex required for transport (ESCRT), then targets CXCR4 for lysosomal degradation. This process is dependent also on prior Ser-/Thr-phosphorylation in the C-terminal of CXCR4. Also binding of ARRB1 to STAM negatively regulates CXCR4 sorting to lysosomes though modulating ubiquitination of SFR5S. Sulfation is required for efficient binding of CXCL12/SDF-1alpha and promotes its dimerization. O- and N-glycosylated. N-glycosylation can mask coreceptor function. The O-glycosylation chondroitin sulfate attachment does not affect interaction with CXCL12/SDF-1alpha nor its coreceptor activity. Half of the embryos die by 17.5 dpc-18.5 dpc and neonates die within a few hours. Mutants display defective vascular development, cerebellar development, B-lymphopoiesis, myelopoiesis, and cardiogenesis with defective formation of the large vessels supplying the gastrointestinal tract. Belongs to the G-protein coupled receptor 1 family.