Endothelial cell-specific membrane protein involved in the formation of the diaphragms that bridge endothelial fenestrae. It is also required for the formation of stomata of caveolae and transendothelial channels. Functions in microvascular permeability, endothelial fenestrae contributing to the passage of water and solutes and regulating transcellular versus paracellular flow in different organs. Plays a specific role in embryonic development. Homodimer. Membrane-associated protein of caveolae (PubMed:10366592). Found in fenestral and stomatal diaphragms in fenestrated endothelia and transendothelial channels (PubMed:10557298). Also colocalized with CAV1 in perinuclear region (PubMed:15640522). Expressed in lung (alveolar endothelial and bronchial epithelial cells), kidney (endothelium of peritubular capillaries), spleen, liver, adrenal (endothelial cells of the zona reticularis of the cortex and chromaffin cells in the medulla), pancreas (islets of Langerhans), testis (germ cells, interstitial cells in neonatal testis and spermatids), ovary (stromal endothelial, thecal layer of developing follicles, luteal cells within the corpus luteum), intestine (endothelium of capillaries of the intestinal villi) and pituitary (pituicyte cells in the neural lobe) (at protein level). Expressed in lung, kidney, spleen, liver, adrenal, testis, heart, muscle, pituitary, thyroid and ovary. Expressed in endothelial cells of the lung at 12 dpc.