Functions as cell surface receptor for TIMP1 and plays a role in the activation of cellular signaling cascades. Plays a role in the activation of ITGB1 and integrin signaling, leading to the activation of AKT, FAK/PTK2 and MAP kinases. Promotes cell survival, reorganization of the actin cytoskeleton, cell adhesion, spreading and migration, via its role in the activation of AKT and FAK/PTK2. Plays a role in VEGFA signaling via its role in regulating the internalization of KDR/VEGFR2. Plays a role in intracellular vesicular transport processes, and is required for normal trafficking of the PMEL luminal domain that is essential for the development and maturation of melanocytes. Plays a role in the adhesion of leukocytes onto endothelial cells via its role in the regulation of SELP trafficking. May play a role in mast cell degranulation in response to Ms4a2/FceRI stimulation, but not in mast cell degranulation in response to other stimuli. Interacts with TIMP1 and ITGB1 and recruits TIMP1 to ITGB1. Interacts with CD9. Identified in a complex with CD9 and ITGB3. Interacts with PMEL. Interacts with KDR/VEGFR2; identified in a complex with ITGB1 and KDR/VEGFR2 and is required to recruit KDR to ITGB1 complexes (By similarity). Interacts with SYT7 (PubMed:21041449). Also found in Weibel-Palade bodies of endothelial cells. Located in platelet dense granules. Detected in a subset of pre-melanosomes. Detected on intralumenal vesicles (ILVs) within multivesicular bodies. Ubiquitous. Strongly expressed in kidney. Detected in spleen, bone marrow, peripheral blood mononuclear cells and macrophages. Palmitoylated at a low, basal level in unstimulated platelets. The level of palmitoylation increases when platelets are activated by thrombin (in vitro) (By similarity). No visible phenotype. Mutant mice are viable and fertile, but display impaired water homeostasis, with increased urinary flow, increased water intake, reduced urine osmolality and increased fecal water content. Mice display inclusions in the principal cells in the renal collecting duct (PubMed:19075008). Mutant mice display variable graying of their coat color and a dramatic reduction in the number of melanosomes in the retinal pigment epithelium (PubMed:21962903). According to PubMed:21803846, mutant mice display a defect in the Selp-dependent attachment of leukocytes to endothelial cells. According to PubMed:23945142, mast cells from mutant mice show decreased degranulation and decreased release of TNF in response to Ms4a2/FceRI stimulation, but no difference in mast cell degranulation in response to other stimuli and no change in the release of IL6 and leukotriene C4. Belongs to the tetraspanin (TM4SF) family.