Lipid transporter that specifically mediates export of sphingosine-1-phosphate (sphing-4-enine 1-phosphate, S1P) and sphinganine-1-phosphate in the lymph, thereby playing a role in lymphocyte trafficking. S1P is a bioactive signaling molecule that regulates many physiological processes important for the development and for the immune system. Regulates levels of S1P and the S1P gradient that exists between the high circulating concentrations of S1P and low tissue levels that control lymphocyte trafficking. Required for the egress of T-cells from lymph nodes during an immune response by mediating S1P secretion, which generates a gradient that enables activated T-cells to access lymph (By similarity). Also required for the egress of immature B-cells from the bone marrow (By similarity). In contrast, not involved in S1P release from red blood cells (By similarity). Involved in auditory function. S1P release in the inner ear is required for maintenance of the endocochlear potential in the cochlea (By similarity). In addition to export, also able to mediate S1P import (By similarity).